Question

Why are the atomic masses printed on the Periodic Table not whole numbers?

Answer 1

There is a concept called isotopes. Isotopes are different atoms of the same element with the same number of protons and electrons but a different number of neutrons.

The atomic weight is the mass relative to 1/12 the mass of an atom of carbon 12. To calculate it, both the isotopes of an element and their relative abundances must be considered.

Using chlorine as an example;

75% of naturally occurring chlorine is Chlorine 35 (has mass 35g)

25% of naturally occurring chlorine is Chlorine 37 (has mass 37g)

To calculate atomic weight, multiply the percentage by the mass.

(0.75)*(35) + (0.25)*(37) = 26.25 + 9.25 = 35.5

This is why chlorine has an atomic weight of 35.5g, and this method can be applied for all elements.

Hope this helps.