Atoms, Elements & the Periodic Table

From protons and neutrons to elements and compounds

All matter is made of atoms. Each atom has a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in shells. The number of protons (atomic number, Z) determines which element the atom is. Carbon always has 6 protons; oxygen always has 8. The mass number (A) = protons + neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Ions have an unequal number of protons and electrons (charged). The periodic table organizes elements by atomic number and groups elements with similar properties in the same column. Elements in the same group have the same number of valence (outer shell) electrons, giving them similar chemical behavior.