The atomic weight of elements is typically not a whole number because it takes into account the abundance of different isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei, resulting in different mass numbers. Since isotopes occur in different abundances in nature, their contribution to the atomic weight of an element must be considered.
The atomic weight of an element is calculated by taking the weighted average of the masses of its isotopes, where the weights correspond to the relative abundance of each isotope. The abundance of isotopes can be determined through various methods, such as mass spectrometry or isotopic analysis.
For example, let's consider the element chlorine (Cl). Chlorine has two stable isotopes: chlorine-35 (^35Cl) and chlorine-37 (^37Cl). The atomic weight of chlorine is the weighted average of the masses of these isotopes, considering their natural abundance. Since chlorine-35 is more abundant in nature, it contributes more to the overall atomic weight of chlorine.
The atomic weight is calculated using the following formula:
Atomic weight = (mass of isotope 1 x abundance of isotope 1) + (mass of isotope 2 x abundance of isotope 2) + ...
The resulting atomic weight may not be a whole number because it depends on the specific masses and abundances of the isotopes present in nature. The atomic weight is usually expressed as a decimal value to reflect the average mass accurately.
It's important to note that some elements have only one naturally occurring isotope, such as fluorine (F) or aluminum (Al), which means their atomic weight is essentially equal to the mass of that isotope and may be a whole number. However, for elements with multiple isotopes, the atomic weight will generally be a decimal value due to the weighted average calculation.