Terbium at a glance
- Atomic number
- 65
- Symbol
- Tb
- Atomic mass
- 158.93 u
- Category
- Lanthanide
- Period
- 6
- Electron configuration
- [Xe] 4f 9 6s 2
- State at 25 °C
- Solid
- Melting point
- 1356 °C
- Boiling point
- 3230 °C
- Density
- 8.229 g/cm³
- Electronegativity
- 1.1
- 1st ionization energy
- 566 kJ/mol
- Oxidation states
- +3, +4
- Discovered
- 1843
Atomic structure
A neutral terbium atom has 65 protons and 65 electrons, and its most common isotope carries about 94 neutrons. One atom weighs about 2.64 × 10⁻²⁵ kg, so a mole of terbium (6.022 × 10²³ atoms) weighs 158.93 g. That nucleus holds 1.45 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 27, O 8, P 2), which gives the configuration [Xe] 4f 9 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. It has only one stable isotope, Tb-159. No other element has a stable isotope with mass number 159, so Tb-159 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) terbium is a solid. It melts at 1356 °C and boils at 3230 °C, so it stays liquid across a 1874-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 8.229 g/cm³, 8.23 times as dense as water. Its atoms have a radius of about 175 pm (Gadolinium next door is 180 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 566 kJ/mol. A bond between terbium and chlorine would be mostly ionic, with an electronegativity difference of 2.1. Its common oxidation states are +3, +4, so in compounds you will usually meet it as Tb³⁺ and Tb⁴⁺. It shares much of its chemistry with the other lanthanides, especially its neighbours Europium, Gadolinium, Dysprosium and Holmium.
Where it comes from
It makes up 1.2 mg/kg of Earth's crust, the 59th most abundant element there. It was first identified in 1843, making it the 56th element to be discovered (counting the 11 known since antiquity together).