Question

Explain where most of the mass of an atom is located. Also, explain why some particles that make up the atom do not contribute much to its mass.

Answer 1

Most of the mass of an atom is located in its nucleus.
That's where the protons and neutrons stay.

The other particles that make up the atom are the electrons,
buzzing around in a cloud outside of the nucleus.  They hardly
contribute much to the atom's mass, because no atom has more
than roughly 100 of any one particle, but it takes more than 1,800
electrons to make as much mass as ONE proton or neutron !