Exercise 3 solution

Word equation: magnesium + oxygen -> magnesium oxide

Formula: magnesium is in group 2, so it forms Mg2+; oxygen forms O2-. One of each
balances the charge, so the formula is MgO. (Ion charges from the Chapter 4 group
rule; the formula MgO is my application, check.) Oxygen gas is the molecule O2 (Chapter 5).

Unbalanced: Mg(s) + O2(g) -> MgO(s)
  left Mg = 1, O = 2; right Mg = 1, O = 1. Oxygen is out.
Fix oxygen: Mg(s) + O2(g) -> 2MgO(s). Now O = 2 / 2 but Mg = 1 / 2.
Fix magnesium: 2Mg(s) + O2(g) -> 2MgO(s).

Atom-count table (run in Python):
         left   right
  Mg      2      2
  O       2      2

State symbols (s) and (g) follow the notation in the Chemical equation article. That
magnesium oxide is a solid is given in the exercise.

Not told by the equation: how fast the burning goes, whether it will start on its own,
or what conditions or heat it needs. Also not shown: how much mass of each
substance is involved (a coefficient counts atoms and molecules, not grams).
Burning in oxygen also makes this a combustion as well as a synthesis (the type labels
are my classification (check)).