Ionic Compounds

Sodium and chlorine combine to form the compound sodium chloride, which is not reactive. The sodium and chlorine atoms form ions that have the same electron arrangement as the nearest noble gas.

Binary Ionic Compounds Binary ionic compounds are composed of two kinds of monatomic ions. For example, table salt is composed of the Na cation and the Cl anion. From the predicted ion charges, you can explain the formation of unreactive ionic compounds such as sodium chloride, calcium chloride, and Aluminum chloride. The net electrical charge in a theoretical chemical formula is zero, that is the sum of the charges on the positive ions is equal to the sum of the charges on the negative ions.

Polyatomic Ions Laboratory work yielded formulas such as NaNO 3, K 2 CO 3 and CaSO 4. When scientists tried to explain these formulas in terms of charges on monatomic ions, the results were not logically consistent. It was assumed that all ions are monatomic, however this theory needed to be revised and a new category of ions was established.

Polyatomic Ions A polyatomic ion is a cation or anion composed of a group of atoms with a net positive or negative charge. Examples are NO 3 -, CO 3 2- and SO 4 2- A large number of polyatomic ions exist, most of which have a negative charge. The formulas for the previous compounds can now be explained by using the ion charge for the simple cations and for the polyatomic anions.

Multi-Valent Metals There was another result chemists could not explain. They discovered that some metals combined with nonmetals in multiple proportions. For example they found two compounds of iron and oxygen, FeO and Fe 2 O 3 and two compounds of copper and oxygen, CuO and Cu 2 O. It was more than 100 years before an explanation emerged.

Multi-Valent Metals Chemists now accept that some metals are multi-valent, that is they can form more than one ion, each with its own particular charge. For example, iron can form both the ions Fe2+ and Fe3+ This provides and exp,lanation of the two emprical formulas for ion oxide.

Hydrates Hydrates are compounds that decompose at low temperatures to produce water and an associated compound, usually an ionic compound. Example: bluestone crystals are formed when water is added to the white powder copper sulfate. When the water of hydrations is removed from the hydrate, the product is referred to as anhydrous.

Hydrates The water molecules are assumed electrically neutral. To indicate the presence of water in a hydrate, the formula of the compound is written first, followed by a raised dot and the number of water molecules Example: CuSO 4 ° 5H 2 O