chemistry help...plsz!!?

Question

why are the atomic masses listed on the periodic table decimals values , and not whole numbers ... ?

Answer 1

They're a weighed average, that take account all isotopes of an element that exist in nature.

for instance, Hydrogen's most common form is 1 proton 1 electron 0 neutron (mass of 1).

But there's a deuturium, which is hydrogen with 1 proton, 1 electron, and 1 neturon (with a mass of 2)

They average the 2's and the 1's (but give more weight to the 1's since H1 is like 99% of all H's). So atomic mass is above 1, but not by much.