Elements (such as hydrogen, oxygen, and carbon) are types of matter composed of different combinations of protons (positive charges), electrons (negative charges), and neutrons (no charges). The smallest quantity of an element is a single atom, which can be understood as a central nucleus composed of protons and neutrons surrounded by a certain number of orbiting electrons. These particles have positive, negative, and neutral (no) charges, respectively. The protons and neutrons are in the nucleus of the atom and the electrons spin around the nucleus in paths called orbitals. Sets of electrons occupy concentric shells around the nucleus, like planetary orbits around the sun. Atoms are electrically neutral, having the same number of electrons as protons, except when they exist as ions.