Fig; 1 Fig; 2 Atoms are the building blocks of matter, and everything around us is made up of atoms. They are the basic units of chemical elements, and are composed of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus, which is located at the center of the atom, while electrons orbit the nucleus in shells or energy levels. The number of protons in an atom's nucleus is what determines its identity as a specific element. For example, an atom with six protons is a carbon atom, while an atom with eight protons is an oxygen atom. The number of neutrons in the nucleus can vary, g...