Orbital hybridisation

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Orbital_hybridisation

In chemistry, orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory. For example, in a carbon atom which forms four single bonds, the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp³ mixtures in a tetrahedral arrangement around the carbon to bond to four different atoms. Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties and are symmetrically disposed in space. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies.

History and uses

Chemist Linus Pauling first developed the hybridisation theory in 1931 to explain the structure of simple molecules such as methane (CH₄) using atomic orbitals. Pauling pointed out that a carbon atom forms four bonds by using one s and three p orbitals, so that "it might be inferred" that a carbon atom would form three bonds at right angles (using p orbitals) and a fourth weaker bond using the s orbital in some arbitrary direction. In reality, methane has four C–H bonds of equivalent strength. The angle between any two bonds is the tetrahedral bond angle of 109°28' (around 109.5°). Pauling supposed that in the presence of four hydrogen atoms, the s and p orbitals form four equivalent combinations which he called hybrid orbitals. Each hybrid is denoted sp³ to indicate its composition, and is directed along one of the four C–H bonds. This concept was developed for such simple chemical systems, but the approach was later applied more widely, and today it is considered an effective heuristic for rationalizing the structures of organic compounds. It gives a simple orbital picture equivalent to Lewis structures.

Hybridisation theory is an integral part of organic chemistry, one of the most compelling examples being Baldwin's rules. For drawing reaction mechanisms sometimes a classical bonding picture is needed with two atoms sharing two electrons. Hybridisation theory explains bonding in alkenes and methane. The amount of p character or s character, which is decided mainly by orbital hybridisation, can be used to reliably predict molecular properties such as acidity or basicity.

Overview

Orbitals are a model representation of the behavior of electrons within molecules. In the case of simple hybridization, this approximation is based on atomic orbitals, similar to those obtained for the hydrogen atom, the only neutral atom for which the Schrödinger equation can be solved exactly. In heavier atoms, such as carbon, nitrogen, and oxygen, the atomic orbitals used are the 2s and 2p orbitals, similar to excited state orbitals for hydrogen.

Hybrid orbitals are assumed to be mixtures of atomic orbitals, superimposed on each other in various proportions. For example, in methane, the C hybrid orbital which forms each carbon–hydrogen bond consists of 25% s character and 75% p character and is thus described as sp³ (read as s-p-three) hybridised. Quantum mechanics describes this hybrid as an sp³ wavefunction of the form ${\displaystyle N(s+\sqrt{3}p\sigma )}$, where N is a normalisation constant (here 1/2) and pσ is a p orbital directed along the C-H axis to form a sigma bond. The ratio of coefficients (denoted λ in general) is ${\displaystyle \sqrt{3}}$ in this example. Since the electron density associated with an orbital is proportional to the square of the wavefunction, the ratio of p-character to s-character is λ² = 3. The p character or the weight of the p component is N²λ² = 3/4.

Types of hybridisation

sp³

Four sp³ orbitals.

See also: tetrahedral molecular geometry

Hybridisation describes the bonding of atoms from an atom's point of view. For a tetrahedrally coordinated carbon (e.g., methane CH₄), the carbon should have 4 orbitals directed towards the 4 hydrogen atoms.

Carbon's ground state configuration is 1s² 2s² 2p² or more easily read:

C	↑↓	↑↓	↑	↑
	1s	2s	2p	2p	2p

This diagram suggests that the carbon atom could use its two singly occupied p-type orbitals to form two covalent bonds with two hydrogen atoms in a methylene (CH₂) molecule, with a hypothetical bond angle of 90° corresponding to the angle between two p orbitals on the same atom. However the true H–C–H angle in singlet methylene is about 102° which implies the presence of some orbital hybridisation.

The carbon atom can also bond to four hydrogen atoms in methane by an excitation (or promotion) of an electron from the doubly occupied 2s orbital to the empty 2p orbital, producing four singly occupied orbitals.

C*	↑↓	↑	↑	↑	↑
	1s	2s	2p	2p	2p

The energy released by the formation of two additional bonds more than compensates for the excitation energy required, energetically favoring the formation of four C-H bonds.

According to quantum mechanics the lowest energy is obtained if the four bonds are equivalent, which requires that they are formed from equivalent orbitals on the carbon. A set of four equivalent orbitals can be obtained that are linear combinations of the valence-shell (core orbitals are almost never involved in bonding) s and p wave functions, which are the four sp³ hybrids.

C*	↑↓	↑	↑	↑	↑
	1s	sp3	sp3	sp3	sp3

In CH₄, four sp³ hybrid orbitals are overlapped by hydrogen 1s orbitals, yielding four σ (sigma) bonds (that is, four single covalent bonds) of equal length and strength.

The following :

translates into :

sp²

Three sp² orbitals.

Ethene structure

See also: trigonal planar molecular geometry

Other carbon compounds and other molecules may be explained in a similar way. For example, ethene (C₂H₄) has a double bond between the carbons.

