Indium at a glance
- Atomic number
- 49
- Symbol
- In
- Atomic mass
- 114.82 u
- Category
- Post-transition metal
- Group / period
- 13 / 5
- Electron configuration
- [Kr] 4d 10 5s 2 5p 1
- State at 25 °C
- Solid
- Melting point
- 156.6 °C
- Boiling point
- 2072 °C
- Density
- 7.31 g/cm³
- Electronegativity
- 1.78
- 1st ionization energy
- 558 kJ/mol
- Oxidation states
- +3
- Discovered
- 1863
Atomic structure
A neutral indium atom has 49 protons and 49 electrons, and its most common isotope carries about 66 neutrons. One atom weighs about 1.91 × 10⁻²⁵ kg, so a mole of indium (6.022 × 10²³ atoms) weighs 114.82 g. That nucleus holds 1.35 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 18, O 3), which gives the configuration [Kr] 4d 10 5s 2 5p 1. With 3 electrons in its outer shell, it tends to lose electrons and form positive ions. It has only one stable isotope, In-113, plus the long-lived In-115. Its most common isotope, In-115, has the same mass number as Sn-115: isobars, nuclei of different elements with 115 nucleons each.
Physical properties
At room temperature (25 °C) indium is a solid. It melts at 156.6 °C and boils at 2072 °C, so it stays liquid across a 1915-degree range. It stays solid in boiling water but would melt in a hot kitchen oven. Its density is 7.31 g/cm³, 7.31 times as dense as water. Its atoms have a radius of about 155 pm (Cadmium next door is 155 pm). In the solid, its atoms pack into a tetragonal lattice.
Chemical behaviour
Its electronegativity is 1.78 on the Pauling scale. Pulling off its outermost electron takes 558 kJ/mol, less than Gallium above it (579 kJ/mol). A bond between indium and chlorine would be polar covalent, with an electronegativity difference of 1.4. It releases 37 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as In³⁺. Group 13 also holds Boron, Aluminium, Gallium, Thallium and Nihonium, which share its outer-shell pattern. Its Lewis symbol is In surrounded by 3 dots.
Where it comes from
It makes up 0.25 mg/kg of Earth's crust, the 64th most abundant element there. It was first identified in 1863, making it the 62nd element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 451.1 nm and 410.2 nm. In a flame test its compounds colour the flame indigo blue.