Skip to notes
wise otter.Chemistry
Contents
Bonding and Structure

Topic 2 of 5

Ionic bonding and properties

Follow electron transfer, build the idea of a lattice, and explain when ions can carry charge.

O-Level 2026 SEC G3 2027

Ionic bonding: transfer, then attraction

Electron transfer creates ions. Attraction between opposite charges holds them together.

A metal atom usually loses outer-shell electrons to form a positive ion. A non-metal atom usually gains electrons to form a negative ion. The resulting ions commonly have the electron arrangement of a noble gas: a full outer shell. The nucleus does not change.

Sodium chloride

Na (2,8,1) loses one electron. Cl (2,8,7) gains that electron.

Sodium chloride: ions after electron transferA sodium ion with charge plus one and a chloride ion with charge minus one. Each has a full outer shell of eight electrons. The chloride has seven dots from chlorine and one cross transferred from sodium. Sodium's remaining outer shell has eight crosses, which were previously in an inner shell.Na+Cl-
One Na+ for each Cl-. The total charge is zero.

Magnesium chloride

Mg (2,8,2) loses two electrons. Two Cl atoms each gain one.

Magnesium chloride: ions after electron transferOne magnesium ion with charge plus two and two chloride ions, each with charge minus one. All three ions have full outer octets. Each chloride shows seven dots from chlorine and one cross transferred from magnesium. Magnesium's eight remaining outer-shell electrons were previously in an inner shell.Mg2+Cl-Cl-
One Mg2+ for every two Cl- ions: +2 - 1 - 1 = 0.
  • Electron from chlorine
  • Electron from the metal

Each bracket shows the ion's full outer shell. The eight electrons on Na+ and Mg2+ were previously in an inner shell. Dots and crosses track origin; all electrons are the same kind of particle.

These groups show the ion ratio, not separate NaCl or MgCl2 molecules. The ions form an extended lattice.

NaCl: one sodium electron transfers to chlorine. MgCl2: magnesium transfers one electron to each of two chlorine atoms. Show brackets and charges on the resulting ions.
Follow the electrons and the charge
Starting atomElectron changeResulting ion
Na: 2,8,1Loses one electronNa+: 2,8
Mg: 2,8,2Loses two electronsMg2+: 2,8
Cl: 2,8,7Gains one electronCl-: 2,8,8

An ionic bond is the strong electrostatic attraction between oppositely charged ions. Na+ and Cl- balance in a 1:1 ratio: NaCl. One Mg2+ needs two Cl- ions: MgCl2. The compound has no overall charge.

Why salt melts high but conducts only when ions move

The attractions explain melting. The mobility of charged particles explains conductivity.

Solid sodium chloride has a giant ionic lattice: a repeating arrangement of positive and negative ions extending throughout the crystal. Each ion attracts surrounding ions of the opposite charge. There are no separate NaCl molecules; its formula gives the simplest ion ratio.

Solid: ions fixed in place

Sodium chloride solid: can the ions carry charge?Eight alternating sodium and chloride ions occupy fixed positions in a regular two-dimensional slice. Positive and negative electrodes show an applied voltage, but no ion movement is drawn. The real sodium chloride lattice extends in three dimensions; each ion has six nearest neighbours of opposite charge.+-Na+Cl-Na+Cl-Cl-Na+Cl-Na+
The ions cannot move through the solid, so they cannot carry charge through it.

Molten: mobile ions

Sodium chloride molten: can the ions carry charge?Four sodium ions and four chloride ions have a disordered liquid arrangement. Arrows on two sodium ions point towards the negative electrode on the right. Arrows on two chloride ions point towards the positive electrode on the left. Sodium chloride remains ionic on melting; it does not become separate neutral atoms or molecules.+-Na+Cl-Na+Cl-Cl-Na+Cl-Na+
The lattice is disrupted. Mobile ions carry charge through the liquid.

Dissolved in water: mobile ions

Sodium chloride dissolved in water: can the ions carry charge?Four sodium ions and four chloride ions are separated in a pale blue region representing water. Sodium ions move towards the negative electrode on the right and chloride ions towards the positive electrode on the left when a voltage is applied. Water molecules and hydration shells are omitted. Electrode products are not shown.+-Cl-Na+Cl-Na+Na+Cl-Na+Cl-Water
Water separates ions from the lattice. The dissolved ions can carry charge.

Conductivity needs charged particles that can move. In molten and aqueous sodium chloride, those particles are ions.

The signed bars are electrodes under an applied voltage. Arrows show ion movement, not bond formation. These are simplified slices, not to scale; water molecules and electrode products are omitted.

Solid: ions vibrate about fixed positions. Molten: ions can move. Aqueous: dissolved ions can move through water. The lattice view is a simplified slice of a three-dimensional crystal.
Link the property to its cause
PropertyExplanation
High melting and boiling pointsStrong electrostatic attractions act throughout the lattice; much energy is needed to overcome them.
Does not conduct when solidThe ions are charged but cannot move through the solid to carry charge.
Conducts when molten or dissolvedMobile ions carry charge through the liquid or solution.
Check your understandingWhy is "solid salt has no charged particles" an incorrect explanation for its lack of conductivity?Think it through, then reveal the answer
Solid salt contains charged ions. They are held in the lattice and cannot move through it. Conductivity requires mobile charged particles.