Question

Which of the following represents the correct order of increasing first ionisation enthalpies of $Ca,Ba,S,Se$ and $Ar$?

• A. $Ca<S<Ba<Se<Ar$
• B. $S<Se<Ca<Ba<Ar$
• C. $Ba<Ca<Se<S<Ar$
• D. $Ca<Ba<S<Se<Ar$

Answer 1

The correct option is C $Ba<Ca<Se<S<Ar$

On moving down the group, ionisation enthalpy decreases.
Hence, the ionisation energy of $Ba<Ca$ and $Se<S$.
In general, ionisation energy increases from left to right in a period. Hence, $S<Ar$ and $Ca<Se$.
Hence, the correct order of increasing ionisation enthalpies is $Ba<Ca<Se<S<Ar$.