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, Cl2, O2, H2 … 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 CH4, NH3 and H2O.
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 H2O2 , Na2O2 , the oxidation number = -1.
in: Oxygen di-fluoride (OF2), 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,
CO32- = -2 , PO4 3+ = +3
1.What is the oxidation number of Aluminum in the compound Al2O3?
The oxidation number of the compound Al2O3 = 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 CCl4 ?
The oxidation number of the compound CCl4 = 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 ICl3 ?
The oxidation number of the compound ICl3 = 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
CO32- ?
The oxidation number of the ion CO32-) = -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 = -2 + 6 = +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 =0
+1 + Oxid. Cl = 0
Oxid. Cl = -1
The oxidation number of the compound HClO = 0
Oxid. H + Oxid. Cl + Oxid. O =0
+1 + Oxid. Cl + (-2) =0
+1 + 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 Zn2+ 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 Zn2+ ion.
Thus, the formula of the compound will be ZnBr2.
Predict the chemical formula for the binary ionic compounds
formed by Aluminum and sulfur?
The oxidation number of aluminum in the Al3+ ion is +3.
The oxidation number of Sulfur in the S2- ion is -2.
In order to form a compound in which the sum of the oxidation
numbers is zero, we should need 2 Al3+ and 3 S2- ions.
Thus the formula of the compound is Al2S3.
Polyatomic ions
Name
Formula
Name
Formula
Acetate
CH3CO2-
Cyanide
CN-
Ammonium
NH4+
Hydroxide
HO-
Carbonate
CO32-
Nitrate
NO3-
Chlorate
ClO3-
Permanganate
MnO4-
Chromate
CrO42-
Phosphate
PO43-
Dichromate
Cr2O72-
Sulfate
SO42-
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: PCl3 and PCl5 .
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 H2S rather than as SH2 . why?
Both sulfur and hydrogen are nonmetals. Hydrogen is the more
electropositive element and is written first; H2S 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 prefix
Number
Greek prefix
Number
Mono-
1
Hexa-
6
Di-
2
Hepta-
7
Tri-
3
Octa-
8
Tetra-
4
Nona-
9
Penta-
5
Deca-
10
Example: PCl3 is called phosphorus trichloride and PCl5 is phosphorus
pentachloride.
Binary ionic compounds
The name calcium chloride for CaCl2 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:
Metal
Ion of lower oxidation number Ion of higher oxidation number
Iron
Ferrous, Fe2+
Ferric, Fe3+
Copper
Cuprous, Cu+
Cupric, Cu2+
Tin
Stannous, Sn2+
Stannic, Sn4+
Lead
Plumbous, Pb2+
Plumbic, Pb4+
Mercury
Mercurous, Hg+
Mercuric, Hg2+
Classical name for ions of some of the metals
that form more than one cation
FeCl2
FeCl3
ferrous chloride
ferric chloride
IUPAC name for ions of some of the metals that
form more than one cation
AlCl3
aluminum chloride
SnCl4
tin(IV) chloride
SnCl2
tin(II) chloride
Hg2O
mercury(I) oxide