Iron at a glance
- Atomic number
- 26
- Symbol
- Fe
- Atomic mass
- 55.845 u
- Category
- Transition metal
- Group / period
- 8 / 4
- Electron configuration
- [Ar] 3d 6 4s 2
- State at 25 °C
- Solid
- Melting point
- 1538 °C
- Boiling point
- 2861 °C
- Density
- 7.874 g/cm³
- Electronegativity
- 1.83
- 1st ionization energy
- 763 kJ/mol
- Oxidation states
- +2, +3
- Discovered
- Antiquity
Atomic structure
A neutral iron atom has 26 protons and 26 electrons, and its most common isotope carries about 30 neutrons. One atom weighs about 9.27 × 10⁻²⁶ kg, so a mole of iron (6.022 × 10²³ atoms) weighs 55.845 g. That nucleus holds 1.15 neutrons for every proton. Its symbol Fe comes from ferrum. The electrons fill 4 shells (K 2, L 8, M 14, N 2), which gives the configuration [Ar] 3d 6 4s 2. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. It has 4 stable isotopes: Fe-54, Fe-56, Fe-57 and Fe-58. No other element has a stable isotope with mass number 56, so Fe-56 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) iron is a solid. It melts at 1538 °C and boils at 2861 °C, so it stays liquid across a 1323-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 7.874 g/cm³, 7.87 times as dense as water. Its atoms have a radius of about 140 pm (Manganese next door is 140 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.83 on the Pauling scale. Pulling off its outermost electron takes 763 kJ/mol. A bond between iron and chlorine would be polar covalent, with an electronegativity difference of 1.3. It releases 15 kJ/mol when it gains an extra electron. Its common oxidation states are +2, +3, so in compounds you will usually meet it as Fe²⁺ and Fe³⁺. Group 8 also holds Ruthenium, Osmium and Hassium, which share its outer-shell pattern.
Where it comes from
It makes up 5.63 % of Earth's crust, the 4th most abundant element there. In the human body it accounts for about 0.006 % of body mass. It has been known since ancient times, one of only 11 elements people used before modern chemistry. When excited it glows at its own set of colours; its brightest visible lines are at 527.0 nm, 438.4 nm and 430.8 nm.