Question

why is the mass of an atom located in the nucleus?

Answer 1

What is in the nucleus of an atom? When you answer that you will also answer why the mass of an atom is in the nucleus

Answer 2

In the nucleus you have the protons and the neutrons. That is my hint. :)

Answer 3

The particles (like protons and neutrons) that make up nucleus is massive compared to the mass of electron, hence most of the mass that contributes to the mass of atom comes from the nucleus.

Mass of proton & neutron is about 1.67e-27 Kg, while the mass of electron is 9.1e-31 Kg.