Symbols and formulae

Elements and symbols

Elements

An element is a substance that is made of only one kind of atom, and cannot be split into simpler substances. Some examples of elements are oxygen, hydrogen, gold, and iron.

Symbols Elements are shown by letters from the alphabet. These letters are also called symbols. The symbol for an element is either a single capital letter or a capital letter with a lower case letter after it.

For example:

• H = Hydrogen
• He = Helium
• Li = Lithium
• Be = Beryllium
• N = Nitrogen
• O = Oxygen
• F = Fluorine

The periodic table

There are 118 chemical elements listed in the periodic table:

• 94 chemical elements occur naturally on Earth and in the universe.
• 24 elements are made using nuclear reactions.

Compounds and molecules

Compounds

A compound is a substance that has atoms of two or more different elements.

The atoms of the elements are joined together by chemical bonds in a fixed ratio of atoms.

The formula of a compound shows how many atoms of each element are in the compound.

The formula is made of symbols and numbers.

The numbers are written as subscript, which means they are smaller and lower than the symbol. For example, the formula for water is H2O.

For example:

• Water is a compound of hydrogen and oxygen. It has the symbol H for hydrogen and the symbol O for oxygen. The formula for water is H2O.
• Oxygen gas is a molecule of two oxygen (O) atoms. It has the symbol O for oxygen. The formula for oxygen gas is O2.

H2O = Two atoms of hydrogen, one atom of oxygen = 3 atoms in total
CO2 = One atom of carbon, two atoms of oxygen = 3 atoms in total

Molecules

Most molecules are made of two or more non-metal atoms bonded together. For example:

Water molecule