Metals and non-metals; their positions in the periodic table; the ions they form; the link between electronic structure and reactivity.

What this dot point is asking

AQA wants you to explain where metals and non-metals sit in the periodic table, describe the typical properties of each, state the type of ions they form, and use electronic structure to explain why metals tend to lose electrons and non-metals tend to gain electrons. The unifying idea is that atoms react to reach a full, stable outer shell, and whether they lose or gain depends on how many outer electrons they start with.

Where they sit

Most elements are metals, on the left and centre of the table. Non-metals are on the right-hand side, including the noble gases in Group 0. The boundary runs roughly diagonally down the right side. Metals are typically shiny, good conductors of heat and electricity, malleable and have high melting points; non-metals are typically dull, poor conductors and often have low melting points.

The ions they form

When metals react, they lose their outer electrons to become positive ions. For example, sodium ($2,8,1$) loses one electron to form $Na^+$ ($2,8$), and magnesium ($2,8,2$) loses two to form $Mg^{2+}$. Non-metals gain electrons to fill their outer shell, for example chlorine ($2,8,7$) gains one to form $Cl^-$ ($2,8,8$), and oxygen ($2,6$) gains two to form $O^{2-}$.

The charge on the ion equals the number of electrons lost (positive) or gained (negative), which is why Group 1 forms $+1$ ions, Group 2 forms $+2$, Group 6 forms $-2$ and Group 7 forms $-1$.

Linking to reactivity

The number of outer electrons decides how an element reacts. Group 1 metals (one outer electron) lose it readily and are reactive; Group 7 non-metals (seven outer electrons) gain one readily and are reactive. The noble gases already have full outer shells, so they are unreactive and do not normally form ions.

Why metals lose and non-metals gain

The reason comes down to how many electrons each atom must move to reach a full shell. A metal has only a few outer electrons, so the easiest route to a full outer shell is to lose those few; a non-metal has nearly a full outer shell, so the easiest route is to gain the small number of electrons it is missing. Losing electrons leaves the metal ion with more protons than electrons, hence a positive charge; gaining electrons gives the non-metal ion more electrons than protons, hence a negative charge. This is why metals and non-metals always form oppositely charged ions, which then attract to form ionic compounds. The size of the charge follows the same logic: a Group 2 metal loses two electrons to form a $2+$ ion, and a Group 6 non-metal gains two to form a $2-$ ion.

Try this

Q1. State the type of ion a metal forms and explain why. [2 marks]

• Cue. A positive ion, because it loses its outer electrons.

Q2. Write the ion formed by oxygen ($2,6$) and explain how it forms. [2 marks]

• Cue. $O^{2-}$, formed by gaining two electrons to complete its outer shell.

Q3. Write the ion formed by potassium ($2,8,8,1$). [1 mark]

• Cue. $K^+$, formed by losing its single outer electron.