ATOMS, COMPOUNDS AND IONS

a) Atoms and compounds are electrically neutral which means that there are equal numbers of positive and negative charges b) Ions are not neutral and may be either positively or negatively charged. An ion that contains more protons than electrons has an overall positive charge and is called a(n) cation. An ion that contains less protons than electrons has an overall negative charge and is called a(n) anion. c) Polyatomic Ions (PAI): a group of atoms covalently bonded together possessing a charge. Parentheses are used to enclose PAIwhen there is more than one of the ions in a unit of a compound. The subscript written after the parentheses tells how many of the ions are present in the compound. **The subscript refers to each of the elements in the ion. Example: (NH4)+ (PO3)- d) Hydrates: when water from some ionic solutions evaporates, the solute forms a crystal lattice that binds water within the structure. Such a compound is called a hydrate. These crystals have a definite number of water molecules for each unit of the compound. These water molecules are loosely held to the compound. Example: Barium Chloride dihydrate BaCl2 • 2H2O *The anhydrous (not hydrated) compound can be obtained by heating the crystals to drive off the water. In a chemical reaction, the water in a hydrate does not react. However, it adds mass to the compound.

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Chem Regents formulas and equations