Atoms and chemical reactions
Balance a reaction by conserving atoms, without changing chemical formulae.
Before you begin
- Elements and compounds
- Simple whole-number ratios
Distinguish atoms from molecules
An element contains one type of atom. A compound contains atoms of different elements chemically combined in fixed proportions. A molecule is a group of bonded atoms, which may be the same element or different elements. The subscript in H₂O states its composition: two hydrogen atoms for each oxygen atom. Changing a subscript changes the substance represented.
Conserve atoms when balancing
Chemical reactions rearrange atoms rather than create or destroy them. Count each element on both sides of the equation, then adjust the coefficients in front of formulae. Coefficients multiply the entire formula. Conservation of atoms explains conservation of mass in a closed system, although a gas leaving an open container can make the measured contents lighter.
Worked example
Balance H₂ + O₂ → H₂O.
Show the worked solution
- Oxygen has two atoms on the left but one on the right; place 2 before H₂O.
- The right side now has four hydrogen atoms; place 2 before H₂.
- Check that each side contains four H atoms and two O atoms.
2H₂ + O₂ → 2H₂O.
Try it yourself
How many oxygen atoms are represented by 3CO₂?
Common mistakes
- Never change a chemical subscript merely to balance an equation.
What can you explain now?
Balance Mg + O₂ → MgO without changing the formula of magnesium oxide.
Compare with the explanation
2Mg + O₂ → 2MgO.
Put 2 before MgO to conserve the two oxygen atoms, then put 2 before Mg to conserve magnesium.
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