Lanthanum at a glance
- Atomic number
- 57
- Symbol
- La
- Atomic mass
- 138.91 u
- Category
- Lanthanide
- Period
- 6
- Electron configuration
- [Xe] 5d 1 6s 2
- State at 25 °C
- Solid
- Melting point
- 919.9 °C
- Boiling point
- 3464 °C
- Density
- 6.145 g/cm³
- Electronegativity
- 1.1
- 1st ionization energy
- 538 kJ/mol
- Oxidation states
- +3
- Discovered
- 1839
Atomic structure
A neutral lanthanum atom has 57 protons and 57 electrons, and its most common isotope carries about 82 neutrons. One atom weighs about 2.31 × 10⁻²⁵ kg, so a mole of lanthanum (6.022 × 10²³ atoms) weighs 138.91 g. That nucleus holds 1.44 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 6 shells (K 2, L 8, M 18, N 18, O 9, P 2), which gives the configuration [Xe] 5d 1 6s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. It has only one stable isotope, La-139, plus the long-lived La-138. No other element has a stable isotope with mass number 139, so La-139 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) lanthanum is a solid. It melts at 919.9 °C and boils at 3464 °C, so it stays liquid across a 2544-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 6.145 g/cm³, 6.14 times as dense as water. Its atoms have a radius of about 195 pm (Barium next door is 215 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.1 on the Pauling scale. Pulling off its outermost electron takes 538 kJ/mol. A bond between lanthanum and chlorine would be mostly ionic, with an electronegativity difference of 2.1. It releases 54 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as La³⁺. It shares much of its chemistry with the other lanthanides, especially its neighbours Cerium and Praseodymium.
Where it comes from
It makes up 39 mg/kg of Earth's crust, the 28th most abundant element there. It was first identified in 1839, making it the 55th element to be discovered (counting the 11 known since antiquity together).