Symbols, Formula and Equations
Chemical formula

The chemical formula of a compound is the combination of the symbols of the combining elements in a definite ratio. A chemical formula is derived by the exchange of radicals of combining elements.
Compounds Formula
Sodium Chloride NaCl
Nitrogen (IV) oxide NO2
Iron (II) oxide FeO
Potassium Chloride KCl
Lead (II) Chloride PbCl2
Trioxonitrate(V) acid HNO3

Radicals

A radical is a group of atoms bearing a common charge or valency.

Radical Charge Valency
Trioxo
carbonate (IV) ion (\(\ {CO}_{ 3}^{2-} \)) 		-2 2
Ammonium ion (\(\ {NH}_{4}^{+} \)) +1 1
Hydroxide ion (OH-) -1 1
Tetraoxo
sulphate (VI) ion (\(\ {SO}_{4}^{2-} \) )		 -2 2
Heptaoxo
dichromate (VI) ion (\(\ {Cr}_{2}O_{7}^{2-} \) )		 -2 2
Trioxonitrate (V) ion( \(\ {NO}_{3}^{-} \)) -1 1
Tetraoxo
phosphate (V) ion (\(\ {PO}_{4}^{3-} \))		 -3 3
Valency and oxidation number

The valency of an element is the combining power of that element. It corresponds to the number of valence electrons an element can donate or receive to attain stable configuration state. It has no charge.

The oxidation number of an element is the actual charge of the element in combined state. The oxidation number of an element can differ as it conbines with other elements to form compounds.

Ions Valency Oxidation number
Ca2+ 2 +2
K+ 1 +1
O2- 2 or 1 -2 or -1 (in peroxides)
N3- 3 or 5 -3 or +5 in nitric acid
Cr3+ 3 +3
Fe3+ 3 +3
H+ 1 +1 when combined with non-metals and -1 when combined with metals to form hydrides
All halogens 1 -1
Zn 2+ 2 +2
Cu2+ 2 +2
Sn2+ 2 +2
Al3+ 3 +3
Mg2+ 2 +2
P-3 3 or 5 -3 or +5
Deriving chemical formula

A chemical formula is derived by exchange of valencies of the combining elements or radicals.

Examples

Write the chemical formula of the following

  1. Potassium hydroxide
  2. Potassium heptaoxodichromate(VI)
  3. Iron (III) oxide
  4. Phosphorus (V) oxide
  5. Calcium trioxonitrate (V)
  6. Lead (II) trioxocarbonate(IV)

Solution

  1. Potassium hydroxide: consists of potassium and hydroxide ion $$\text{Writing out the valencies} $$ $$ K = 1 $$ $$ OH = 1 $$ $$ \text{After exchange of valencies} $$ $$ Formula = KOH $$
  2. Potassium heptaoxodichromate (VI) consists of potassium and heptaoxodichromate (VI) ion $$ K = 1 $$ $$ Cr_{2}O_{7}^{-2} = 2 $$ $$ \text{After exchange of valencies} $$ $$ Formula = K_2Cr_2O_7 $$
  3. Iron (III) oxide consists of Iron and oxygen. The (III) shows the iron has a valency of 3. $$ Fe = 3 $$ $$ O = 2 $$ $$\text{After exchange of valencies} $$ $$ Formula = Fe_2O_3 $$
  4. Phosphorus (V) oxide is composed of phosphorus and oxygen. The (V) means phosphorus has a valency of 5. $$ P = 5 $$ $$ O = 2 $$ $$ \text{After exchange of valencies} $$ $$ Formula = P_2O_5 $$
  5. Calcium trioxonitrate V) is composed of Calcium and the radical trioxonitrate(V) ion $$ Ca = 2 $$ $$ NO_3^{-} = 1 $$ $$ \text{After exchange of radicals} $$ $$ Formula = Ca(NO_3)_2 $$
  6. Lead (II) trioxocarbonate(IV) is composed of Lead and the radical trioxocarbonate(IV) ion. The (II) means lead has a valency of 2 $$ Pb = 2 $$ $$ CO_3^{2-} = 2 $$ $$ \text{After exchange of valencies} $$ $$ Formula = PbCO_3 $$ $$\text{Note: Same values can cancel out} $$
IUPAC naming of compounds

Compounds can be named by using oxidation number. The rules that govern oxidation number of an atom or radical are as follows:

Examples

  1. Find the oxidation number of Chlorine in NaClO and name the compound

Solution

Let the oxidation number of Chlorine be x The oxidation number of Na = +1 and that of oxygen = -2

$$ NaClO = 0 $$ $$ +1 + x + (-2) = 0 $$ $$ +1 + x -2 = 0 $$ $$ x = 2 - 1 $$ $$ x = +1 $$ $$ \text{Oxidation of chlorine = 1} $$

Name the compound from first, to last before the middle element which is the unknown.
\(\therefore \) The name of the compound is Sodium oxochlorate (I).


  1. Find the oxidation number of Aluminum in LiAlH4

Solution

In this example, Hydrogen is combining with a metal hence its oxidation number is -1. Lithium has an oxidation number of +1. Let the oxidation number of Al be x. $$ +1 + x + (-1 × 4) = 0 $$ $$ x + 1 - 4 = 0 $$ $$ x = +3 $$ The name of the compound is given as Lithium tetrahydridoaluminate(III).

  1. Find the oxidation number of Chromium
    1. \(\ Cr_2O_7^{-2} \)
    2. \(\ K_2Cr_2O_7 \)

Solution

  1. Let the oxidation number of Chromium be x $$ 2x + (-2 × 7) = -2 $$ $$ 2x - 14 = -2 $$ $$ 2x = -2 + 14 $$ $$ x = \frac{12}{2} $$ $$ x = +6 $$ The name is heptaoxodichromate (VI) ion
  2. The oxidation number of potassium is +1 $$ (+1 × 2) + 2x + (-2 × 7) = 0 $$ $$ 2 + 2x -14 = 0 $$ $$ 2x = 12 $$ $$ x = +6 $$

The oxidation number of chromium is +6.
The name is Potassium heptaoxodichromate (VI)

Summary