Matter is made of very small particles. An atom is the smallest unit of an element that retains its chemical identity. A molecule contains atoms bonded together. Some substances, such as sodium chloride, are better described as networks of ions rather than separate molecules.
Read a formula carefully
A chemical symbol identifies an element. Capital letters matter: Co is cobalt, while CO is a formula for carbon monoxide. A subscript tells you how many atoms of the preceding element appear in the formula. H₂O contains two hydrogen atoms and one oxygen atom per molecule.
A coefficient multiplies the whole formula. Thus 3H₂O represents three water molecules containing six hydrogen atoms and three oxygen atoms in total.
- A missing subscript means 1.
- In Ca(OH)₂, the subscript 2 multiplies both O and H inside the brackets.
Add relative atomic masses
To find relative molecular mass, add the relative atomic masses of all atoms in a molecule, counting the subscripts. Using H = 1 and O = 16 gives water a relative molecular mass of 18. The approximate mass of one water molecule is therefore 18 u.
Relative molecular mass itself is a ratio and has no unit. Molecular mass may be expressed in u, while molar mass is expressed in units such as g/mol. Keeping these terms distinct prevents a correct number being given with the wrong unit.
Put it into practice
A worked example
Find the relative molecular mass of CO₂ using C = 12 and O = 16.
- There is one carbon atom and two oxygen atoms.
- Calculate 12 + 2 × 16 = 44.
Answer: Relative molecular mass = 44; approximate molecular mass = 44 u.
Try it yourself
Have a go before revealing the answer.
How many hydrogen atoms are represented by 4H₂?
Show answer
8 hydrogen atoms.
Answer: 8 hydrogen atoms.
Count the atoms in one formula unit of Ca(OH)₂.
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One calcium, two oxygen and two hydrogen atoms: five atoms in total.
Answer: One calcium, two oxygen and two hydrogen atoms: five atoms in total.
Watch out for these
- Do not change a subscript to balance an equation; that changes the substance.
- A coefficient applies to the entire formula, not just its first element.
Continue with official resources
Match these concept notes to your current textbook and syllabus. They are independent explanations, not official board publications.
