Question

Arrange the elements ${\text{Se}}$, ${\text{Cl}}$ and ${\text{S}}$ in the increasing order of ionisation energy.
(A) ${\text{Se}} > {\text{S}} > {\text{Cl}}$
(B) ${\text{Se}} < {\text{S}} < {\text{Cl}}$
(C) ${\text{Se}} < {\text{S}} > {\text{Cl}}$
(D) None of the above.

Answer 1

We know that the amount of energy which is required to remove an electron from an isolated gaseous atom or a molecule is known as ionisation energy or ionisation potential. Ionisation energy gives an idea about the chemical reactivity of atoms or molecules.

Complete answer: We know that the amount of energy which is required to remove an electron from an isolated gaseous atom or a molecule is known as ionisation energy or ionisation potential. Ionisation energy gives an idea about the chemical reactivity of atoms or molecules.
We know that the atomic number increases as we move from left to right in the periodic table. As the atomic number increases ionisation energy increases.Down the group from top to bottom, the ionisation energy decreases.We can decide the order of ionisation energies of the given elements ${\text{Se}}$, ${\text{Cl}}$ and ${\text{S}}$ from their positions in the periodic table.Now, we know that ${\text{Cl}}$ and ${\text{S}}$ belong to the third period of the periodic table. The ${\text{S}}$ is on the right left side while the ${\text{Cl}}$ is on the right side. Thus, the ionisation energy of ${\text{Cl}}$ is higher than that of ${\text{S}}$.Now, we know that ${\text{Se}}$ and ${\text{S}}$ belong to the 16 of the periodic table. But ${\text{S}}$ lies above ${\text{Se}}$. Thus, the ionisation energy of ${\text{S}}$ is higher than that of ${\text{Se}}$.
Thus, the increasing order of ionisation energy is as follows:
${\text{Se}} < {\text{S}} < {\text{Cl}}$

Thus, the correct option is (B) ${\text{Se}} < {\text{S}} < {\text{Cl}}$.

Note: There are types of ionisation energy: First ionisation energy, second ionisation energy and third ionisation energy. The amount of energy required to remove the first electron is known as the first ionisation energy. The amount of energy required to remove the second electron is known as the second ionisation energy. The amount of energy required to remove the third electron is known as the third ionisation energy.