Question

ie increases across a period. which element will have the smaller ie? fluorine oxygen

Explanation:

Step1: Recall ionization - energy trend

Ionization energy (IE) generally increases across a period from left - to - right. However, there are some exceptions. Oxygen has a lower first - ionization energy than fluorine. This is because oxygen has an electron configuration of $1s^{2}2s^{2}2p^{4}$, and when one electron is removed, it results in a half - filled $2p$ sub - shell ($2p^{3}$) which is relatively stable. Fluorine has an electron configuration of $1s^{2}2s^{2}2p^{5}$, and removing an electron disrupts its near - noble gas configuration.

Answer:

Oxygen