For this molecule, carbon sp² hybridises, because one π (pi) bond is required for the double bond between the carbons and only three σ bonds are formed per carbon atom. In sp² hybridisation the 2s orbital is mixed with only two of the three available 2p orbitals, usually denoted 2p_x and 2p_y. The third 2p orbital (2p_z) remains unhybridised.

C*	↑↓	↑	↑	↑	↑
	1s	sp2	sp2	sp2	2p

forming a total of three sp² orbitals with one remaining p orbital. In ethene, the two carbon atoms form a σ bond by overlapping one sp² orbital from each carbon atom. The π bond between the carbon atoms perpendicular to the molecular plane is formed by 2p–2p overlap. Each carbon atom forms covalent C–H bonds with two hydrogens by s–sp² overlap, all with 120° bond angles. The hydrogen–carbon bonds are all of equal strength and length, in agreement with experimental data.

sp

Two sp orbitals

See also: linear molecular geometry

The chemical bonding in compounds such as alkynes with triple bonds is explained by sp hybridization. In this model, the 2s orbital is mixed with only one of the three p orbitals,

C*	↑↓	↑	↑	↑	↑
	1s	sp	sp	2p	2p

resulting in two sp orbitals and two remaining p orbitals. The chemical bonding in acetylene (ethyne) (C₂H₂) consists of sp–sp overlap between the two carbon atoms forming a σ bond and two additional π bonds formed by p–p overlap. Each carbon also bonds to hydrogen in a σ s–sp overlap at 180° angles.

Hybridisation and molecule shape

Shapes of the different types of hybrid orbitals

Hybridisation helps to explain molecule shape, since the angles between bonds are approximately equal to the angles between hybrid orbitals. This is in contrast to valence shell electron-pair repulsion (VSEPR) theory, which can be used to predict molecular geometry based on empirical rules rather than on valence-bond or orbital theories.

sp^x hybridisation

As the valence orbitals of main group elements are the one s and three p orbitals with the corresponding octet rule, sp^x hybridization is used to model the shape of these molecules.

Coordination number	Shape	Hybridisation	Examples
2	Linear	sp hybridisation (180°)	CO2
3	Trigonal planar	sp2 hybridisation (120°)	BCl3
4	Tetrahedral	sp3 hybridisation (109.5°)	CCl4
Interorbital angles	${\displaystyle \theta =\arccos \left(-\frac{1}{x}\right)}$

sp^xd^y hybridisation

As the valence orbitals of transition metals are the five d, one s and three p orbitals with the corresponding 18-electron rule, sp^xd^y hybridisation is used to model the shape of these molecules. These molecules tend to have multiple shapes corresponding to the same hybridization due to the different d-orbitals involved. A square planar complex has one unoccupied p-orbital and hence has 16 valence electrons.

sd^x hybridisation

In certain transition metal complexes with a low d electron count, the p-orbitals are unoccupied and sd^x hybridisation is used to model the shape of these molecules.

Coordination number	Shape	Hybridisation	Examples
3	Trigonal pyramidal	sd2 hybridisation (90°)	CrO3
4	Tetrahedral	sd3 hybridisation (70.5°, 109.5°)	TiCl4
5	Square pyramidal	sd4 hybridisation (65.9°, 114.1°)	Ta(CH3)5
6	C3v Trigonal prismatic	sd5 hybridisation (63.4°, 116.6°)	W(CH3)6
Interorbital angles	${\displaystyle \theta =\arccos \left(\pm \sqrt{\frac{1}{3}\left(1-\frac{2}{x}\right)}\right)}$

Hybridisation of hypervalent molecules

Main article: Hypervalent molecule

Octet expansion

In some general chemistry textbooks, hybridization is presented for main group coordination number 5 and above using an "expanded octet" scheme with d-orbitals first proposed by Pauling. However, such a scheme is now considered to be incorrect in light of computational chemistry calculations.

Coordination number	Molecular shape	Hybridisation	Examples
5	Trigonal bipyramidal	sp3d hybridisation	PF5
6	Octahedral	sp3d2 hybridisation	SF6
7	Pentagonal bipyramidal	sp3d3 hybridisation	IF7

In 1990, Eric Alfred Magnusson of the University of New South Wales published a paper definitively excluding the role of d-orbital hybridisation in bonding in hypervalent compounds of second-row (period 3) elements, ending a point of contention and confusion. Part of the confusion originates from the fact that d-functions are essential in the basis sets used to describe these compounds (or else unreasonably high energies and distorted geometries result). Also, the contribution of the d-function to the molecular wavefunction is large. These facts were incorrectly interpreted to mean that d-orbitals must be involved in bonding.

Resonance

In light of computational chemistry, a better treatment would be to invoke sigma bond resonance in addition to hybridisation, which implies that each resonance structure has its own hybridisation scheme. All resonance structures must obey the octet rule.

