Thallium Chemical Properties (25 Facts You Should Know)

Thallium is a component or element manufactured artificially. Let us discuss the facts of thallium.

Thallium is a delicate grey metal from the post-transition elements. The smelting of zinc and lead sulphide ores is another method of obtaining Thallium. It is taken from pyrite, which is found as a trace element, as a byproduct of roasting the material to make sulfuric acid. 

The chemical characteristics of thallium, including its block, ionisation energy, allotropic forms, oxidation states, and electron negativity, should be discussed in detail below.

Thallium symbol

In the periodic table, thallium is represented by the letter “Tl.” The term “Thallium,” which means green shoot or twig in Greek, is the source of the name.

thallium chemical properties
Thallium symbol in the periodic table

Thallium group in periodic table

The boron family includes the element thallium, which is a member of group 13.

Thallium period in periodic table

Thallium belongs to the 6th period that lies in between Mercury and Lead in the periodic table.

Thallium block in periodic table

Thallium is a p-block element because it has one p-orbital electron and an electronic configuration of 6p1

Thallium atomic number

Thallium has an atomic number of 81 due to 81 protons, 123 neutrons, and 81 electrons in its nucleus.

Thallium atomic Weight

The atomic weight of thallium is 204.38 g/mol.

Thallium Electronegativity according to Pauling

Thallium has an electronegativity of 1.62 on the Pauling scale. Nuclei and electrons are attracted to each other, more powerfully, as electronegativity increases from Al to Tl. As nuclear charge increases down the group, the electronegativity is impacted by the inert pair effect.

Thallium atomic Density

Thallium has a higher density of 11.85 g/cm3. Thallium has a liquid-state density of 11.22 g/cm3 at 304°C (m.p.).

Thallium melting point

The melting point of thallium is 577 K (304 °C, 579 °F). Thallium has a much lower melting point.

Thallium boiling point

The boiling point of thallium is 1746 K (1473 °C, 2683 °F).

Thallium Vanderwaals radius

Thallium has a van der Waals radius of 196 pm(1pm=1*10‑12 m).

Thallium ionic/covalent radius

Thallium has a covalent radius of 145 pm and an ionic radius of (+1) 159 (Tl). The table below shows that thallium has a large number of coordination centres and ionic radii:

Ion(Tln+ )
Ionic Radius
(1pm=1*10‑12 m)
The ionic radius of thallium

Thallium isotopes

There are different types of isotopes, which are chemical elements with the same nuclei or nuclear charges. Let us go over the thallium isotopes in more depth.

Thallium is composed of 41 isotopes, ranging from 176 to 216 atomic weights. A long half-life of 3.78 years makes 204-Tl the most stable radioisotope, while 203-Tl and 205-Tl are the only stable isotope. Only the common isotopes of thallium are mentioned in the table below:

Isotope of
204Tl203.97386353.78 yβ
207Tl206.9774194.77 minβ207Pb
207mTl1348.11.3 sIT (99.9%)
β (0.1%) 
Isotopes of thallium

Thallium electronic shell

A shell of electrons is an energy state that corresponds to electrons is called it’s electronic shell. Let us determine how many electronic shells there are in thallium.

The electron structure of thallium has six electronic shells. Each of its electrical shells has 2, 8, 18, 32, 3, and 18 electrons, respectively.

Thallium electron configurations

The electron configurations of thallium is: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10 6s2 6p1 or [Xe] 4f14 5d10 6s2 6p1. The period 6 elements’ shells are filled in that order: 4f, 5d, 6s, and 6p. 

The thallium electron configuration reveals that this element has three valence electrons, two of which are located in the 6s orbital and one in the 6p orbital.

Thallium energy of first ionisation

Thallium’s first ionisation energy is 589.4 kJ/mol. It is necessary to remove the initial electrons of thallium as follows: Tl + IE → Tl+ + e ([Xe] 4f14 5d10 6s2 6p0; Tl→Tl1+).

Thallium energy of second ionisation

Thallium has a second ionisation energy of 2878 kJ/mol. Due to the inert 6s orbital configuration, Tl+ + IE → Tl2+ + e( [Xe] 4f14 5d10 6s1; Tl1+→Tl2+), which results from removing electrons from the 6p orbital. The second ionisation of an electron is significantly larger than the first.

Thallium energy of third ionisation

Thallium has a third ionisation energy of 2704 kJ/mol. The third electron must be removed from the outermost shell using this energy as follows: Tl2+ + IE → Tl3+ + e([Xe] 4f14 5d10; Tl2+→Tl3+). The third ionisation energy is relatively high because of Tl2+.

Thallium oxidation states

Thallium from group III A exhibits a less stable +3 and a more stable +1 oxidation state due to the inert pair effect which is a result of its general valency electronic configuration of ns2np1. It has a range of oxidation states, including 1, 2, and 3. 

Thallium CAS number

Thallium’s CAS registry number is 7440-74-6.

Thallium ChemSpider ID

The ChemSpider ID for thallium is 4514408.

Thallium allotropic forms

In terms of their physical properties, allotropes are different from their chemical counterparts. Let us discuss the allotropic form of thallium.

Thallium has two allotropic forms: body-centred cubic thallium, stable at 226°C, and hexagonal close-packed thallium, stable at ambient temperature. On solidification, there is an increase in the volume of 3.23%.

Thallium chemical classification 

Following are some of thallium’s chemical classifications.

  1. Due to its softness, thallium can be easily cut.
  2. The very toxic metal thallium soon tarnishes when exposed to air.
  3. Thallium (III) compounds are similar to aluminium (III).
  4. Thallium is the first of the 13 elements in the group whose compounds spontaneously degrade from an oxidation state of +3 to a state of +1.
  5. At low temperatures, thallium amalgam is used in thermometers because it freezes at -58 °C.

Thallium state at room temperature

Thallium is a solid at room temperature. Thallium has an artificial hexagonal crystal shape.

Is Thallium paramagnetic?

Paramagnetism is the name given to the phenomena that occur when the components are pulled into the magnetic field. Let us check to see if thallium has a paramagnetic property.

Thallium should be paramagnetic due to its unpaired electrons, however, it is diamagnetic. The molar magnetic susceptibility of thallium is −50.9×10−6 cm3/mol (298 K).


Thallium is a group 13 and exhibits weak metal behaviour. The hexagonal crystal structure describes it. The stable isotopes of this element are 203 and 205. When left alone, it has the appearance of tin, but exposure to air may cause it to change colour.