Question

Which has maximum ionisation potential?
(A) N
(B) O
(C) ${{O}^{-}}$
(D) Na

Answer 1

Hint : Try to recall that ionization enthalpy is also known as ionization potential. Ionization energy is the minimum amount of energy required to remove the most loosely bound electron from an isolated neutral gaseous atom. Now, by using this you can easily find the correct option from the given ones.

Complete step by step solution :
As it is known, the ionization enthalpy is also known as ionization potential since it is the minimum potential difference (in a discharge tube) required to remove the most loosely bound electron from an isolated gaseous atom to form gaseous cation.
Ionisation potential is measured in units of electron volts (eV) per atom.
Also, we know that in general, as we move from left to right in a period, the ionization potential or ionization enthalpy increases with increasing atomic numbers and decreases on going down in periodic table. This is because across a period the size of the atom decreases and down the group atomic size increases.
Now, since nitrogen (N) and oxygen (O) belong to the same period and will have more ionization enthalpy compared to sodium (Na) which belongs to the third period and among nitrogen and oxygen, Oxygen (O) will have more ionization potential compared to nitrogen (N).
The last comparison is between O and${{O}^{-}}$. The electronic configuration of oxygen is-$1{{s}^{2}}2{{s}^{2}}2{{p}^{4}}$. Which means there are one paired and two unpaired orbitals in the “$2p$” subshell. As a half or full configuration is more stable than anything in between, the isolated oxygen atom would readily lose an electron to form-
$1{{s}^{2}}2{{s}^{2}}2{{p}^{3}}$. But this isn’t the case with ${{O}^{-}}$ as even it can only reach a stable electronic configuration if it gains another electron.
So we can conclude that, ${{O}^{-}}$ has the highest ionization potential among all others.

Note : To find which atom has the highest ionization enthalpy, you have to treat the atom in two cases. First you have to check its present stability and second, the stability when it ionizes. Writing the electronic configuration in both the cases eases the work and makes thing pretty clear.