Coordination number	Resonance structures
5	Trigonal bipyramidal
6	Octahedral
7	Pentagonal bipyramidal

Hybridisation in computational VB theory

Further information: Modern valence bond theory

While the simple model of orbital hybridisation is commonly used to explain molecular shape, hybridisation is used differently when computed in modern valence bond programs. Specifically, hybridisation is not determined a priori but is instead variationally optimized to find the lowest energy solution and then reported. This means that all artificial constraints, specifically two constraints, on orbital hybridisation are lifted:

• that hybridisation is restricted to integer values (isovalent hybridisation)
• that hybrid orbitals are orthogonal to one another (hybridisation defects)

This means that in practice, hybrid orbitals do not conform to the simple ideas commonly taught and thus in scientific computational papers are simply referred to as sp^x, sp^xd^y or sd^x hybrids to express their nature instead of more specific integer values.

Isovalent hybridisation

Main article: Isovalent hybridisation

Although ideal hybrid orbitals can be useful, in reality, most bonds require orbitals of intermediate character. This requires an extension to include flexible weightings of atomic orbitals of each type (s, p, d) and allows for a quantitative depiction of the bond formation when the molecular geometry deviates from ideal bond angles. The amount of p-character is not restricted to integer values; i.e., hybridizations like sp^2.5 are also readily described.

The hybridization of bond orbitals is determined by Bent's rule: "Atomic s character concentrates in orbitals directed towards electropositive substituents".

For molecules with lone pairs, the bonding orbitals are isovalent sp^x hybrids. For example, the two bond-forming hybrid orbitals of oxygen in water can be described as sp^4.0 to give the interorbital angle of 104.5°. This means that they have 20% s character and 80% p character and does not imply that a hybrid orbital is formed from one s and four p orbitals on oxygen since the 2p subshell of oxygen only contains three p orbitals.

Hybridisation defects

Hybridisation of s and p orbitals to form effective sp^x hybrids requires that they have comparable radial extent. While 2p orbitals are on average less than 10% larger than 2s, in part attributable to the lack of a radial node in 2p orbitals, 3p orbitals which have one radial node, exceed the 3s orbitals by 20–33%. The difference in extent of s and p orbitals increases further down a group. The hybridisation of atoms in chemical bonds can be analysed by considering localised molecular orbitals, for example using natural localised molecular orbitals in a natural bond orbital (NBO) scheme. In methane, CH₄, the calculated p/s ratio is approximately 3 consistent with "ideal" sp³ hybridisation, whereas for silane, SiH₄, the p/s ratio is closer to 2. A similar trend is seen for the other 2p elements. Substitution of fluorine for hydrogen further decreases the p/s ratio. The 2p elements exhibit near ideal hybridisation with orthogonal hybrid orbitals. For heavier p block elements this assumption of orthogonality cannot be justified. These deviations from the ideal hybridisation were termed hybridisation defects by Kutzelnigg.

However, computational VB groups such as Gerratt, Cooper and Raimondi (SCVB) as well as Shaik and Hiberty (VBSCF) go a step further to argue that even for model molecules such as methane, ethylene and acetylene, the hybrid orbitals are already defective and nonorthogonal, with hybridisations such as sp^1.76 instead of sp³ for methane.

Photoelectron spectra

One misconception concerning orbital hybridization is that it incorrectly predicts the ultraviolet photoelectron spectra of many molecules. While this is true if Koopmans' theorem is applied to localized hybrids, quantum mechanics requires that the (in this case ionized) wavefunction obey the symmetry of the molecule which implies resonance in valence bond theory. For example, in methane, the ionised states (CH₄⁺) can be constructed out of four resonance structures attributing the ejected electron to each of the four sp³ orbitals. A linear combination of these four structures, conserving the number of structures, leads to a triply degenerate T₂ state and an A₁ state. The difference in energy between each ionized state and the ground state would be ionization energy, which yields two values in agreement with experimental results.

Two distinct states for CH₄⁺ exist (A₁ and T₂), both of which result from the ionization of CH₄. This gives rise to the two unique peaks on the photoelectron spectrum of methane.

Localized vs canonical molecular orbitals

Main articles: Localized molecular orbitals and Natural bond orbital

Bonding orbitals formed from hybrid atomic orbitals may be considered as localized molecular orbitals, which can be formed from the delocalized orbitals of molecular orbital theory by an appropriate mathematical transformation. For molecules in the ground state, this transformation of the orbitals leaves the total many-electron wave function unchanged. The hybrid orbital description of the ground state is, therefore equivalent to the delocalized orbital description for ground state total energy and electron density, as well as the molecular geometry that corresponds to the minimum total energy value.

Two localized representations

Main article: Sigma-pi and equivalent-orbital models

The symmetry-adapted and hybridized lone pairs of H₂O

Molecules with multiple bonds or multiple lone pairs can have orbitals represented in terms of sigma and pi symmetry or equivalent orbitals. Different valence bond methods use either of the two representations, which have mathematically equivalent total many-electron wave functions and are related by a unitary transformation of the set of occupied molecular orbitals.

