Formation of Ions

An atom has the same number of protons (+) as electrons (−), so it has no overall charge. An ion is what you get when an atom loses or gains electrons — this leaves an unequal number of protons and electrons, so the particle becomes charged.

Why do atoms do this? Atoms are most stable when they have a full outer shell of electrons — the same arrangement as the nearest noble gas (Group 0). By losing or gaining a few electrons, an atom can reach this stable noble-gas configuration.

• If an atom loses electrons it has more protons than electrons → it becomes positive (+). A positive ion is called a cation.
• If an atom gains electrons it has more electrons than protons → it becomes negative (−). A negative ion is called an anion.

Important: only the electrons move — the nucleus (protons and neutrons) never changes. The number of protons stays the same, so it is still the same element.

The size of the charge tells you how many electrons were transferred:

• lose 1 electron → 1+    lose 2 → 2+    lose 3 → 3+
• gain 1 electron → 1−    gain 2 → 2−    gain 3 → 3−

Metals are on the left of the periodic table (Groups 1, 2 and 3). They have only a few electrons in their outer shell, so the easiest way to get a full outer shell is to lose those outer electrons. When the outer shell empties, the next shell down (which is already full) becomes the new outer shell.

Group	Atom	Outer e⁻	Loses	Ion formed
1	Na (2,8,1)	1	1 e⁻	Na⁺ (2,8)
1	K (2,8,8,1)	1	1 e⁻	K⁺
2	Mg (2,8,2)	2	2 e⁻	Mg²⁺ (2,8)
2	Ca (2,8,8,2)	2	2 e⁻	Ca²⁺
3	Al (2,8,3)	3	3 e⁻	Al³⁺ (2,8)

You can show electron loss as a simple equation, writing the lost electrons on the right of the arrow:

• Sodium:   Na → Na⁺ + e⁻
• Magnesium:   Mg → Mg²⁺ + 2e⁻
• Aluminium:   Al → Al³⁺ + 3e⁻

Quick rule for metals. The charge equals the group number: Group 1 → 1+, Group 2 → 2+, Group 3 → 3+.

Non-metals are on the right of the periodic table (Groups 5, 6 and 7). They have an outer shell that is nearly full, so the easiest way to reach a full outer shell of 8 is to gain a few electrons.

Group	Atom	Outer e⁻	Gains	Ion formed
5	N (2,5)	5	3 e⁻	N³⁻ (2,8)
6	O (2,6)	6	2 e⁻	O²⁻ (2,8)
6	S (2,8,6)	6	2 e⁻	S²⁻ (2,8,8)
7	F (2,7)	7	1 e⁻	F⁻ (2,8)
7	Cl (2,8,7)	7	1 e⁻	Cl⁻ (2,8,8)

You can show electron gain as a simple equation, writing the gained electrons on the left of the arrow:

• Chlorine:   Cl + e⁻ → Cl⁻
• Oxygen:   O + 2e⁻ → O²⁻
• Nitrogen:   N + 3e⁻ → N³⁻

Notice the name of a non-metal ion usually ends in -ide: chloride (Cl⁻), oxide (O²⁻), nitride (N³⁻), sulfide (S²⁻).

Quick rule for non-metals. The charge magnitude = (8 − group number) and the sign is negative: Group 7 → 8 − 7 = 1−; Group 6 → 8 − 6 = 2−; Group 5 → 8 − 5 = 3−.

You must be able to recall the charge of an ion just from its group number. Every element in the same group forms an ion with the same charge, because they all have the same number of outer electrons.

Group	Outer electrons	Lose or gain?	Ion charge	Example
1	1	lose 1	1+	Na⁺, K⁺, Li⁺
2	2	lose 2	2+	Mg²⁺, Ca²⁺
3	3	lose 3	3+	Al³⁺
5	5	gain 3	3−	N³⁻
6	6	gain 2	2−	O²⁻, S²⁻
7	7	gain 1	1−	Cl⁻, F⁻, Br⁻
0	8 (full)	neither	no ion	Ne, Ar

Notice the pattern: the charges go 1+, 2+, 3+ ... then 3−, 2−, 1−. Group 4 is the turning point — its elements (like carbon) usually share electrons (covalent bonding) rather than form ions. Group 0 noble gases already have full outer shells, so they form no ions at all.

A common exam task is to work out the number of protons and electrons in an ion. The key rule:

• Protons stay the same as the atom — the nucleus is untouched, so the ion is still the same element with the same atomic number.
• Electrons change by the size of the charge:
  • a positive ion has fewer electrons than the atom (it lost some);
  • a negative ion has more electrons than the atom (it gained some).

Worked logic.

• Na⁺: sodium has atomic number 11, so 11 protons. The atom has 11 electrons; the 1+ ion has lost 1, so 10 electrons. (11 protons − 10 electrons = +1.)
• O²⁻: oxygen has atomic number 8, so 8 protons. The atom has 8 electrons; the 2− ion gained 2, so 10 electrons. (8 protons − 10 electrons = −2.)
• Al³⁺: aluminium has atomic number 13, so 13 protons. The 3+ ion has lost 3 electrons → 10 electrons.

Notice Na⁺, O²⁻ and Al³⁺ all end up with 10 electrons — the same as neon. That is the whole point: forming an ion gives the particle a noble-gas electron configuration.