Question

the element with the smallest ionization energy in period 6 is -

Explanation:

Ionization energy generally decreases down a group and increases across a period. In Period 6, cesium (Cs) is the first - element in the period (an alkali metal). Alkali metals have the lowest ionization energy in their respective periods as they have only one valence electron which is relatively far from the nucleus and easily removed. Tm (thulium) is a lanthanide, Os (osmium) is a transition metal, and Rn (radon) is a noble gas. Noble gases have high ionization energies due to their stable electron configurations.

Answer:

Cs