Question

why is the atomic mass of an element not given as a whole number on the periodic table even though each individual atom of the element has a whole number of protons and neutrons?
a because electrons have a very small amount of weight
b because there are parts of protons and neutrons that do not add up to a whole number
c because chemists these days have very precise instruments to measure the atomic mass
d because atomic mass is the average relative atomic mass of all isotopes of the element

Explanation:

Atomic mass on the periodic table is the average relative atomic mass of all isotopes of an element. Isotopes have the same number of protons but different numbers of neutrons, and their abundances vary, resulting in a non - whole number average.

Answer:

D. because atomic mass is the average relative atomic mass of all isotopes of the element