Thallium

Thallium (Tl) is a post-transition metal with atomic number 81 and an atomic mass of 204.38. Its electron configuration is [Xe] 4f14 5d10 6s2 6p1, which gives it 3 valence electrons.

Thallium key data

Symbol	Tl
Atomic number	81
Atomic mass (standard atomic weight)	204.38
Molar mass	204.38 g/mol
Protons	81
Neutrons (most abundant isotope, thallium-205)	124
Electrons (neutral atom)	81
Valence electrons	3
Category	Post-transition metal
Period	6
Group	13
Block	p-block
Noble gas notation	[Xe] 4f14 5d10 6s2 6p1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1
Electrons per shell	2, 8, 18, 32, 18, 3

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of thallium

From PubChem, the US National Institutes of Health. Public domain. The electron affinity is the measured value for thallium, from Wikipedia’s electron affinity data page, in place of PubChem’s 0.2 eV.

Melting point	577 K (303.85 °C), 74th of 103 by melting point
Boiling point	1746 K (1472.9 °C), 62nd of 93 by boiling point
Density	11.8 g/cm³, 20th of 96 by density
State at room temperature	Solid
Electronegativity	1.62 (Pauling), 48th of 95 by electronegativity
Van der Waals radius	196 pm, 65th of 99 by atomic radius
First ionisation energy	6.108 eV (589.3 kJ/mol), joint 73rd of 102 by first ionisation energy
Electron affinity	0.320053 eV (30.8804 kJ/mol), 50th of 57 by electron affinity
Common oxidation states	+3, +1
Discovered	1861

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Thallium electron configuration

The full electron configuration of thallium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1. With the noble-gas core collapsed it is [Xe] 4f14 5d10 6s2 6p1, and the outermost subshell is 6p1.

Bohr model of thallium (Tl): a nucleus and six shells holding 2, 8, 18, 32, 18 and 3 electrons from the inside out. — Bohr model of thallium (Tl). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of thallium (Tl), [Xe] 4f14 5d10 6s2 6p1, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 6p. — Orbital diagram of thallium (Tl). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one thallium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 204.38 / (6.02214076 × 10²³)
= 3.3938 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 204.38
= 2.9465 × 10²¹ atoms

Thallium on the periodic table

Thallium is in period 6, group 13, in the p-block.

The other elements of group 13 are boron, aluminium, gallium, indium and nihonium.