For multiple bonds, the sigma-pi representation is the predominant one compared to the equivalent orbital (bent bond) representation. In contrast, for multiple lone pairs, most textbooks use the equivalent orbital representation. However, the sigma-pi representation is also used, such as by Weinhold and Landis within the context of natural bond orbitals, a localized orbital theory containing modernized analogs of classical (valence bond/Lewis structure) bonding pairs and lone pairs. For the hydrogen fluoride molecule, for example, two F lone pairs are essentially unhybridized p orbitals, while the other is an sp^x hybrid orbital. An analogous consideration applies to water (one O lone pair is in a pure p orbital, another is in an sp^x hybrid orbital).

Iron Age

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Iron_Age

The Iron Age (c. 1200 – c. 550 BC) is the final epoch of the three historical Metal Ages, after the Chalcolithic and Bronze Age. It has also been considered as the final age of the three-age division starting with prehistory (before recorded history) and progressing to protohistory (before written history). In this usage, it is preceded by the Stone Age (subdivided into the Paleolithic, Mesolithic and Neolithic) and Bronze Age. These concepts originated for describing Iron Age Europe and the Ancient Near East. The indigenous cultures of the New World did not develop an iron economy before 1500.

Although meteoric iron has been used for millennia in many regions, the beginning of the Iron Age is defined locally around the world by archaeological convention when the production of smelted iron (especially steel tools and weapons) replaces their bronze equivalents in common use.

In Anatolia and the Caucasus, or Southeast Europe, the Iron Age began during the late 2nd millennium BC (c. 1300 BC). In the Ancient Near East, this transition occurred simultaneously with the Late Bronze Age collapse, during the 12th century BC (1200–1100 BC). The technology soon spread throughout the Mediterranean Basin region and to South Asia between the 12th and 11th century BC. Its further spread to Central Asia, Eastern Europe, and Central Europe was somewhat delayed, and Northern Europe was not reached until about the start of the 5th century BC (500 BC).

The Iron Age in India is stated as beginning with the ironworking Painted Grey Ware culture, dating from the 15th century BC, through to the reign of Ashoka in the 3rd century BC. The term "Iron Age" in the archaeology of South, East, and Southeast Asia is more recent and less common than for Western Eurasia. Africa did not have a universal "Bronze Age", and many areas transitioned directly from stone to iron. Some archaeologists believe that iron metallurgy was developed in sub-Saharan Africa independently from Eurasia and neighbouring parts of Northeast Africa as early as 2000 BC.

The concept of the Iron Age ending with the beginning of the written historiographical record has not generalized well, as written language and steel use have developed at different times in different areas across the archaeological record. For instance, in China, written history started before iron smelting began, so the term is used infrequently for the archaeology of China. For the Ancient Near East, the establishment of the Achaemenid Empire c. 550 BC is used traditionally and still usually as an end date; later dates are considered historical according to the record by Herodotus despite considerable written records now being known from well back into the Bronze Age. In Central and Western Europe, the Roman conquests of the 1st century BC serve as marking the end of the Iron Age. The Germanic Iron Age of Scandinavia is considered to end c. AD 800, with the beginning of the Viking Age.

History of the concept

The three-age method of Stone, Bronze, and Iron Ages was first used for the archaeology of Europe during the first half of the 19th century, and by the latter half of the 19th century, it had been extended to the archaeology of the Ancient Near East. Its name harks back to the mythological "Ages of Man" of Hesiod. As an archaeological era, it was first introduced to Scandinavia by Christian Jürgensen Thomsen during the 1830s. By the 1860s, it was embraced as a useful division of the "earliest history of mankind" in general and began to be applied in Assyriology. The development of the now-conventional periodization in the archaeology of the Ancient Near East was developed during the 1920s and 1930s.

Definition of "iron"

Main articles: Ferrous metallurgy § Iron smelting and the Iron Age, and Archaeometallurgical slag

Willamette Meteorite, the sixth largest in the world, is an iron–nickel meteorite.

Meteoric iron, a natural iron–nickel alloy, was used by various ancient peoples thousands of years before the Iron Age. The earliest-known meteoric iron artifacts are nine small beads dated to 3200 BC, which were found in burials at Gerzeh in Lower Egypt, having been shaped by careful hammering.

The characteristic of an Iron Age culture is the mass production of tools and weapons made not just of found iron, but from smelted steel alloys with an added carbon content. Only with the capability of the production of carbon steel does ferrous metallurgy result in tools or weapons that are harder and lighter than bronze.

Smelted iron appears sporadically in the archeological record from the middle Bronze Age. Whilst terrestrial iron is abundant naturally, temperatures above 1,250 °C (2,280 °F) are required to smelt it, impractical to achieve with the technology available commonly until the end of the second millennium BC. In contrast, the components of bronze—tin with a melting point of 231.9 °C (449.4 °F) and copper with a relatively moderate melting point of 1,085 °C (1,985 °F)—were within the capabilities of Neolithic kilns, which date back to 6000 BC and were able to produce temperatures greater than 900 °C (1,650 °F).

In addition to specially designed furnaces, ancient iron production required the development of complex procedures for the removal of impurities, the regulation of the admixture of carbon, and the invention of hot-working to achieve a useful balance of hardness and strength in steel. The use of steel has also been regulated by the economics of the metallurgical advancements.

