Elements, compounds, mixtures

  • An element has only one type of atom: Fe, O2, S8. About 100 are known, each with a symbol in the periodic table.
  • A compound has two or more elements chemically combined in fixed proportions: H2O, NaCl. Only a chemical reaction can split it up.
  • A mixture has two or more substances not chemically joined: air, sea water. Physical methods separate it.

Reading a formula

The small number after a symbol counts the atoms just before it. A number after a bracket multiplies everything inside.

  1. Ca(OH)₂: 1 calcium, 2 oxygen, 2 hydrogen
  2. Al₂(SO₄)₃: 2 aluminium, 3 sulfur, 12 oxygen
  3. Total atoms in Al₂(SO₄)₃ = 2 + 3 + 12 = 17

Balancing equations

Atoms are not created or destroyed in a reaction, so each side of a symbol equation has the same number of each type of atom. Balance by changing the big numbers in front, never the formulae.

  1. H₂ + O₂ → H₂O: 2 O on the left, 1 on the right
  2. H₂ + O₂ → 2H₂O: now 4 H on the right
  3. 2H₂ + O₂ → 2H₂O: balanced (4 H and 2 O on each side)

Changing H₂O to H₂O₂ would "balance" it, but that is a different substance. Only the numbers in front can change.

Count the atoms

Open the Molar Mass Calculator and type Al2(SO4)3. It lists how many atoms of each element are in the formula: check it against the steps above.

Open the Molar Mass Calculator in a new tab