I am having a problem solving for example
if Hg atomic # is 204 and atomic mass is 80 what is Proton, neutron, and electron I thought the atomic# is the same as proton so why the answer is 80 is proton is = atomic #

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First of all, we have the information backwards.

The atomic number would be 80
and the atomic mass would be 204

You are right, the atomic number is the same as the number of protons.
It is also the same as the number of electrons so long as we are talking about a neutral atom.
The number of neutrons is equal to the mass number minus the atomic number so in this case we get

204 - 80 = 124

Mercury-204 has 124 neutrons.

The mass number represents the total number of protons and neutrons in the nucleus.

We don't factor the weight of the electrons because it is so small that it does not contribute to the mass of the atom in a significant way.

Kind of like finding the mass of an elephant with or without 2 dozen flies on its back. It doesn't really make much difference does it?