Chronology

Earliest evidence

The earliest tentative evidence for iron-making is a small number of iron fragments with the appropriate amounts of carbon admixture found in the Proto-Hittite layers at Kaman-Kalehöyük in modern-day Turkey, dated to 2200–2000 BC. Akanuma (2008) concludes that "The combination of carbon dating, archaeological context, and archaeometallurgical examination indicates that it is likely that the use of ironware made of steel had already begun in the third millennium BC in Central Anatolia". Souckova-Siegolová (2001) shows that iron implements were made in Central Anatolia in very limited quantities about 1800 BC and were in general use by elites, though not by commoners, during the New Hittite Empire (≈1400–1200 BC).

Similarly, recent archaeological remains of iron-working in the Ganges Valley in India have been dated tentatively to 1800 BC. Tewari (2003) concludes that "knowledge of iron smelting and manufacturing of iron artifacts was well known in the Eastern Vindhyas and iron had been in use in the Central Ganga Plain, at least from the early second millennium BC". By the Middle Bronze Age increasing numbers of smelted iron objects (distinguishable from meteoric iron by the lack of nickel in the product) appeared in the Middle East, Southeast Asia and South Asia.

African sites are revealing dates as early as 2000–1200 BC. However, some recent studies date the inception of iron metallurgy in Africa between 3000 and 2500 BC, with evidence existing for early iron metallurgy in parts of Nigeria, Cameroon, and Central Africa, from as early as around 2,000 BC. The Nok culture of Nigeria may have practiced iron smelting from as early as 1000 BC, while the nearby Djenné-Djenno culture of the Niger Valley in Mali shows evidence of iron production from c. 250 BC. Iron technology across much of sub-Saharan Africa has an African origin dating to before 2000 BC. These findings confirm the independent invention of iron smelting in sub-Saharan Africa.

Beginning

Copy of The Warrior of Hirschlanden (German: Krieger von Hirschlanden), a statue of a nude ithyphallic warrior made of sandstone, the oldest known Iron Age life-size anthropomorphic statue north of the Alps.

Modern archaeological evidence identifies the start of large-scale global iron production about 1200 BC, marking the end of the Bronze Age. The Iron Age in Europe is often considered as a part of the Bronze Age collapse in the ancient Near East.

Anthony Snodgrass suggests that a shortage of tin and trade disruptions in the Mediterranean about 1300 BC forced metalworkers to seek an alternative to bronze. Many bronze implements were recycled into weapons during that time, and more widespread use of iron resulted in improved steel-making technology and lower costs. When tin became readily available again, iron was cheaper, stronger and lighter, and forged iron implements superseded cast bronze tools permanently.

In Central and Western Europe, the Iron Age lasted from c. 800 BC to c. 1 BC, beginning in pre-Roman Iron Age Northern Europe in c. 600 BC, and reaching Northern Scandinavian Europe about c. 500 BC.

The Iron Age in the Ancient Near East is considered to last from c. 1200 BC (the Bronze Age collapse) to c. 550 BC (or 539 BC), roughly the beginning of historiography with Herodotus, marking the end of the proto-historical period.

In China, because writing was developed first, there is no recognizable prehistoric period characterized by ironworking, and the Bronze Age China transitions almost directly into the Qin dynasty of imperial China. "Iron Age" in the context of China is used sometimes for the transitional period of c. 900 BC to 100 BC during which ferrous metallurgy was present even if not dominant.

Ancient Near East

The Iron Age in the Ancient Near East is believed to have begun after the discovery of iron smelting and smithing techniques in Anatolia, the Caucasus or Southeast Europe during the late 2nd millennium BC (c. 1300 BC). The earliest bloomery smelting of iron is found at Tell Hammeh, Jordan about 930 BC (determined from ¹⁴C dating).

The Early Iron Age in the Caucasus area is divided conventionally into two periods, Early Iron I, dated to about 1100 BC, and the Early Iron II phase from the tenth to ninth centuries BC. Many of the material culture traditions of the Late Bronze Age continued into the Early Iron Age. Thus, there is a sociocultural continuity during this transitional period.

In Iran, the earliest actual iron artifacts were unknown until the 9th century BC. For Iran, the best studied archaeological site during this time period is Teppe Hasanlu.

West Asia

In the Mesopotamian states of Sumer, Akkad and Assyria, the initial use of iron reaches far back, to perhaps 3000 BC. One of the earliest smelted iron artifacts known is a dagger with an iron blade found in a Hattic tomb in Anatolia, dating from 2500 BC. The widespread use of iron weapons which replaced bronze weapons rapidly disseminated throughout the Near East (North Africa, southwest Asia) by the beginning of the 1st millennium BC.

