Common Compounds

A compound is a chemical substance made of two or more different elements chemically joined together in fixed proportions. There are over 200 million known compounds (and new ones being made every day in labs), but a handful of common ones are essential to everyday life. Water (H2O), table salt (NaCl), sugar (C12H22O11), carbon dioxide (CO2), methane (CH4) and ammonia (NH3) are familiar examples. Understanding compounds means understanding most of the chemistry around you.

• Known compoundsOver 200 millionMost are organic
• Most commonWater (H2O)Covers 71% of Earth's surface
• In everyday lifeSalt (NaCl)Same atoms in sea, food, body
• Energy sourceSugar (C12H22O11)What plants store from photosynthesis
• Greenhouse gasCO2Currently approximately 420 ppm in atmosphere
• Element vs compoundSingle vs combined atoms

Compounds vs elements

Two key terms to keep straight:

• An element is a pure substance made of one kind of atom. Oxygen, carbon, gold and iron are all elements.
• A compound is a substance made of two or more different elements chemically joined together. Water (oxygen + hydrogen) and salt (sodium + chlorine) are compounds.

A compound has completely different properties from its elements. Sodium is a soft metal that explodes on contact with water. Chlorine is a poisonous green gas. Combine them and you get harmless table salt that you sprinkle on your chips. That is why chemistry is so powerful: the combinations matter much more than the individual ingredients.

Famous compounds

• Water (H2O): covers 71% of Earth's surface. Essential for all known life.
• Salt (NaCl): sodium chloride. In sea water, your blood, your sweat and your food.
• Sugar (C12H22O11): sucrose. The energy currency plants store after photosynthesis.
• Carbon dioxide (CO2): 0.04% of the atmosphere; what plants take in, what we breathe out, the main greenhouse gas of climate change.
• Methane (CH4): natural gas. The simplest hydrocarbon, used to heat homes and a powerful greenhouse gas.
• Ammonia (NH3): nitrogen + hydrogen. Used in fertilisers and cleaning products.
• Vinegar (CH3COOH): acetic acid in water. Used in cooking and cleaning.

How compounds form

Atoms join together when they need to fill or empty their outer electron shells. Different combinations produce different types of bond.

• Ionic bonds: one atom completely gives an electron to another. The two atoms become charged ions and stick together by electric attraction. Common in salts.
• Covalent bonds: two atoms share electrons. Common in molecules of gases, water, sugars, plastics, almost all organic compounds.
• Metallic bonds: metal atoms share a sea of electrons that can flow freely. This is why metals conduct electricity.

Fact The total number of known chemical compounds has grown from approximately 1 million in 1965 to over 200 million today. Most of the new compounds are designed in laboratories: medicines, plastics, materials, dyes and many others. Chemists at universities and pharmaceutical companies add thousands of new compounds to the records every single day.

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