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K326 / K328 / 2027
Bonding and Structure

Lesson 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

  • I can form ions by electron transfer.Syllabus

    Syllabus K326 / K328, 3.1(a). Form ions by electron transfer

  • I can represent ionic bond formation.Syllabus

    Syllabus K326 / K328, 3.1(b). Represent ionic bond formation

  • I can explain ionic physical properties.Syllabus

    Syllabus K326 / K328, 3.1(c). Explain ionic physical properties

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.

Check your understanding

Which ion has the same electron arrangement as a neon atom, 2.8?

  1. S2-
  2. Al3+
  3. K+
  4. Cl-
Show the answer

Al3+

Aluminium (2.8.3) loses its three outer electrons, leaving 2.8, like neon.

Worked exampleDot-and-cross diagram for sodium oxide

Draw a dot-and-cross diagram for sodium oxide, Na2O. Show outer electrons only. Then explain why the formula has two sodium ions.

Try it first. Then show the worked stepsHide the worked steps
  1. Each sodium atom (2,8,1) gives its one outer electron to the oxygen atom (2,6). Oxygen needs two electrons to fill its outer shell, so it takes one from each of two sodium atoms.
  2. Each Na+ ion is now 2,8; its emptied third shell is not drawn. The O2- ion has 8 outer electrons: 6 of its own (dots) and 2 from sodium (crosses). Use square brackets and write the charge on every ion. Two Na+ charges balance one O2- charge, so the formula is Na2O.
Answer

Each Na gives one electron, and O needs two, so two Na+ balance one O2-.

Check your understanding

Why does sodium oxide have the formula Na2O?

  1. Each Na gives one electron, and O needs two, so two Na+ balance one O2-.
  2. Na gives two electrons, and O needs one, so two Na balance the extra charge.
  3. Two Na atoms share a pair of electrons with O, forming two covalent bonds.
Show the answer

Each Na gives one electron, and O needs two, so two Na+ balance one O2-.

Sodium (2.8.1) loses one electron and oxygen (2.6) gains two. Two Na+ charges balance one O2- charge, so the formula is Na2O.

Worked exampleDot-and-cross diagram for the hydride ion

Magnesium hydride, MgH2, is an ionic compound that contains the hydride ion, H-. Draw a dot-and-cross diagram for H-, and explain why this ion is stable.

Try it first. Then show the worked stepsHide the worked steps
  1. A hydrogen atom has one electron. It gains one electron from magnesium to form H-.
  2. The first shell holds only two electrons, so H- has a full shell, the same arrangement as helium. That is why hydrogen can form a negative ion with a metal as reactive as magnesium. Each Mg atom gives one electron to each of two H atoms, so the formula is MgH2.
Answer

Its first shell holds two electrons, so H- has a full shell like helium.

Check your understanding

Magnesium hydride, MgH2, contains the hydride ion, H-. Why is H- stable?

  1. Its first shell holds two electrons, so H- has a full shell like helium.
  2. It has lost its single electron, so it has an empty shell like a bare proton.
  3. It has eight outer electrons, so it has the arrangement of a noble gas.
Show the answer

Its first shell holds two electrons, so H- has a full shell like helium.

Hydrogen has one electron and gains one from magnesium. The first shell holds only two, so H- has the stable arrangement of helium. Each Mg gives one electron to each of two H atoms: MgH2.

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.

Watch out for this

A student says: "Salt disappears in water because it melts, and every ionic compound dissolves like this." What is wrong with this?

Show the answer

Melting uses heat to turn the compound into a liquid. Dissolving separates its ions into a solvent. Many ionic compounds dissolve in water, but some are insoluble; ionic bonding alone does not guarantee solubility.

Check your understanding

"Solid salt has no charged particles, so it does not conduct." What is wrong with this explanation?

  1. Solid salt has charged particles, but they are electrons, not ions.
  2. Nothing; the ions form when salt is melted or dissolved, not before.
  3. Solid salt has ions, but they are fixed in the lattice and cannot move.
Show the answer

Solid salt has ions, but they are fixed in the lattice and cannot move.

Conduction needs charged particles that can move. Solid NaCl contains Na+ and Cl- ions, but they cannot move until it is melted or dissolved.