The development of iron smelting was once attributed to the Hittites of Anatolia during the Late Bronze Age. As part of the Late Bronze Age-Early Iron Age, the Bronze Age collapse saw the slow, comparatively continuous spread of iron-working technology in the region. It was long believed that the success of the Hittite Empire during the Late Bronze Age had been based on the advantages entailed by the "monopoly" on ironworking at the time. Accordingly, the invading Sea Peoples would have been responsible for spreading the knowledge through that region. The idea of such a "Hittite monopoly" has been examined more thoroughly and no longer represents a scholarly consensus. While there are some iron objects from Bronze Age Anatolia, the number is comparable to iron objects found in Egypt and other places of the same time period; and only a small number of these objects are weapons.

Early examples and distribution of non-precious metal finds

Date	Crete	Aegean	Greece	Cyprus	Sub-totals	Anatolia	Totals
1300–1200 BC	5	2	9	0	16	33	49
Total Bronze Age	5	2	9	0	16	33	49
1200–1100 BC	1	2	8	26	37	N/A	37
1100–1000 BC	13	3	31	33	80	N/A	80
1000–900 BC	37+	30	115	29	211	N/A	211
Total Iron Age [Columns don't sum precisely]	51	35	163	88	328	N/A	328

Dates are approximate; consult particular article for details.

•    Prehistoric (or Proto-historic) Iron Age   Historic Iron Age

Egypt

Main article: Third Intermediate Period of Egypt

Iron metal is singularly scarce in collections of Egyptian antiquities. Bronze remained the primary material there until the conquest by the Neo-Assyrian Empire in 671 BC. The explanation of this would seem to be that the relics are in most cases the paraphernalia of tombs, the funeral vessels and vases, and iron being considered an impure metal by the ancient Egyptians it was never used in their manufacture of these or for any religious purposes. It was attributed to Seth, the spirit of evil who according to Egyptian tradition governed the central deserts of Africa. In the Black Pyramid of Abusir, dating before 2000 BC, Gaston Maspero found some pieces of iron. In the funeral text of Pepi I, the metal is mentioned. A sword bearing the name of pharaoh Merneptah as well as a battle axe with an iron blade and gold-decorated bronze shaft were both found in the excavation of Ugarit. A dagger with an iron blade found in Tutankhamun's tomb, 13th century BC, was examined recently and found to be of meteoric origin.

Europe

Main article: Iron Age Europe

Maiden Castle, Dorset, England. More than 2,000 Iron Age hillforts are known in Britain.

In Europe, the Iron Age is the last stage of prehistoric Europe and the first of the protohistoric periods, which initially means descriptions of a particular area by Greek and Roman writers. For much of Europe, the period came to an abrupt local end after conquest by the Romans, though ironworking remained the dominant technology until recent times. Elsewhere it may last until the early centuries AD, and either Christianization or a new conquest during the Migration Period.

Iron working was introduced to Europe during the late 11th century BC, probably from the Caucasus, and slowly spread northwards and westwards over the succeeding 500 years. The Iron Age did not start when iron first appeared in Europe but it began to replace bronze in the preparation of tools and weapons. It did not happen at the same time throughout Europe; local cultural developments played a role in the transition to the Iron Age. For example, the Iron Age of Prehistoric Ireland begins about 500 BC (when the Greek Iron Age had already ended) and finishes about 400 AD. The widespread use of the technology of iron was implemented in Europe simultaneously with Asia. The prehistoric Iron Age in Central Europe is divided into two periods based on the Hallstatt culture (early Iron Age) and La Tène (late Iron Age) cultures. Material cultures of Hallstatt and La Tène consist of 4 phases (A, B, C, D).

Culture	Phase A	Phase B	Phase C	Phase D
Hallstatt	1200–700 BC Flat graves	1200–700 BC Pottery made of polychrome	700–600 BC Heavy iron and bronze swords	600–475 BC Dagger swords, brooches, and ring ornaments, girdle mounts
La Tène	450–390 BC S-shaped, spiral and round designs	390–300 BC Iron swords, heavy knives, lanceheads	300–100 BC Iron chains, iron swords, belts, heavy spearheads	100–15 BC Iron reaping-hooks, saws, scythes and hammers

A sword of the Iron Age Cogotas II culture in Spain.

The Iron Age in Europe is characterized by an elaboration of designs of weapons, implements, and utensils. These are no longer cast but hammered into shape, and decoration is elaborate and curvilinear rather than simple rectilinear; the forms and character of the ornamentation of the northern European weapons resemble in some respects Roman arms, while in other respects they are peculiar and evidently representative of northern art.

Citânia de Briteiros, located in Guimarães, Portugal, is one of the examples of archaeological sites of the Iron Age. This settlement (fortified villages) covered an area of 3.8 hectares (9.4 acres), and served as a Celtiberian stronghold against Roman invasions. İt dates more than 2500 years back. The site was researched by Francisco Martins Sarmento starting from 1874. A number of amphoras (containers usually for wine or olive oil), coins, fragments of pottery, weapons, pieces of jewelry, as well as ruins of a bath and its pedra formosa (lit. 'handsome stone') revealed here.

Asia

Central Asia

The Iron Age in Central Asia began when iron objects appear among the Indo-European Saka in present-day Xinjiang (China) between the 10th century BC and the 7th century BC, such as those found at the cemetery site of Chawuhukou.

