Understanding Thorium’s Atomic Structure
Thorium (Th) is a radioactive element with atomic number 90. It is a silvery metal that tarnishes black when exposed to air. Named after Thor, the Norse god of thunder, thorium is one of the naturally occurring radioactive elements. It is particularly relevant in India, where it is found in abundance in the monazite sands along the coastal regions, notably in Kerala. These sands represent one of the world’s largest reserves of thorium.
Protons, Neutrons, and Electrons in Thorium
The atomic number of an element defines the number of protons in its nucleus. For Thorium:
- Atomic Number (Z): 90
- Number of Protons: 90
In a neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons. Therefore, for a neutral Thorium atom:
- Number of Electrons: 90
The most common isotope of thorium found naturally is Thorium-232 ($\text{^{232}Th}$). The mass number (A) represents the total number of protons and neutrons in the nucleus.
- Mass Number (A): 232
- Number of Neutrons: Mass Number - Atomic Number = 232 - 90 = 142
Thus, a neutral atom of Thorium-232 contains 90 protons, 90 electrons, and 142 neutrons.
Electron Configuration of Thorium
Electron configuration describes the arrangement of electrons in an atom’s orbitals. For Thorium (atomic number 90), the electron configuration helps understand its chemical behaviour. Due to its large number of electrons, a shorthand notation using the preceding noble gas is often employed. The noble gas preceding Thorium is Radon (Rn), which has an atomic number of 86.
The electron configuration of a neutral Thorium atom is:
$[Rn] 6d^2 7s^2$
This configuration indicates that the first 86 electrons are arranged like those in a Radon atom. The subsequent four electrons are distributed as two electrons in the $6d$ subshell and two electrons in the $7s$ subshell. It is noteworthy that, unlike most other actinides, the neutral thorium atom’s ground state does not typically fill the $5f$ subshell with electrons; instead, it populates the $6d$ subshell.
Valence Electrons in Thorium
Valence electrons are the electrons located in the outermost shell of an atom, which are involved in chemical bonding and determine an element’s chemical properties. For Thorium, based on its electron configuration $[Rn] 6d^2 7s^2$, the highest principal energy level is 7, containing two $7s$ electrons. Additionally, the $6d$ electrons are close in energy and can participate in bonding.
Therefore, Thorium typically exhibits 4 valence electrons (two $7s$ electrons and two $6d$ electrons). This explains why Thorium most commonly forms ions with a +4 oxidation state in its compounds, such as in thorium dioxide ($\text{ThO}_2$), a refractory material used in various applications.