Lithium at a glance
- Atomic number
- 3
- Symbol
- Li
- Atomic mass
- 6.94 u
- Category
- Alkali metal
- Group / period
- 1 / 2
- Electron configuration
- [He] 2s 1
- State at 25 °C
- Solid
- Melting point
- 180.5 °C
- Boiling point
- 1330 °C
- Density
- 0.534 g/cm³
- Electronegativity
- 0.98
- 1st ionization energy
- 520 kJ/mol
- Oxidation states
- +1
- Discovered
- 1817
Atomic structure
A neutral lithium atom has 3 protons and 3 electrons, and its most common isotope carries about 4 neutrons. One atom weighs about 1.15 × 10⁻²⁶ kg, so a mole of lithium (6.022 × 10²³ atoms) weighs 6.94 g. That nucleus holds 1.33 neutrons for every proton. The electrons fill 2 shells (K 2, L 1), which gives the configuration [He] 2s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. It has 2 stable isotopes: Li-6 and Li-7. No other element has a stable isotope with mass number 7, so Li-7 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) lithium is a solid. It melts at 180.5 °C and boils at 1330 °C, so it stays liquid across a 1149-degree range. It stays solid in boiling water but would melt in a hot kitchen oven. Its density is 0.534 g/cm³, lighter than water. Its atoms have a radius of about 145 pm (Beryllium next door is 105 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.98 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 520 kJ/mol, less than Hydrogen above it (1312 kJ/mol). A bond between lithium and chlorine would be mostly ionic, with an electronegativity difference of 2.2. It releases 60 kJ/mol when it gains an extra electron. Its common oxidation states are +1, so in compounds you will usually meet it as Li⁺. Group 1 also holds Hydrogen, Sodium, Potassium, Rubidium, Caesium and Francium, which share its outer-shell pattern. Its Lewis symbol is Li surrounded by 1 dot. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 20 mg/kg of Earth's crust, the 32nd most abundant element there. It was first identified in 1817, making it the 48th 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 670.8 nm, 610.4 nm and 460.3 nm. In a flame test its compounds colour the flame crimson red.