Why are the atomic masses of elements usually decimal numbers?

The answer by John suggests that it is the result of each atom having such a small mass (they do) but I assume your question is why are the masses not whole numbers. That's because the value in the periodic table are the averages of all of the different isotopes of the element weighted for the percent of each isotope in nature. For example, the atomic mass of Cl35 is approximately 35 and all of the Cl on earth is about 75% Cl 35. That of Cl37 is aproximately 37 and it is about 25% of the total.

35*0.75 + 37*0.25 = about 35.5.

The atomic masses of elements are usually decimal numbers because they are calculated based on the average weights of the different isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons in their nucleus, resulting in different atomic weights.

Most elements have multiple isotopes, each with a different abundance or occurrence in nature. These isotopes' atomic masses are considered when calculating the average atomic mass of an element. The atomic mass is determined by multiplying the mass of each isotope by its abundance and then summing these values.

Since isotopic abundances are expressed as percentages or proportions, the resulting atomic mass is an average and usually falls between the masses of the individual isotopes. Consequently, the atomic masses of elements are generally expressed as decimal numbers reflecting the weighted average mass of the isotopes.

The atomic masses of elements are usually decimal numbers because they take into account the isotopic abundance of different isotopes of the element.

To understand this, we need to know that elements can exist in different isotopic forms, which have the same number of protons but different numbers of neutrons. Isotopes of an element have different masses, and their abundance varies in nature. For example, naturally occurring carbon has three isotopes with atomic masses of approximately 12, 13, and 14.

The atomic mass of an element is calculated by taking the weighted average of the masses of its isotopes, considering their relative abundance. This average is expressed as a decimal number, which represents the most commonly occurring average mass for that element in nature.

The atomic mass values provided in the periodic table are based on a weighted average rather than being whole numbers because they reflect the actual distribution of isotopes in nature. The abundance of different isotopes can vary, leading to non-whole numbers in the atomic masses.

I think it is because atoms are so small, you have to measure many of them.. so we are actually taking an average which results in decimals.

Does that make sense?