The element having lowest first ionization potential among Sr, As, S and F and why ?

• First ionisation potential increases from left to right across the period due to increase in the effective nuclear charge.
• And it decreases from top to bottom in column.
• The element which is down and left will have lower 1st ionization potential.
• Sr belongs to the s- block elements. As, S and F belong to the p- block elements.
• The increasing order of ionisation energy is Sr < As < S < F.
• Hence, Sr will have lowest 1st ionization potential.