Why Elements React &
Ionic Bonding

Atoms have electrons arranged in shells (energy levels). The outermost shell is called the highest energy level. An atom has no overall charge because it has equal numbers of protons and electrons. Elements in Group 8 (the noble gases — also labelled Group 18 on modern periodic tables) already have a full outermost shell, so they are stable and do not react. All other elements react to try to get a full outer shell too — either by losing or gaining electrons (ionic bonding) or by sharing electrons (covalent bonding, which you'll meet later).

Metal atoms (Groups 1 & 2) lose electrons from their outer shell to form positive ions — e.g. sodium (2,8,1) loses 1 electron to become Na⁺ (2,8). Non-metal atoms (e.g. Group 7) gain electrons to fill their outer shell, forming negative ions — e.g. chlorine (2,8,7) gains 1 electron to become Cl⁻ (2,8,8). An ion is a charged particle formed when an atom loses or gains electrons.

Ionic bonding is the strong electrostatic attraction between oppositely charged ions, e.g. Na⁺ and Cl⁻ in sodium chloride, NaCl. In any ionic compound, the total positive charge must balance the total negative charge. Magnesium (Mg²⁺) needs two Cl⁻ ions to balance its +2 charge, forming MgCl₂.

Group 1 (lithium, sodium, potassium): reactivity increases down the group. The outer electron is further from the nucleus in bigger atoms, so it is held less strongly by the protons and is easier to lose. Group 7 (fluorine, chlorine): reactivity decreases down the group. The electron being gained enters a shell closer to the nucleus in smaller atoms, so it is attracted more strongly.