CHEMICAL FORMULAS & NOMENCLATURE

Oxidation Number The oxidation number of an element indicates the number of electrons lost, gained, or shared as a result of chemical bonding. To determine the oxidation number of an atom in a compound or an ion, a certain number of rules must be made. Rule (1): The oxidation number of an atom in the elemental state is zero. The oxidation no. of He, Ne, Al, Fe, Cl 2, O 2, H 2 … is zero. Rule (2): The oxidation number of a monatomic ion is equal to its charge. The oxidation number of Na + = +1 F - = -1 Ca ++ = +2 S -- = -2 Rule (3): The sum of the oxidation numbers of all the atoms in a neutral compound is zero. The sum of the oxidation no. of NaF = (+1) + (-1) = 0.

Rule (4): The oxidation number of H = +1 when combined with a nonmetal as in CH 4, NH 3 and H 2 O. H = -1 when combined with a metal such as sodium hydride (NaH) Rule (5): The oxidation number of oxygen is always -2 except: in: peroxides, such as H 2 O 2, Na 2 O 2, the oxidation number = -1. in: Oxygen di-fluoride (OF 2 ), the oxidation number = +2. Rule (6): The oxidation number of halogens such as F, Cl, Br, I =-1 Rule (7): The sum of the oxidation numbers of all the atoms in a polyatomic ion is equal to the net charge on the ion, CO 3 2- = -2, PO 4 3+ = +3

1.What is the oxidation number of Aluminum in the compound Al 2 O 3 ? The oxidation number of the compound Al 2 O 3 = 0 (2 × Oxidation number of Al) + (3 × Oxidation number of O)= 0 [2 × Al + [3 × (-2)] = 0 2Al + (-6) = 0 2Al -6 = 0 2Al =+6 Al =+3 2.What is the oxidation number of Carbon in the compound CCl 4 ? The oxidation number of the compound CCl 4 = 0 (1 × Oxidation number of C) + (4 x Oxidation number of Cl) = 0 Oxidation number of C + (4 x -1) = 0 Oxidation number of C = +4 2.What is the oxidation number of Carbon in the compound ICl 3 ? The oxidation number of the compound ICl 3 = 0 (1 × Oxidation number of C) + (3 x Oxidation number of Cl) = 0 Oxidation number of I + (3 x -1) = 0 Oxidation number of I = +3

3.What is the oxidation number of Carbon in the compound CO 3 2- ? The oxidation number of the ion CO 3 2- ) = -2 (Oxidation number of C) + (3 × Oxidation number of O) = -2 (Oxidation number of C) + 3(-2) = -2 (Oxidation number of C) -6 = -2, thus: Oxidation number of C = = +4 What is the oxidation number of chlorine in the compounds HCl and HClO? The oxidation number of the compound HCl = 0 Oxid. H + Oxid. Cl = Oxid. Cl = 0 Oxid. Cl = -1 The oxidation number of the compound HClO = 0 Oxid. H + Oxid. Cl + Oxid. O = Oxid. Cl + (-2) = Oxid. Cl -2 =0 Oxid. Cl -1 =0 Oxid. Cl = +1

The Formulas of Compounds The formulas of ionic and covalent compounds could be predicted by using the oxidation numbers as a follow: Ionic compounds How to use the oxidation numbers to predict the formula of the binary ionic compound formed by zinc (Zn) and bromine (Br) ? The zinc forms Zn 2+ ions, its oxidation = +2 in its ionic compounds; The bromine will form Br - ions (an oxidation number of -1). Since the sum of the oxidation numbers must be zero for the ionic compound, we will need two Br - ions for every Zn 2+ ion. Thus, the formula of the compound will be ZnBr 2.

Predict the chemical formula for the binary ionic compounds formed by Aluminum and sulfur? The oxidation number of aluminum in the Al 3+ ion is +3. The oxidation number of Sulfur in the S 2- ion is -2. In order to form a compound in which the sum of the oxidation numbers is zero, we should need 2 Al 3+ and 3 S 2- ions. Thus the formula of the compound is Al 2 S 3. Polyatomic ions NameFormulaNameFormula AcetateCH 3 CO 2 - CyanideCN - AmmoniumNH 4 + HydroxideHO - CarbonateCO 3 2- NitrateNO 3 - ChlorateClO 3 - PermanganateMnO 4 - ChromateCrO 4 2- PhosphatePO 4 3- DichromateCr 2 O 7 2- SulfateSO 4 2-

Covalent compounds Although predicting the formula of ionic compounds is fairly easy, covalent compounds present more of a problem. This is because covalent compounds consist of two (or more) nonmetals which may exhibit a variety of oxidation numbers. Predict the formula of the compound formed by P and Cl ? Chlorine is the more electronegative, it has its normal oxidation number of -1. Phosphorus has two common positive oxidation numbers, +3 and +5. Thus, two compounds are possible: PCl 3 and PCl 5.

Order of Symbols in a chemical Formula For compounds containing only two elements, the more electropositive element is usually written first. Potassium chloride is written as KCl rather than as ClK. why? Metals have a lower electronegativity than nonmetals. Thus, potassium (K) is the more electropositive element and is written before chlorine (Cl); the formula KCl is correct. Hydrogen sulfide is written as H 2 S rather than as SH 2. why? Both sulfur and hydrogen are nonmetals. Hydrogen is the more electropositive element and is written first; H 2 S is the correct formula.

Chemical Nomenclature Binary covalent compounds Due to the ability of nonmetals to exhibit more than one oxidation number, two nonmetals can combine to form more than one compound. The number of atoms present is indicated by the Greek prefixes: mono-, di-, tri-,…etc Greek prefixNumberGreek prefixNumber Mono-1Hexa-6 Di-2Hepta-7 Tri-3Octa-8 Tetra-4Nona-9 Penta-5Deca-10 Example: PCl 3 is called phosphorus trichloride and PCl 5 is phosphorus pentachloride.

Binary ionic compounds The name calcium chloride for CaCl 2 is adequate, the name calcium dichloride would be unnecessary and incorrect. Classical names for ions of some of the metals that form more than one cation: MetalIon of lower oxidation numberIon of higher oxidation number IronFerrous, Fe 2+ Ferric, Fe 3+ CopperCuprous, Cu + Cupric, Cu 2+ TinStannous, Sn 2+ Stannic, Sn4 + LeadPlumbous, Pb 2+ Plumbic, Pb 4+ MercuryMercurous, Hg + Mercuric, Hg 2+

Classical name for ions of some of the metals that form more than one cation FeCl 2 ferrous chloride FeCl 3 ferric chloride IUPAC name for ions of some of the metals that form more than one cation AlCl 3 aluminum chloride SnCl 4 tin(IV) chloride SnCl 2 tin(II) chloride Hg 2 O mercury(I) oxide