What is inert pair effect

The inert pair effect is the tendency of the two electrons in the outermost atomic s-orbital to remain unshared in compounds of post-transition metals. The term inert pair effect is often used in relation to the increasing stability of oxidation states that are two less than the group valency for the heavier elements of groups 13, 14, 15 and 16. .[1] The name suggests that the outermost s electrons are more tightly bound to the nucleus in these atoms, and therefore more difficult to ionize or share

the atomic mass is the bottom number on the element.

-steel jelly

the answer is 0.42 is that is an option. this question provides no info