Iron
Iron is a chemical element with symbol Fe (from Latin:ferrum) and atomic number 26. It is a metal in the first transition series. It is by mass the most common element on Earth, forming much of Earth's outer and inner core.
At a glance
- Atomic number
- 26
- Atomic mass
- 55.8452 u
- Phase
- Solid
- Density
- 7.874 g/cm³
- Melting point
- 1,811 K
- Boiling point
- 3,134 K
- First ionization energy
- 762.5 kJ/mol
- Electronegativity
- 1.83
- Electron affinity
- 14.785 kJ/mol
- Molar heat capacity
- 25.1 J/(mol·K)
- Period
- 4
- Appearance
- lustrous metallic with a grayish tinge
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Iron’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
18 is the [Ar] core total across three shells.
Preparing the orbital view…
26 protons30 neutrons
Iron-56: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, iron is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
FeNeutral atom · no charge
A neutral iron atom. Removing its first electron takes 763 kJ/mol.
Iron’s outer shell holds 2 electrons. Electron 3 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 1.9× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 2, then 14, then 8, then 2 bars. Read backwards, that’s 2, 8, 14, 2: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The jump after iron’s outer electrons is small (×1.9), because the electrons in the shell below have similar energies. That’s typical of transition metals, and it’s why iron forms both Fe²⁺ and Fe³⁺ ions.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 4 | 763 | – | Measured |
| 2 | 4 | 1,563 | 2 | Measured |
| 3 | 3 | 2,957 | 1.9 | Measured |
| 4 | 3 | 5,298 | 1.8 | Measured |
| 5 | 3 | 7,236 | 1.4 | Estimated |
| 6 | 3 | 9,551 | 1.3 | Measured |
| 7 | 3 | 12,057 | 1.3 | Measured |
| 8 | 3 | 14,575 | 1.2 | Measured |
| 9 | 3 | 22,539 | 1.5 | Estimated |
| 10 | 3 | 25,289 | 1.1 | Estimated |
| 11 | 3 | 28,068 | 1.1 | Estimated |
| 12 | 3 | 31,917 | 1.1 | Estimated |
| 13 | 3 | 34,831 | 1.1 | Estimated |
| 14 | 3 | 37,842 | 1.1 | Estimated |
| 15 | 3 | 44,017 | 1.2 | Measured |
| 16 | 3 | 47,211 | 1.1 | Estimated |
| 17 | 2 | 121,832 | 2.6 | Estimated |
| 18 | 2 | 131,008 | 1.1 | Estimated |
| 19 | 2 | 140,869 | 1.1 | Estimated |
| 20 | 2 | 152,022 | 1.1 | Measured |
| 21 | 2 | 162,771 | 1.1 | Estimated |
| 22 | 2 | 173,519 | 1.1 | Estimated |
| 23 | 2 | 188,185 | 1.1 | Estimated |
| 24 | 2 | 197,386 | 1 | Calculated |
| 25 | 1 | 851,790 | 4.3 | Calculated |
| 26 | 1 | 895,161 | 1.1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
Heat it and the atoms rearrange
Solid iron is a crystal: its atoms sit in a repeating pattern. The smallest box that repeats is the unit cell. Heat iron up and the pattern rearranges itself twice before it melts, and one of those rearrangements is the secret of steel.
20 °C293 K
α-iron ferrite
- Arrangement
- Body-centred cubic (BCC)
- Atoms per cell
- 2
- Space filled
- 68%
- Cell edge
- 0.287 nm
- Magnetic
- Yes
- Carbon it can hold
- up to 0.022% by mass at 727 °C
Why this makes steel possible
Heat it into austenite. Above about 912 °C, iron’s face-centred cells have roomy gaps, so the carbon in steel dissolves evenly through the metal.
Orange-hot iron on the anvil, at roughly 1,000 °C: austenite, soft enough to hammer into shape and able to dissolve carbon.
Photo: MichalPL, CC BY-SA 4.0, via Wikimedia CommonsCool it slowly and iron switches back to the body-centred arrangement, which has no room for the carbon. It is pushed out into thin layers of iron carbide: steel that is tough but fairly soft.
Slowly cooled steel with 0.8% carbon under an electron microscope. The stripes are alternating layers of iron and iron carbide, a structure called pearlite: the carbon that was pushed out as the cells switched back. Scale bar: 10 micrometres.
Photo: Edward Pleshakov, Public domain, via Wikimedia CommonsCool it fast (quench it in water) and the carbon has no time to escape. Trapped inside, it strains the iron into a distorted, very hard structure called martensite. That’s how blacksmiths harden a blade.
Quenched steel (SAE 1045) under a microscope. The fine needles are martensite: carbon trapped in a strained, distorted iron structure, which makes the steel very hard. Scale bar: 50 micrometres.
Photo: koganam, Ort Braude College, CC BY 3.0, via Wikimedia Commons
At normal pressure. Cell sizes are typical for each range and change a little with temperature. Atoms are drawn smaller than their true size so the arrangement is visible; “True atom size” shows how closely they pack. The yellow arrows show each atom’s magnetism, lined up below 770 °C and jumbled above.
How does it compare?
Density
7.87 g/cm³43rd highest of 99
Melting point
1,811 K24th highest of 96
Boiling point
3,134 K38th highest of 97
Electronegativity
1.8342nd highest of 96
First ionization energy
763 kJ/mol34th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- IronRoyal Society of Chemistry
- IronWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database