Symbols and formulae

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Key points

  • Letters from the alphabet are used to represent chemical .
  • An element’s chemical is a single capital letter or a capital letter followed by a lower case letter.
  • The shows the chemical symbols for all the different elements.

The formula H₂O is used to represent water. What does the formula mean?

The formula H₂O shows that water is made from two atoms of hydrogen (H) bonded to one atom of oxygen (O).


Watch this video to learn more about the periodic table, chemical symbols and .

What are the symbols for the elements magnesium and oxygen?

The chemical symbol for oxygen is O. The chemical symbol for magnesium is Mg.

Symbols and elements


An element is a substance that cannot be broken down into any other substance. Every element is made up of its own type of . Oxygen, hydrogen, gold, and iron are all elements.


Elements are represented by letters from the alphabet. These letters are also known as symbols. The symbol for an element is either a single capital letter or a capital letter followed by a lower case letter.

For example:


A lot of the element names have come from Latin. The Latin name for sodium was 'natrium'.

Which element has the symbol N?


The periodic table

There are 118 chemical elements and they are listed on the periodic table in a specific order.

The periodic table. Metals in red squares and non-metals in yellow squares.
  • 94 chemical elements occur naturally on Earth and in the universe.
  • 24 elements are made using nuclear reactions.

Scientist Marie Curie discovered two elements: radium and polonium. She discovered that radium could help the body fight cancer cells. Marie became the first woman to win a Nobel Prize.

What are the only two letters of the alphabet to not appear on the periodic table?

Q and J.

Compounds and molecules


Watch this video where Dr Tim explains how can be represented by chemical formulae.

The formula for carbon dioxide is CO₂. What atoms are in one molecule of carbon dioxide?

Each molecule of carbon dioxide contains one atom of carbon (C) and two atoms of oxygen (O₂).


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

The elements are chemically bonded together in a specific of atoms.

The formula of a compound shows how many atoms of each element are bonded together.

The formula is made up of symbols and numbers.

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


  • Water is a compound of hydrogen and oxygen. The symbols H for hydrogen and O for oxygen are used to represent H₂O.

  • Oxygen gas is a made of two oxygen (O) atoms represented as O₂.

  • Sulfuric acid is a combination of hydrogen (H), sulfur (S) and oxygen (O) represented as H₂SO₄.

H₂OTwo hydrogen atoms, one oxygen atom3 atoms in total
CO₂One carbon atom, two oxygen atoms3 atoms in total
C₄H₁₀Four carbon atoms, ten hydrogen atoms14 atoms in total

The plural of is ‘formulae’.


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

Five red circles, each with two small white circles attached
Water molecules
Three molecules, each represented by two red circles attached to one black circle
Carbon dioxide molecules

The structure of compounds

Compounds made from a metal and a non-metal, like sodium chloride, do not form molecules.

They form giant structures like this.

For compounds made from metals and non-metals, the ratio is simplified down as much as possible.

In this example, the formula is NaCl instead of Na₈Cl₈.

Calcium chloride has the formula CaCl₂, so there are twice as many chlorine atoms present as calcium atoms.

Non-metal elements

Many non-metal are made from , and these have a chemical formula which shows how many are bonded to each other in their molecules.

Here are some common examples. Most are made from two atoms in each molecule.

  • H₂ – hydrogen
  • O₂ – oxygen
  • Cl₂ – chlorine
  • N₂ – nitrogen

Metals don’t form molecules, so always use just the element symbol. For example, Mg for magnesium, not Mg₂ or Mg₄.

Which elements does MgCl₂ contain? How many of each type of atom are present?

MgCl₂ is the formula for magnesium chloride, which is a compound containing magnesium (Mg) and chlorine (Cl). There are two atoms of chlorine for every one atom of magnesium.

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