Question

Why are the atomic masses of elements usually decimal numbers? Provide an answer using 3 – 4 sentences in your own words.

Answer 1

The atomic masses of elements are usually written in decimal numbers because they are non-terminating. The numbers usually are rounded to the nearest thousandths place. The atomic masses are more exact in decimal numbers.

Answer 2

Explanation: An element does not exist alone in the nature. It exist in the form of isotopes of its own. Atomic mass of that element is the average atomic masses of the isotopes.

Average atomic mass of the element is defined as the sum of atomic masses of isotopes each multiplied with their respective natural fractional abundance.

Mathematically,

$\text{Average atomic mass}=\sum_{i=1}^n\text{(atomic mass of an isotopes)}_i\times \text {(fractional abundance)}_i$

This is the reason, why an element does not have a whole number atomic mass, but exist in decimal numbers.