Chemical Formulae

Chemical formulas are used to represent elements and compounds in chemical equations.

Chemical Symbols

Each element can be represented by a unique chemical symbol, which is shown in the periodic table. A chemical symbol is always one or two letters. It starts with a capital letter and the following letter (if present) is lowercase.

For example, the chemical symbol for magnesium is ‘Mg‘.

The chemical formula for most elements is just their symbol. For example:

  • Helium (He)
  • Lithium (Li)
  • Potassium (K)
  • Argon (Ar)
  • Carbon (C)
  • Fluorine (F)
  • Sodium (Na)

There are non-metal elements that exist as molecules, with two atoms joined together. They are represented by adding a subscript ‘2’ after the chemical symbol. For example:

  • Oxygen – O2
  • Iodine – I2
  • bromine – Br2
  • chlorine – Cl2
  • hydrogen – H2

Chemical formulae of compounds

Compounds can be represented by chemical formulae. The chemical formula of a compound includes:

  • The chemical symbol for each element in the compound
  • The number of atoms of each element in the compound

For example, the chemical formula for carbon dioxide is CO2. This formula shows that the compound contains one carbon atom and two oxygen atoms. The subscript ‘2’ is used to show that there are two oxygen atoms.

Here are some examples of common compounds and their chemical formulae:

Name of compoundChemical Formula
WaterH2O
AmmoniaNH3
Magnesium oxideMgO
Sodium chlorideNaCl
MethaneCH4

Chemical formulae of Ions

We can work out the formulae of simple ionic compounds if we know the charges of the ions. The charge of an ion is shown in superscript. Here are some examples of common ions and their charges.

Name of ionCharge
OxideO²⁻
Copper (II)Cu²⁺
SulfateSO₄²⁻
HydroxideOH
CarbonateCO3²⁻

We can predict the chemical formula of an ionic compound by using the formulae of the ions it contains. The positive and negative charges in the compound must balance, resulting in an overall charge of 0.

For example, if we want to determine the chemical formula of an ionic compound formed from the combination of sodium (Na+) and chloride (Cl) ions, we can write the chemical formula as NaCl. The positive and negative charges in this compound balance, resulting in an overall charge of 0.

Example

What is the chemical formula of magnesium chloride?

Magnesium chloride is a chemical compound composed of magnesium (Mg) and chloride (Cl) ions. The charge of the magnesium ion in this compound is +2 while that of the chloride ion is -1. To balance these charges, we need to use two chloride ions for every magnesium ion.

Therefore, the chemical formula for magnesium chloride is represented as MgCl2. This simple formula tells us that there is one magnesium ion and two chloride ions in every molecule of magnesium chloride.

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