sentences: the nucleus of an atom is usually made of both protons and neutrons & the nucleus is the core of an atom ->  the core of an atom is made of protons and neutrons  electrons surround the nucleus of an atom & the nucleus is the core of an atom ->  electrons surround the core of an atom  the core of an atom is made of protons and neutrons & electrons surround the core of an atom -> an atom is made of a core of protons and neutrons surrounded by electrons