Thorium (Th)

Atomic Model Details

Thorium (Th) is a radioactive chemical element with atomic number 90.

Atomic Number (Z)

The atomic number of Thorium is 90. This indicates 90 protons in the nucleus of every Thorium atom.

Mass Number (A)

The most stable and abundant isotope of Thorium is Thorium-232 ($^{232}\text{Th}$), with a mass number of 232.

Number of Protons

In a neutral Thorium atom, the number of protons is equal to its atomic number, which is 90.

Number of Electrons

For a neutral Thorium atom, the number of electrons is equal to the number of protons, which is 90.

Number of Neutrons

For the most common isotope, Thorium-232: Number of Neutrons = Mass Number (A) - Atomic Number (Z) = 232 - 90 = 142.

Atomic Size (Atomic Radius)

Atomic Radius (empirical): Approximately 179 pm (picometers).
Metallic Radius: Approximately 180 pm.
Covalent Radius: Approximately 165 pm.

Electronic Configuration & Shell Diagram Representation

Ground State Electronic Configuration

The ground state electronic configuration of Thorium (Z=90) is an exception among actinides, typically shown as:

Full Configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^{10} 6p^6 7s^2 6d^2$
Noble Gas Shorthand: $[\text{Rn}] 6d^2 7s^2$
- Note: Although Thorium is an actinide, its ground state atomic configuration does not contain any 5f electrons. The 5f orbitals become occupied in subsequent actinides.

Shell Diagram Representation (Textual Explanation)

The electrons in a neutral Thorium atom are distributed in seven electron shells (K, L, M, N, O, P, Q):

K-shell (n=1): 2 electrons ($1s^2$)
L-shell (n=2): 8 electrons ($2s^2 2p^6$)
M-shell (n=3): 18 electrons ($3s^2 3p^6 3d^{10}$)
N-shell (n=4): 32 electrons ($4s^2 4p^6 4d^{10} 4f^{14}$)
O-shell (n=5): 18 electrons ($5s^2 5p^6 5d^{10}$)
P-shell (n=6): 10 electrons ($6s^2 6p^6 6d^2$)
Q-shell (n=7): 2 electrons ($7s^2$) The outermost shell (Q-shell) contains 2 electrons, and the penultimate P-shell contains 10 electrons (including the $6d^2$ electrons).

Periodic Trends

Thorium is the second element in the actinide series, located in Group 3, Period 7.

Ionization Enthalpy

First Ionization Enthalpy: 587 kJ/mol
Second Ionization Enthalpy: 1110 kJ/mol
Trend: Actinides generally have relatively low ionization enthalpies, indicating that they readily lose electrons to form positive ions, characteristic of metals. Thorium’s values are consistent with its metallic character.

Electronegativity

Pauling Scale: 1.3
Trend: Thorium has a relatively low electronegativity value, indicating a weak tendency to attract electrons in a chemical bond. This is typical for electropositive metallic elements.

Electron Gain Enthalpy

Thorium, being a metal, does not readily accept electrons. Its electron gain enthalpy is expected to be positive or near zero, indicating that energy would be required to add an electron to a neutral thorium atom. Precise experimental values are scarce but align with its metallic nature.

Atomic Radius

Atomic Radius (empirical): 179 pm.
Trend: Thorium has one of the largest atomic radii among the actinides, as atomic radius generally decreases across the actinide series (actinide contraction).

Key Physical Properties

Density

Density: 11.72 g/cm³ (at 20 °C)

State at STP

State: Solid

Color

Color: Silvery-white metal, which tarnishes to black upon exposure to air due due to the formation of thorium dioxide ($\text{ThO}_2$).

Melting Point

Melting Point: 1750 °C (2023 K, 3182 °F)

Boiling Point

Boiling Point: 4788 °C (5061 K, 8650 °F)

Thorium (Th) - Atomic Structure and Properties