Atomic masses are often expressed in decimal form rather than whole numbers because they take into account the existence of isotopes. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Since neutrons contribute to the mass of an atom but not its chemical properties, different isotopes of an element will have slightly different atomic masses.
The atomic mass of an element is determined by calculating the weighted average of the masses of its naturally occurring isotopes, taking into account their relative abundances. This average is expressed as a decimal number on the periodic table.
For example, let's consider carbon. Carbon has two stable isotopes: carbon-12 and carbon-13. Carbon-12 is more abundant, making up about 98.9% of naturally occurring carbon, while carbon-13 makes up the remaining 1.1%. The atomic mass of carbon is calculated as follows:
(12 amu x 0.989) + (13 amu x 0.011) = 12.01 amu
The resulting atomic mass of carbon is 12.01 atomic mass units (amu), which is the average mass of carbon atoms taking into account the presence of both carbon-12 and carbon-13 isotopes.
By expressing atomic masses in decimal form, it provides a more accurate representation of the actual mass of an atom, accounting for the natural variations in isotopic composition among different samples of an element.