Question

has the lowest first - ionization energy
a cs
b pb
c br
d se

Explanation:

First - ionization energy generally decreases down a group and increases across a period. Cs (cesium) is an alkali metal in Group 1. Pb (lead) is in Group 14, Br (bromine) is in Group 17, and Se (selenium) is in Group 16. Alkali metals have relatively low first - ionization energies because they have one valence electron that is easily removed. Among the given elements, Cs is the largest (in terms of atomic radius among these options) and has the least attraction for its outermost electron, so it has the lowest first - ionization energy.

Answer:

A. Cs