Which from following elements has highest value of ionization enthalpy $\left(\Delta_1 \mathrm{H}_1\right)$ ?
Which from following elements has highest value of ionization enthalpy $\left(\Delta_1 \mathrm{H}_1\right)$ ?
He
Ar
Cl
I
Solution
The ionization enthalpy (I.E.) is highest for the inert gases. Therefore I.E of Ar and He will be higher than the halogens Cl and I .
Down the group, I.E decreases due to increase in the atomic size.
Therefore, ionization enthalpy of $\mathrm{He}\gt\mathrm{Ar}$. varies as: $\mathrm{I} \lt \mathrm{Cl}$ and $\mathrm{Ar} \lt \mathrm{He}$.
Across a period, I.E increases with increase in atomic number.
Therefore, I.E of Cl and Ar is in the order: $\mathrm{Cl} \lt \mathrm{Ar}$.
Hence, the correct increasing order of ionization enthalpy for the given elements is:
$\mathrm{I} \lt \mathrm{Cl} \lt \mathrm{Ar} \lt \mathrm{He}$
He has the highest I.E.