Question

why atomic number and atomic mass number of element always a whole number?

Answer 1

Atomic Number of an element is the number of electrons OR  the number of protons present in atom

Since the protons or electrons are never split into half (in normal conditions) , the atomic number is always a whole number.

The same holds for Atomic Mass Number.
Atomic Mass Number= Number of protons + Number of neutrons

Since the neutrons are also not split in normal conditions, atomic mass number is also a whole number.