How can the atomic mass be a decimal?

Based on the percentages of each atom's occurrences in nature, the atomic mass displayed on the periodic table is actually an average of all the masses of the numerous distinct isotopes (atoms with varying numbers of neutrons).

For instance, C-12 and C-13 have atomic masses of precisely 12 and 13.00 u, respectively.

However, the average is 12.01 u because C-12 occurs in nature in 98.89% of cases, while C-13 occurs in 1.11% of cases (there is one other natural isotope).

In any case, even though these aren't exact numbers, they're still close enough to allow a mass number to be a decimal.

I recall all of this from our class.

Since the masses of all naturally occurring isotopes of an element are weighted averages based on their relative abundances, the atomic mass of an element can be a decimal. Additionally, since different isotopes of an element can have different masses due to different numbers of neutrons, the atomic mass is not always a whole number.