Question

if protons+neutrons=atomic mass, and we also know that they are present in a nucleus of an atom so it must be the mass of the nucleus, than why we say that it is the mass of the atom?

Answer 1

The atom consist of protons,electrons and neutrons in which only the proton and neutron constitute the entire mass of the atom . If there are no electrons ,I could not defined as an atom because the valance electrons constitute the properties of the atoms .As atomic mass is also a charecteristic property of an element ,it is said as mass of the atom

Answer 2

you knew definition of the atomic number that is total no of protons or electrons in an atom this will clear your question