5.4 Chemical Formulas of Compounds

The chemical formula expresses the combination of atoms that form a molecule of a compound. It uses the symbols of elements and numbers to represent the atoms involved. These numbers, indicating the quantity of atoms, are written in subscript.

System of Writing Chemical Formula

1. Writing the Number of Atoms:

• In a molecule, the number of atoms for each element is written as a subscript after its symbol.

• Example: A nitrogen molecule consists of two nitrogen atoms, written as N2.

• Example: An ozone molecule has three oxygen atoms, written as O3.

• Some elements, like metals, do not form molecules and are expressed by their symbol only.

• Example: Iron is written as Fe.

• Example: Helium is written as He (since it is an inert gas).

2. Compounds Made of Two Elements:

• When a molecule consists of atoms from two different elements and their valencies (combining capacities) are not divisible by a common number, the elements are written side by side, each followed by the valency of the other.

• Example: For a compound of aluminum (Al) and oxygen (O):

• Valency of aluminum = 3, and valency of oxygen = 2.

• The formula is written as Al2O3 because the valency of aluminum (3) and oxygen (2) are not divisible by a common number.

• Example: In calcium chloride, the valency of calcium (Ca) is 2, and chlorine (Cl) is 1.

• The formula is CaCl2 (the number 1 for chlorine is not written explicitly).

3. Radicals in the Compound:

• When a compound contains a radical (a group of atoms acting as a single unit), and the number of radicals is greater than 1, the radical’s formula is written in brackets followed by the number of radicals.

• Example: In ammonium phosphate, the radical ammonium is NH4, and the radical phosphate is PO4.

• The formula is written as (NH4)3PO4.

• Example: In aluminum sulfate, the radical sulfate is SO4, and aluminum is Al.

• The formula is written as Al2(SO4)3.

4. Divisible Valencies:

• When the valencies of two elements are divisible by a common number, the valency is divided by that number, and the quotient is written as part of the formula.

• Example: In carbon dioxide, carbon has a valency of 4 and oxygen has a valency of 2.

• These valencies can be divided by 2, so the formula becomes CO2 (quotient 2).

• Example: In ferrous sulfate, the valency of ferrous (Fe) is 2, and the valency of sulfate (SO4) is 2.

• Dividing by 2, the formula becomes FeSO4.

• Example: In boron nitride, the valency of boron (B) is 3, and the valency of nitrogen (N) is 3.

• Since the valencies are the same, the formula becomes BN.