Monday, November 30th, 2009

Covalent-Network Compounds

• Some covalent compounds do not consist of individual molecules.
• Instead, Each atom is joined to all its neighbors in a covalently bonded, three-dimensional network
• The subscripts in a formula for a covalent-network compound indicate the smallest whole-number ratio of the atoms in the compound.

Ex: SiC-Silicon carbide

SiO2-Silicon dioxide

Si3N4-Trisilicon tetranitride

Acids and Salts

• Acid is a certain type of molecular compound.
• Most acids used in the laboratory can be classified as either binary acids or oxyacids.
• Binary Acids- Are acids that consist of two elements, usually hydrogen and one of the halogens-Fluorine, chlorine, bromine, iodine.
• Oxyacids- Are acids that contain hydrogen, oxygen and a third element (usually a nonmetal)
• In chemical nomenclature, the term acid usually refers to a solution in water of one of these special compounds rather than to the compound itself.
• For example, hydrochloric acid refers to a water solution of the molecular compound hydrogen chloride, HCl.
• Many polyatomic ions are produced by the loss of hydrogen ions from oxyacids.

Ex. Sulfuric acid- H2SO4 Sulfate-SO2-/4

Nitric acid- HNO3 Nitrate-NO-/3

Phosphoric acid- H3PO4 Phosphate- PO3-/4

• An ionic compound composed of a cation and the anion from an acid is often referred to as a salt. Table salt, NaCl, contains the anion from hydrochloric acid. Calcium Sulfate, CaSO4, is a salt containing an anion from sulfuric acid. HCO-/3, Hydrogen carbonate ion bicarbonate ion.

Oxidation Numbers

• The charges on the ions composing an ionic compound reflect the electron distribution of the compound
• In order to indicate the general distribution of elections among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers, also called oxidation states, are assigned to the atoms composing the compound or ion.
• Unlike ionic charges, oxidation numbers do not have an exact physical meaning.

Assigning Oxidation Numbers

• As a general rule in assigning oxidation numbers, shared electrons are assumed to belong to the more electronegative atom in each bond.
• More specific rules for determining oxidation numbers are provided by the following guidelines:

1. The atoms in a pure element have an oxidation number of zero. For example, the atoms in pure sodium, Na, oxygen, O2, phosphorus, P4, and sulfur, S8 all have oxidation numbers of zero.
2. The more-electronegative element in a binary molecular compound is assigned the number equal to the negative charge it would have as an anion. The less-electronegative atom is assigned the number equal to the positive charge it would have as a cation.
3. Fluorine has an oxidation number of -1 in all of its compounds because it is the most electronegative element.
4. Oxygen has an oxidation number of -2 in almost all compounds. Exceptions include when it is in peroxides, such as H2O2 , in which its oxidation number is -1, and when it is in compounds with fluorine, such as OF2, in which its oxidation number is +2.
5. Hydrogen has an oxidation number of +1 in all compounds containing elements that are more electronegative than it; it has an oxidation number of -1 in compounds with metals.
6. The algebraic sum of the oxidation numbers of all atoms in a neutral compound is equal to zero.
7. The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.
8. Although rules 1 through 7 apply to covalently bonded atoms, oxidation numbers can also be assigned to atoms in ionic compounds.