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Medicine LibreTexts

1.03: Compounds and Molecules

molecule is formed when two or more atoms join together. A compound is formed when two or more different elements combine in a fixed ratio by mass. Note that some atoms are never found alone. For example oxygen is always found as molecules of 2 oxygen atoms (represented as O2).

The table below gives some common compounds.

Compound Symbol
Calcium carbonate CaCO3
Carbon dioxide CO2
Copper sulphate CuSO4
Glucose C6H12O6
Hydrochloric acid HCl
Sodium bicarbonate (baking soda) NaHCO3
Sodium chloride (table salt) NaCl
Sodium hydroxide NaOH
Water H2O


  • Ruth Lawson (Otago Polytechnic; Dunedin, New Zealand)