The key to understanding chemical reactions is the octet rule. It states that every atom wishes to achieve a full valence shell, or in layman's terms, wishes to fill its outermost shell with electrons. There is little to remember, as the first shell takes at most two electrons (goal for H and He) and all subsequent shells take eight.

• Helium He has two valence electrons. It has achieved the octet state without losing or gaining electrons.
• Sodium Na has a single valence electron. It has two alternatives: lose one or gain seven electrons. It chooses the easier and wishes to lose one electron.
• Calcium Ca has two valence electrons. It has two alternatives: lose two or gain six electrons. It chooses the easier and wishes to lose two electrons.
• Oxygen O has six valence electrons. It has two alternatives: lose six or gain two electrons. It chooses the easier and wishes to gain two electrons.

From this matchmaking example, we have found a possible reaction! A calcium atom wishes to lose two electrons, and an oxygen atom wishes to gain as many. Therefore, they form an ionic bond and react to form calcium oxide CaO in a bright flash. In other words, Ca + O → CaO.