← Section 1 Introduction to Atoms Export Options Alphabetize Word-Def Delimiter Tab Comma Custom Def-Word Delimiter New Line Semicolon Custom Data Copy and paste the text below. It is read-only. Select All nucleus The central core of an atom containing protons and usually neutrons. proton A small, positively charged particle in the nucleus of the atom. neutron A small particle in the nucleus of the atom, with no electrical charge. electron A tiny negatively charged particle that moves around the nucleus of an atom. atomic number The number of protons in the nucleus of an atom. isotope An atom with the same number of protons and a different number of neutrons from other atoms of the same element. mass number The sum of protons and neutrons in the nucleus of an atom. model In science, a diagram, a mental picture, a mathematical statement, or an object that helps explain ideas about the natural world.