The basic particle from which all elements are made is called an atom. Atoms consist of three primary subatomic particles: protons, neutrons, and electrons. Protons and neutrons reside in the atom’s nucleus, while electrons orbit the nucleus in defined energy levels. The number of protons in the nucleus determines an element's atomic number and its identity. Atoms can combine in various ways to form molecules, creating the diverse substances that make up matter in the universe.