- Atoms are made up of protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons orbit in energy levels around the nucleus. - The number of protons determines the element and is equal to its atomic number. The total number of protons and neutrons determines the atomic mass of an element. - Electrons occupy different energy levels based on their distance from the nucleus, with the first level holding up to 2 electrons and subsequent levels holding more. Valence electrons in the outermost level determine an element's reactivity.