Thorium Properties (25 Facts You Should Know)

Thorium is soft, pliable, and tarnishes black when exposed to air, generating thorium dioxide. Let us discuss some important thorium facts.

Thorium is an actinide metal that occurs in significant quantities in nature as a primordial element. The metal thorium is very electropositive and very reactive. It is a radioactive and extremely reactive metal that can react with halogens, oxygen, hydrogen, and sulphur. 

Let us go over thorium’s chemical characteristics, including its allotropic forms, oxidation states, period, electronegativity, and ionisation energy in detail.

Thorium symbol

Thorium has the chemical atomic symbol “Th” and is a naturally occurring radioactive metal. The term “Thorium”  was honoured by the name of “Thor” which means the God of thunder(the Norse god of thunder).

thorium properties
Thorium symbol in the periodic table

Thorium group in periodic table

The group is not specified for thorium, it belongs to actinide and radioactive metal. 

Thorium period in periodic table

Thorium is given to the 7th period of the periodic table.

Thorium block in periodic table

On the periodic table, thorium is a member of the f-block and is situated to the right of actinium, beneath protactinium, and beneath cerium.

Thorium atomic number

Thorium has an atomic number of 90 due to its nucleus, which contains 90 protons and 90 electrons.

Thorium atomic Weight

A thorium atom possesses an atomic weight/mass (Ar°(Th) of 232.0377 a.m.u. It is a common heavy metal that is weakly radioactive and naturally occurs in our environment.

Thorium Electronegativity according to Pauling

Thorium electronegativity is 1.3 according to Pauling’s scale.

Thorium atomic Density

Thorium’s atomic density is 11.7 g/cm3, at standard temperature.

Thorium melting point

Thorium has a melting point of 2023 K ​(1750 °C, ​3182 °F). Its melting point is high. Of all known oxides, thorium dioxide has the highest melting point.

Thorium boiling point

Thorium has a boiling point of 5061 K (4788 °C, 8650 °F).

Thorium Vanderwaals radius

Thorium has a van der Waals radius of 240 pm(1pm=1*10‑12 m).

Thorium ionic/covalent radius

Thorium has a covalent radius of 206 pm and an ionic radius of 99 pm, in the (+4) oxidation state. Based on the coordination number, the radius of the Th4+ ion can range from 0.95 to 1.14, making it the largest tetrapositive actinide ion.

Here is a list of thorium’s coordination ionic centres:

Ion(Thn+ )
Ionic Radius
(1pm=1*10‑12 m)
Th(VI) 6-coordinate,
Th(Vi) 8-coordinate 105
The ionic radius of thorium

Thorium isotopes

An isotope of a given element is composed of the same number of protons and neutrons but has a different number of neutrons. Let us check the thorium isotopes.

There are 30 isotopes of thorium, each containing 209–238 nucleons. One stable naturally occurring isotope of thorium exists that includes 232-Th. Th-232 is found in nature, and its half-life is 1.41 x 1010 years. The thorium isotopes are all unstable with atomic masses between 223 and 234. 

Isotopes of thorium are listed below:

Isotope of
Half-life Decay 
227Th 227.0277041   18.68 d α  223Ra
228Th 228.0287411   1.9116 y α  224Ra
229Th 229.031762 7917 y α  225Ra
230Th 230.0331338 75400 y α  226Ra
231Th 231.0363043 25.5 h β  231Ra
232Th 232.0380553 1.405×1010y α  228Ra
234Th 234.043601 24.1 d β−  234mPa
Isotopes of thorium

Thorium electronic shell

The electronic shell of an electron corresponds to its energy level. Let us count how many electronic shells there are in thorium.

The thorium electron structure consists of seven electronic shells. Among the electrons it contains, there are 2, 8, 18, 32, 18, 10, and 2 in each of its electrical shells.

Thorium electron configurations

The arrangement of an electron within a shell is referred to as the electron configuration of an element. Let us find the arrangements of the thorium electrons.

The following list includes thorium’s electron configurations:1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s2, 4p6, 4d10, 5s2, 5p5, 4f14, 5d10, 6s2, 6p6, 6d2, 7s2 or [Rn] 6d2 7s2. Atomic orbitals 5f, 6d, 7s, and 7p can all be filled by electron shells, in theory. Compared to its 5f subshells, its 6d subshell has less energy.

Thorium energy of first ionisation

Thorium has the first ionisation energy of 587 kJ/mol or 6.3067 eV. The lowest of the initial electrons of thorium, Th + IE → Th+ + e ([Rn] 6d2 7s1; Th→Th1+), must be eliminated.

Thorium energy of second ionisation

Thorium’s second ionisation energy is 1110 kJ/mol or 11.9 eV. When it is ionised again, a disproportionately larger amount of an electron is created than the initial time. The process follows as: Th+ + IE → Th2+ + e([Rn] 6d2; Th1+→Th2+).

Thorium energy of third ionisation 

The third ionisation energy of thorium is 1930 kJ/mol or 20.0 eV. Th3+ is the cause of the third ionisation energy being abnormally high as of 6d1 orbitals. It is possible to remove the third electron as follows: Th2+ + IE → Th3+ + e([Rn] 6d1; Th2+→Th3+).

Thorium oxidation states

The oxidation state of nearly all thorium-containing compounds is +4. The Th4+ ion can produce a large number of complex ions. It is a weakly basic oxide that can exist in -1, +1, +2, +3, and +4 oxidation states. 

Thorium CAS number

The CAS registration number for thorium is 7440-29-1.

Thorium ChemSpider ID

The ChemSpider ID for thorium is 22399.

Thorium allotropic forms

Allotropic forms of an element have different bonds to one another and have varied physical but the same chemical properties. Here is an overview of thorium’s allotropy.

Thorium does not possess any allotropic form because it is a radioactive element. 

Thorium chemical classification

Thorium can be categorised under some chemical categories in the list below.

  1. Thorium powder has pyrophoric properties.
  2. Thorium is a highly ductile, and fairly soft element.
  3. Thorium is a crucial alloying component of magnesium because it increases the metal’s strength and creeps resistance at high temperatures.
  4. Thorium dioxide was once added to glass to raise the refractive index, generating thoriated glass for use in premium camera lenses. 
  5. Thoriated tungsten, or Thorium, may be used to make electrodes and filaments for incandescent lights.

Thorium state at room temperature

At normal or room temperature, thorium is in a solid phase. This substance’s crystal structure is face-centred cubic.

Is Thorium paramagnetic?

It is considered to be paramagnetic for materials whose electrons are hindered in magnetic fields. Let us examine the paramagnetic characteristics of thorium.

The paramagnetic properties of thorium are caused by the fact that it has two unpaired electrons. The Th has a magnetic moment of 2.83 (spin-only value). The table below displays the magnetic susceptibility value for thorium:

Mass Magnetic
7.2e-9 m3/kg
Molar Magnetic
1.7e-9 m3/mol
Volume Magnetic
The magnetic susceptibility value for thorium


Thorium may be used as a source of nuclear energy and is around three times as common as uranium and about as abundant as lead. Thorium oxide is used as an industrial catalyst, and thoriate tungsten is used in the manufacture of light bulbs.