The atomic weight of an atom reflects the average number of

neutron, proton and electrons

Explanation:

atomic weight is the sum of protons and neutrons. Its the average weight of the Relative atomic masses of all the isotopes of the elements according to their relative abundance on earth. This value is the sum of the proton and neutron.

For example the helium atom has 2 protons and 2 neutrons. Its atomic weight will be 2+2 = 4.