Writing Chemical Formulae

What is Chemical Formula?

A chemical formula shows the elements that make up the compound and the numbers of atoms of each element in the smallest unit of that compound, be it a molecule or a formula unit.

Knowing the names of the elements and a few basic rules allows us to name simple compounds given the chemical formula. We also can reverse the process. That is, if we know the name of the compound, we should be able to write the chemical formula.

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Atoms and Molecules

Atoms group together to make the objects and other materials around us. Matter that contains just one type of atom is called an element. Atoms can also form molecules that have complex shapes. Carbon atoms can join to make spherical molecules of connected hexagons and pentagons.

• Chemists have to write chemical equations all the time and it would take too long to write and read if they had to spell everything out.
• So chemists use symbols just like it’s done in maths. Each element has a symbol. There’s a particular way of writing what’s in a molecule called a chemical formula.
• The chemical formulae for all the elements that form each molecule and uses a small number to the bottom right of an element’s symbol to stand for the number of atoms of that element.
• For example, the chemical formula for water is H₂O. That tells us that a water molecule is made up of two hydrogen atoms and one oxygen atom.

Writing and Balancing Chemical Equations

A chemical equation is a written statement that uses chemical symbols and chemical formulas instead of words to describe the changes that occur in chemical reactions.

The following example, shows the contrast between a word description of a chemical reaction and a chemical equation for the same reaction.

• Word description – Calcium sulfide reacts with water to produce calcium oxide and hydrogen sulfide
• Chemical equation – CaS + H₂O → CaO + H₂S

In the same way that chemical symbols are considered the letters of chemical language and formulas are considered the words of the language, chemical equations can be considered the sentences of chemical language.

The substances present at the start of a chemical reaction are called reactants. A reactant is a starting material in a chemical reaction that undergoes change in the chemical reaction. As a chemical reaction proceeds reactants are consumed and new materials with new chemical properties called products are produced. A product is a substance produced as a result of the chemical reaction.

Convention used in Writing Chemical Equations

Four convention are used to write chemical equations

1. The correct formulas of the reactants are always written on the left side of the equation.

CaS + H₂O → CaO + H₂S

The correct formulas of the products are always written on the right side of the equation.

CaS + H₂O → CaO + H₂S

The reactants and products are separated by an arrow pointing toward the products.

CaS + H₂O → CaO + H₂S

This arrow means “to produce“.

Plus signs are used to separate different reactants or different products.

CaS + H₂O → CaO + H₂S

Plus signs on the reactant side of the equation mean “reacts with” and plus signs on the products side mean “and”.

Writing Chemical Formula of Binary Compounds using Valency

A Chemical formula represents the proportions, by mass of the different elements in a compound.

A binary compound is composed of two different elements only. Formulae of binary compounds can be written using valencies since both types of atom forming the compound must lose, gain or share the same number of electrons.

• To write the chemical formula of a binary compound.
• Determine the valencies of the two elements present.
• Write the chemical formula such that the sums of the valencies of the two elements are equal. Do this by determining the lowest common multiple of the two valencies.
• If a metal is present, always place it first in the formula.

Solved Examples

1. Aluminium oxide

Valencies of elements:

Al = 3 (Group III)

O = 2 (Group VI)

Lowest common multiple of valencies = 6

Al = 2 x 3 = 6

O = 3 x 2 = 6

Therefore the chemical formula = Al₂O₃

2. Phosphorus trichloride

Valencies of elements:

P = 3 (Group V)

Cl = 1 (Group VII)

Lowest common multiple of valencies = 3

P = 1 x 3 = 3

Cl = 3 x 1 = 3

Chemical formula = PCl₃