Ionic bonding: forming ions and balancing charges

How can neutral atoms form a compound containing charged particles? Follow the electrons to identify the ions, then name the electrostatic attraction that holds oppositely charged ions together. Electron transfer forms the ions; that attraction is the ionic bond.

Understand ionic bonding

Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions (cations and anions).

Key ideas

• Metals form positive ions by losing electrons.
• Non-metals form negative ions by gaining electrons.
• Ionic bonds form after electron transfer between a metal and a non-metal.
• Ionic compounds form a giant ionic lattice (not molecules).

Form ions, then describe the attraction

Form ions using electron arrangements

The common ions considered here usually have a noble-gas electron arrangement. Losing or gaining electrons changes the charge, while the number of protons and the element’s identity stay the same. A full outer shell helps you predict common charges; it does not mean an isolated atom spontaneously gives away electrons.

• Cations (positive ions): metal atoms lose valence electrons.
  • Example: lithium (2.1) loses 1 electron → Li⁺ (2).
  • Example: aluminium (2.8.3) loses 3 electrons → Al³⁺ (2.8).
• Anions (negative ions): non-metal atoms gain electrons.
  • Example: oxygen (2.6) gains 2 electrons → O²⁻ (2.8).
  • Example: chlorine (2.8.7) gains 1 electron → Cl⁻ (2.8.8).

Use Electron arrangements to practise counting shell electrons.

Distinguish transfer from attraction

Ionic bonds are formed between metal ions and non-metal ions after electrons are transferred from the metal to the non-metal.

The attraction acts between opposite charges. It does not depend on which individual atom supplied an electron. In a solid, each ion attracts several oppositely charged neighbours.

Example: Sodium Chloride (NaCl)

1. Sodium (2.8.1) transfers 1 valence electron to a chlorine atom (2.8.7).
2. Na⁺ and Cl⁻ ions are formed.
3. The oppositely charged ions attract each other strongly to form an ionic bond.

From ions to a lattice

Ions attract many neighbouring ions in a giant ionic lattice, rather than forming separate NaCl molecules. A formula such as NaCl states the simplest ion ratio. It does not name a molecule or a unique pair of bonded ions.

The next lesson, Ionic structures and properties, develops the lattice model and explains melting and electrical conductivity.

Track transferred electrons

The syllabus can test ionic bonding using dot-and-cross diagrams.

• Use dots and crosses to track the atoms from which the electrons came. Either symbol convention is valid if you use it consistently; the electrons themselves are identical.
• Show the ions in square brackets with the charge outside.
• Account for every transferred electron. For MgCl₂, the two electrons lost by magnesium supply one electron to each of two chlorine atoms.
• These diagrams omit the metal ion’s filled inner shells; Na⁺ and Mg²⁺ each have the electron arrangement 2.8.

Misconceptions to check

• Defining the bond as electron transfer. Transfer forms ions; ionic bonding is electrostatic attraction between oppositely charged ions.
• Writing “molecules of NaCl”. Use giant ionic lattice / formula unit.
• Forgetting charge balance when writing formulas (e.g., writing MgCl instead of MgCl₂).

Write formulas from charges

Worked examples

For the solid-versus-molten worked explanation, continue to Ionic structures and properties.

Try these independently

Compare the same compound in different states in Ionic structures and properties.

Practise and check