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Ionic bonding

Sodium is a soft metal that reacts violently with water; chlorine is a poisonous gas. Together they make the salt on your table. The change comes from one electron moving from one atom to another.

Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

NaClsodium atom (2,8,1)chlorine atom (2,8,7)Na+Cl−sodium ion (2,8)chloride ion (2,8,8)sodium electronchlorine electron
Figure 1. Sodium transfers one electron to chlorine, forming Na⁺ and Cl⁻ ions.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding ionic bonding
  5. Formulae and structure
  6. Ionic compounds in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • explain how metal and non-metal atoms form ions by transferring electrons
  • predict the charges on ions from their position in the periodic table
  • write names and formulae of ionic compounds, including compound ions
  • draw and interpret dot-and-cross and lattice diagrams
  • explain the melting points, conductivity and brittleness of ionic compounds
  • evaluate uses of ionic compounds and their impacts

Before you start

You will use these skills. If any feel shaky, review them first.

  • atomic structure: protons, neutrons, electrons and electron shells
  • groups and periods of the periodic table; metals and non-metals
  • the particle model and states of matter (see States of matter)

Key vocabulary

Ion
An atom or group of atoms with a charge, because electrons have been lost or gained.
Cation / anion
A positive ion (usually a metal) / a negative ion (usually a non-metal).
Ionic bond
The strong electrostatic attraction between oppositely charged ions.
Giant ionic lattice
A regular, repeating three-dimensional arrangement of oppositely charged ions.
Dot-and-cross diagram
A diagram showing electrons in shells, using dots and crosses to show which atom each electron came from.
Compound ion
A charged group of atoms, such as nitrate (NO₃⁻), sulfate (SO₄²⁻) or hydroxide (OH⁻).

Understanding ionic bonding

  1. What is it?

    When a metal reacts with a non-metal, electrons move from the metal atoms to the non-metal atoms. The metal becomes a positive ion and the non-metal a negative ion, and the oppositely charged ions attract strongly to form a giant lattice.

  2. Why does it happen?

    Metal atoms have few outer electrons, which are relatively easy to remove; non-metal atoms release energy when they gain electrons. Most importantly, a large amount of energy is released when the oppositely charged ions come together in the lattice, so forming an ionic compound is strongly exothermic. The ions usually end up with the same electron arrangement as a noble gas.

  3. How do we know?

    Ionic compounds conduct electricity when molten or dissolved, and electrolysis shows that positive and negative particles move to opposite electrodes. X-ray crystallography, first used on sodium chloride by William and Lawrence Bragg in 1913, revealed its regular lattice with no separate molecules.

  4. Why does it matter?

    Ionic compounds are everywhere: table salt, the minerals in rocks and bones, fertilisers, and the electrolytes in our blood that carry nerve signals. Their properties decide how they are used, from de-icing roads to extracting metals.

  5. What does it connect to?

    Ionic bonding links to covalent and metallic bonding, electrolysis, reactivity and the periodic table in chemistry; to nerve impulses and minerals in biology; and to electrostatic forces in physics.

Formulae and structure

Charges from the periodic table: group 1 forms 1+, group 2 forms 2+, group 3 forms 3+; group 6 forms 2−, group 7 forms 1−.

Writing formulae: balance the charges so the total is zero. Mg²⁺ and Cl⁻ give MgCl₂; Al³⁺ and O²⁻ give Al₂O₃. Use brackets for more than one compound ion, as in Ca(NO₃)₂.

−+−+−++−+−+−−+−+−++−+−+−chloride ion, Cl⁻sodium ion, Na⁺
Figure 2. Part of the sodium chloride lattice: every ion is surrounded by ions of the opposite charge.
  • High melting points: many strong attractions in all directions must be overcome.
  • Conduct only when molten or dissolved: the ions must be free to move.
  • Hard but brittle: shifting a layer brings like charges together, and they repel.
  • Often soluble in water: water molecules can surround and separate the ions.

Ionic compounds in the real world

Doctors give oral rehydration solutions containing sodium and potassium salts to replace ions lost through diarrhoea, a treatment that has saved millions of lives. Farmers spread ionic fertilisers such as ammonium nitrate and potassium chloride. Toothpastes contain sodium fluoride, which helps harden tooth enamel.

Worked examples

Example 1: writing a formula

Work out the formula of calcium fluoride.

  1. Calcium is in group 2: Ca²⁺. Fluorine is in group 7: F⁻.
  2. Two F⁻ ions (2−) balance one Ca²⁺ (2+).
  3. Formula: CaF₂.

Example 2: explaining a property

Explain why potassium chloride can conduct electricity in solution but not as a solid.

  1. Conduction needs charged particles that can move.
  2. In the solid, K⁺ and Cl⁻ ions are held in fixed positions in the lattice.
  3. In solution, the ions separate and move freely, carrying the current.

In the eAssessment

Bonding questions test whether you can move between diagrams, formulae and properties. Expect:

  • Interpret diagrams: dot-and-cross diagrams (count electrons, identify charges) and lattice diagrams.
  • Deduce: ion charges from group numbers, and formulae from charges, including unfamiliar elements labelled X and Y.
  • Explain properties from structure, always naming the particles (ions) and the forces (electrostatic attraction).
  • Analyse data: melting points or conductivity, linking trends to ion charge and size.

Common ways to lose marks: describing sodium chloride as molecules; saying ions form by gaining or losing protons; saying electrons carry current in solutions; and explaining melting points with mass instead of the strength of attraction.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: which substances behave like ionic compounds?

Partially guided investigation · about 50 minutes · pairs

Research question
Which of four substances (sodium chloride, potassium iodide, sugar and paraffin wax) show the properties expected of ionic compounds: high melting point, solubility in water, and conduction only when dissolved?
Scientific background
Ionic compounds are giant lattices of oppositely charged ions. Their properties come from strong attractions between ions and whether the ions are free to move.
Hypothesis
Write your own prediction for each substance, with a justification.
Variables
Identify what you will keep the same in each test so the comparison is fair, and explain why.
Apparatus
Four solids, distilled water, beakers, spatula, balance, low-voltage supply with ammeter and two carbon electrodes, test tubes, test-tube holder, Bunsen burner or hot plate.
Method
  1. Test each solid for conductivity by touching the electrodes to a small pile of it.
  2. Dissolve 2 g of each in 50 cm³ of water, noting whether it dissolves, and test the conductivity again.
  3. Gently warm a small amount of each in a test tube for up to two minutes and record whether it melts.
  4. Record all results in a table and classify each substance.

Safety. Wear eye protection. Heat only small amounts, with the tube pointed away from everyone; wax and sugar can catch fire or char, so heat gently and stop if they smoke. Use a low-voltage supply only. Never taste chemicals.

Then evaluate: which test gave the clearest evidence, and why?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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