Answer the following question given this paragraph: 

Electrons have virtually no mass, but protons and neutrons have a lot of mass for their size. As a result, the nucleus has virtually all the mass of an atom. Given its great mass and tiny size, the nucleus is very dense. If an object the size of a penny had the same density as the nucleus of an atom, its mass would be greater than 30 million tons! You can learn more about the size and mass of the nucleus at this URL:.


Q: Where does most of the mass for an atom reside?


A:
nucleus