The Pazyryk culture is an Iron Age archaeological culture (c. 6th to 3rd centuries BC) identified by excavated artifacts and mummified humans found in the Siberian permafrost in the Altay Mountains.

East Asia

Further information: History of metallurgy in China § Iron

 

Dates are approximate; consult particular article for details.

•    Prehistoric (or Proto-historic) Iron Age   Historic Iron Age

In China, Chinese bronze inscriptions are found around 1200 BC, preceding the development of iron metallurgy, which was known by the 9th century BC. The large seal script is identified with a group of characters from a book entitled Shǐ Zhòu Piān (c. 800 BC). Therefore, in China prehistory had given way to history periodized by ruling dynasties by the start of iron use, so "Iron Age" is not used typically to describe a period of Chinese history. Iron metallurgy reached the Yangtse Valley toward the end of the 6th century BC. The few objects were found at Changsha and Nanjing. The mortuary evidence suggests that the initial use of iron in Lingnan belongs to the mid-to-late Warring States period (from about 350 BC). Important non-precious husi style metal finds include iron tools found at the tomb at Guwei-cun of the 4th century BC.

The techniques used in Lingnan are a combination of bivalve moulds of distinct southern tradition and the incorporation of piece mould technology from the Zhongyuan. The products of the combination of these two periods are bells, vessels, weapons and ornaments, and the sophisticated cast.

An Iron Age culture of the Tibetan Plateau has been associated tentatively with the Zhang Zhung culture described by early Tibetan writings.

In Japan, iron items, such as tools, weapons, and decorative objects, are postulated to have entered Japan during the late Yayoi period (c. 300 BC – 300 AD) or the succeeding Kofun period (c. 250–538 AD), most likely from the Korean Peninsula and China.

Distinguishing characteristics of the Yayoi period include the appearance of new pottery styles and the start of intensive rice agriculture in paddy fields. Yayoi culture flourished in a geographic area from southern Kyūshū to northern Honshū. The Kofun and the subsequent Asuka periods are sometimes referred to collectively as the Yamato period; The word kofun is Japanese for the type of burial mounds dating from that era.

Silla chest and neck armour from the National Museum of Korea in Seoul (3rd century AD).

Iron objects were introduced to the Korean peninsula through trade with chiefdoms and state-level societies in the Yellow Sea area during the 4th century BC, just at the end of the Warring States Period but prior to the beginning of the Western Han dynasty. Yoon proposes that iron was first introduced to chiefdoms located along North Korean river valleys that flow into the Yellow Sea such as the Cheongcheon and Taedong Rivers. Iron production quickly followed during the 2nd century BC, and iron implements came to be used by farmers by the 1st century in southern Korea. The earliest known cast-iron axes in southern Korea are found in the Geum River basin. The time that iron production begins is the same time that complex chiefdoms of Proto-historic Korea emerged. The complex chiefdoms were the precursors of early states such as Silla, Baekje, Goguryeo, and Gaya. Iron ingots were an important mortuary item and indicated the wealth or prestige of the deceased during this period.

South Asia

Main article: Iron Age in India

Dates are approximate; consult particular article for details.

•    Prehistoric (or Proto-historic) Iron Age   Historic Iron Age

The earliest evidence of iron smelting predates the emergence of the Iron Age proper by several centuries. Iron was being used in Mundigak to manufacture some items in the 3rd millennium BC such as a small copper/bronze bell with an iron clapper, a copper/bronze rod with two iron decorative buttons, and a copper/bronze mirror handle with a decorative iron button. Artefacts including small knives and blades have been discovered in the Indian state of Telangana which have been dated between 2400 BC and 1800 BC. The history of metallurgy in the Indian subcontinent began prior to the 3rd millennium BC. Archaeological sites in India, such as Malhar, Dadupur, Raja Nala Ka Tila, Lahuradewa, Kosambi and Jhusi, Allahabad in present-day Uttar Pradesh show iron implements in the period 1800–1200 BC. As the evidence from the sites Raja Nala ka tila, Malhar suggest the use of Iron in c. 1800/1700 BC. The extensive use of iron smelting is from Malhar and its surrounding area. This site is assumed as the center for smelted bloomer iron to this area due to its location in the Karamnasa River and Ganga River. This site shows agricultural technology as iron implements sickles, nails, clamps, spearheads, etc., by at least c. 1500 BC. Archaeological excavations in Hyderabad show an Iron Age burial site.

The beginning of the 1st millennium BC saw extensive developments in iron metallurgy in India. Technological advancement and mastery of iron metallurgy were achieved during this period of peaceful settlements. One ironworking centre in East India has been dated to the first millennium BC. In Southern India (present-day Mysore) iron appeared as early as 12th to 11th centuries BC; these developments were too early for any significant close contact with the northwest of the country. The Indian Upanishads mention metallurgy. and the Indian Mauryan period saw advances in metallurgy. As early as 300 BC, certainly by 200 AD, high-quality steel was produced in southern India, by what would later be called the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in a crucible and heated until the iron melted and absorbed the carbon.

