Term 1 review: Units 1 to 4

Review the key ideas. An atom has Z protons and Z electrons. Neutrons = A − Z. Isotopes are atoms of one element with the same Z but different A. Electrons fill energy levels in order, and the number of levels is the period number. For groups 1A to 7A the outer electrons equal the group number. Metals tend to give electrons and nonmetals tend to gain them. When a metal and a nonmetal meet, the electrons move and an ionic bond forms between ions. When two nonmetals meet, they share electrons in a covalent bond. A compound has two or more elements joined chemically, while a mixture can be separated by physical means. Sodium chloride is a crystal of ions, not molecules.