Chemistry - PERIODIC TABLE Question with Solution | TestHub

The correct option is (A) $1s^{2}2s^1$.

Identify the elements: Configuration (A) represents Lithium (Li), an alkali metal, while (B) is Nitrogen (N), and (C) and (D) are halogens (I and At).

Alkali metal property: Alkali metals have the largest atomic radii in their respective periods and only a single electron in their outermost shell.

Low effective nuclear charge: This outermost electron experiences a very weak nuclear attraction, making it highly unstable and very easy to remove.

High IP of others: Conversely, Nitrogen has a stable half-filled $p$-orbital, and halogens have very high nuclear charges, resulting in much higher ionization potentials.