Gallium at a glance
- Atomic number
- 31
- Symbol
- Ga
- Atomic mass
- 69.723 u
- Category
- Post-transition metal
- Group / period
- 13 / 4
- Electron configuration
- [Ar] 3d 10 4s 2 4p 1
- State at 25 °C
- Solid
- Melting point
- 29.76 °C
- Boiling point
- 2204 °C
- Density
- 5.907 g/cm³
- Electronegativity
- 1.81
- 1st ionization energy
- 579 kJ/mol
- Oxidation states
- +3
- Discovered
- 1875
Atomic structure
A neutral gallium atom has 31 protons and 31 electrons, and its most common isotope carries about 38 neutrons. One atom weighs about 1.16 × 10⁻²⁵ kg, so a mole of gallium (6.022 × 10²³ atoms) weighs 69.723 g. That nucleus holds 1.23 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 18, N 3), which gives the configuration [Ar] 3d 10 4s 2 4p 1. With 3 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 2 stable isotopes: Ga-69 and Ga-71. No other element has a stable isotope with mass number 69, so Ga-69 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) gallium is a solid. It melts at 29.76 °C and boils at 2204 °C, so it stays liquid across a 2174-degree range. It would melt in your hand (body temperature). Its density is 5.907 g/cm³, 5.91 times as dense as water. Its atoms have a radius of about 130 pm (Zinc next door is 135 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 1.81 on the Pauling scale. Pulling off its outermost electron takes 579 kJ/mol, more than Aluminium above it (578 kJ/mol). A bond between gallium and chlorine would be polar covalent, with an electronegativity difference of 1.4. It releases 41 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as Ga³⁺. Group 13 also holds Boron, Aluminium, Indium, Thallium and Nihonium, which share its outer-shell pattern. Its Lewis symbol is Ga surrounded by 3 dots.
Where it comes from
It makes up 19 mg/kg of Earth's crust, the 35th most abundant element there. It was first identified in 1875, making it the 64th 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 417.2 nm and 403.3 nm. In a flame test its compounds colour the flame violet.