The protohistoric Early Iron Age in Sri Lanka lasted from 1000 BC to 600 BC. Radiocarbon evidence has been collected from Anuradhapura and Aligala shelter in Sigiriya. The Anuradhapura settlement is recorded to extend 10 ha (25 acres) by 800 BC and grew to 50 ha (120 acres) by 700–600 BC to become a town. The skeletal remains of an Early Iron Age chief were excavated in Anaikoddai, Jaffna. The name "Ko Veta" is engraved in Brahmi script on a seal buried with the skeleton and is assigned by the excavators to the 3rd century BC. Ko, meaning "King" in Tamil, is comparable to such names as Ko Atan and Ko Putivira occurring in contemporary Brahmi inscriptions in south India. It is also speculated that Early Iron Age sites may exist in Kandarodai, Matota, Pilapitiya and Tissamaharama.

The earliest undisputed deciphered epigraphy found in the Indian subcontinent are the Edicts of Ashoka of the 3rd century BC, in the Brahmi script. Several inscriptions were thought to be pre-Ashokan by earlier scholars; these include the Piprahwa relic casket inscription, the Badli pillar inscription, the Bhattiprolu relic casket inscription, the Sohgaura copper plate inscription, the Mahasthangarh Brahmi inscription, the Eran coin legend, the Taxila coin legends, and the inscription on the silver coins of Sophytes. However, more recent scholars have dated them to later periods.

Southeast Asia

Dates are approximate; consult particular article for details.

•   Prehistoric (or Proto-historic) Iron Age   Historic Iron Age

Lingling-o earrings from Luzon, Philippines

Archaeology in Thailand at sites Ban Don Ta Phet and Khao Sam Kaeo yielding metallic, stone, and glass artifacts stylistically associated with the Indian subcontinent suggest Indianization of Southeast Asia beginning in the 4th to 2nd centuries BC during the late Iron Age.

In Philippines and Vietnam, the Sa Huynh culture showed evidence of an extensive trade network. Sa Huynh beads were made from glass, carnelian, agate, olivine, zircon, gold and garnet; most of these materials were not local to the region and were most likely imported. Han-dynasty-style bronze mirrors were also found in Sa Huynh sites. Conversely, Sa Huynh produced ear ornaments have been found in archaeological sites in Central Thailand, as well as the Orchid Island.

Africa

Main article: Iron metallurgy in Africa

See also: Nok culture, Urewe, and Bantu expansion

Examples of African bloomery furnace types

Early evidence for iron technology in Sub-Saharan Africa can be found at sites such as KM2 and KM3 in northwest Tanzania and parts of Nigeria and the Central African Republic. Nubia was one of the relatively few places in Africa to have a sustained Bronze Age along with Egypt and much of the rest of North Africa.

Archaeometallurgical scientific knowledge and technological development originated in numerous centers of Africa; the centers of origin were located in West Africa, Central Africa, and East Africa; consequently, as these origin centers are located within inner Africa, these archaeometallurgical developments are thus native African technologies. Iron metallurgical development occurred 2631–2458 BC at Lejja, in Nigeria, 2136–1921 BC at Obui, in Central Africa Republic, 1895–1370 BC at Tchire Ouma 147, in Niger, and 1297–1051 BC at Dekpassanware, in Togo.

Very early copper and bronze working sites in Niger may date to as early as 1500 BC. There is also evidence of iron metallurgy in Termit, Niger from around this period. Nubia was a major manufacturer and exporter of iron after the expulsion of the Nubian dynasty from Egypt by the Assyrians in the 7th century BC.

Though there is some uncertainty, some archaeologists believe that iron metallurgy was developed independently in sub-Saharan West Africa, separately from Eurasia and neighboring parts of North and Northeast Africa.

Archaeological sites containing iron smelting furnaces and slag have also been excavated at sites in the Nsukka region of southeast Nigeria in what is now Igboland: dating to 2000 BC at the site of Lejja (Eze-Uzomaka 2009) and to 750 BC and at the site of Opi (Holl 2009). The site of Gbabiri (in the Central African Republic) has yielded evidence of iron metallurgy, from a reduction furnace and blacksmith workshop; with earliest dates of 896–773 BC and 907–796 BC, respectively. Similarly, smelting in bloomery-type furnaces appear in the Nok culture of central Nigeria by about 550 BC and possibly a few centuries earlier.

Iron and copper working in Sub-Saharan Africa spread south and east from Central Africa in conjunction with the Bantu expansion, from the Cameroon region to the African Great Lakes in the 3rd century BC, reaching the Cape around 400 AD. However, iron working may have been practiced in central Africa as early as the 3rd millennium BC. Instances of carbon steel based on complex preheating principles were found to be in production around the 1st century CE in northwest Tanzania.

Typical bloomery iron production operational sequence starting with acquiring raw materials through smelting and smithing

Dates are approximate; consult particular article for details

•    Prehistoric (or Proto-historic) Iron Age   Historic Iron Age