Chemical element · reference page

81Tl

Thallium

Thallium is element 81 (Tl), located in period 6, group 13 and the p block. At room temperature its reference phase is solid. Its electron configuration is [Xe] 4f14 5d10 6s2 6p1.

Atomic number81

Standard atomic weight[204.382, 204.385]

Group13

Period6

Blockp

Electron configuration[Xe] 4f14 5d10 6s2 6p1

Electronic structure

What controls its chemistry

Atoms in group 13 have three valence electrons in the outer s and p subshells. The balance between covalent bonding and positive oxidation states changes down the group as atomic size and metallic character increase.

Thallium is therefore best understood by comparing it with the elements immediately above and below it rather than by treating group 13 as chemically uniform.

Chemical forms

Compounds, ions and materials

Group 13 chemistry includes oxides, halides and covalent or ionic compounds in positive oxidation states. The lighter elements show more covalent character, while the heavier members become progressively more metallic.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 11.85 g/cm³. The melting point is 577 K (303.9 °C). The boiling point is 1 746 K (1 473 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where thallium is found and used

In practice, thallium is encountered in electronics, optics and scientific research.

In practice

Examples of real uses

Electronics

Thallium is used in electronic components or materials where conductivity, semiconducting behaviour, optical response or chemical stability is useful.

Optics

Thallium is used in optical materials, coatings or devices because of characteristic absorption, emission or refractive properties.

Scientific research

Thallium is produced, isolated or measured mainly to test atomic, nuclear and periodic trends rather than for bulk commercial use.

Periodic position

What its neighbours tell us

Thallium is in period 6 and group 13. In the neighbourhood shown here, it lies between mercury and lead in the same period; indium is above it and nihonium below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationpost-transition metal

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 32 · 18 · 3

Density11.85 g/cm³

Melting point577 K (303.9 °C)

Boiling point1 746 K (1 473 °C)

Pauling electronegativity1.62

First ionisation energy589.4 kJ/mol

Electron affinity36.4 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of thallium is associated with William Crookes. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is given as the interval [204.382, 204.385]. Natural samples can differ slightly in isotope proportions, so a single universal decimal value would hide real variation between materials.

Thallium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

Thallium compounds are highly toxic and require strict laboratory and industrial controls.