Question

An element has a weight of 131.3. What is the reason the number is not a whole number?

Answer 1

Atomic masses are averages of all the possible isotopes that can have.

Explanation:

You are referring to Xenon. It's Atomic Mass on the periodic table reads 131.3. The mass is a sum of the protons plus the neutrons in the atom/element. The number of neutrons can vary from isotope to isotope however. For example, Hydrogen can have a mass of 1, 2, or 3; but it's atomic mass on the periodic table is 1.008. This means that you are more likely to find Hydrogen-1 in real life, but the 1.008 is an average of all the different isotopes it can have, and their prominence.
The 131.3 is the same way for Xenon, it is the average of the possible isotopes.