Thallium at a glance
- Atomic number
- 81
- Symbol
- Tl
- Atomic mass
- 204.38 u
- Category
- Post-transition metal
- Group / period
- 13 / 6
- Electron configuration
- [Xe] 4f 14 5d 10 6s 2 6p 1
- State at 25 °C
- Solid
- Melting point
- 303.9 °C
- Boiling point
- 1473 °C
- Density
- 11.85 g/cm³
- Electronegativity
- 1.62
- 1st ionization energy
- 589 kJ/mol
- Oxidation states
- +1, +3
- Discovered
- 1861
Atomic structure
A neutral thallium atom has 81 protons and 81 electrons, and its most common isotope carries about 123 neutrons. One atom weighs about 3.39 × 10⁻²⁵ kg, so a mole of thallium (6.022 × 10²³ atoms) weighs 204.38 g. That nucleus holds 1.52 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 32, O 18, P 3), which gives the configuration [Xe] 4f 14 5d 10 6s 2 6p 1. With 3 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 2 stable isotopes: Tl-203 and Tl-205. Its most common isotope, Tl-204, has the same mass number as Hg-204 and Pb-204: isobars, nuclei of different elements with 204 nucleons each.
Physical properties
At room temperature (25 °C) thallium is a solid. It melts at 303.9 °C and boils at 1473 °C, so it stays liquid across a 1169-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 11.85 g/cm³, 11.8 times as dense as water. Its atoms have a radius of about 190 pm (Mercury next door is 150 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.62 on the Pauling scale. Pulling off its outermost electron takes 589 kJ/mol, more than Indium above it (558 kJ/mol). A bond between thallium and chlorine would be polar covalent, with an electronegativity difference of 1.5. It releases 31 kJ/mol when it gains an extra electron. Its common oxidation states are +1, +3, so in compounds you will usually meet it as Tl⁺ and Tl³⁺. Group 13 also holds Boron, Aluminium, Gallium, Indium and Nihonium, which share its outer-shell pattern. Its Lewis symbol is Tl surrounded by 3 dots. Handle with care: as a pure element it is toxic.
Where it comes from
It makes up 0.85 mg/kg of Earth's crust, the 60th most abundant element there. It was first identified in 1861, making it the 61st 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 535.0 nm and 377.6 nm. In a flame test its compounds colour the flame pure green.