The atomic mass of an element is not always a whole number because it takes into account the natural abundance of isotopes. Isotopes are variants of an element that have the same number of protons but differ in the number of neutrons in their nuclei. Since the atomic mass is a weighted average of the masses of different isotopes, it can result in fractional values.
Let's take the element chlorine (Cl) as an example. Chlorine has two stable isotopes: chlorine-35 and chlorine-37, with atomic masses of approximately 34.97 u and 36.97 u, respectively. The natural abundance of these isotopes in the Earth's crust is approximately 75.77% chlorine-35 and 24.23% chlorine-37.
To calculate the atomic mass of chlorine, we multiply the mass of each isotope by its relative abundance and sum the results:
(0.7577 * 34.97 u) + (0.2423 * 36.97 u) = 26.52 u + 8.96 u = 35.48 u
Therefore, the atomic mass of chlorine is approximately 35.48 atomic mass units (u), which is not a whole number.
This fractional atomic mass reflects the weighted average of the masses of the different isotopes of chlorine present in nature, considering their relative abundances.