The document summarizes atomic structure. It explains that atoms are made up of protons, neutrons, and electrons. Protons are positively charged and have a mass of about 1 atomic mass unit. Neutrons are neutral and also have a mass of about 1 atomic mass unit. Electrons are negatively charged and have a negligible mass. The nucleus of an atom contains protons and neutrons, while electrons orbit the nucleus in shells. Hydrogen has 1 proton and 1 electron, while helium has 2 protons, 2 neutrons, and 2 electrons. An element's mass number is the total number of protons and neutrons, while its atomic number refers specifically to the number of protons. Electron shells can hold a maximum of 2, 8