Ch 2 · Ionic bonding · AS & A Level Chemistry 9701
Topic 2 of 7 in Chemical bonding — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Ionic bonding explains why compounds such as sodium chloride form hard, high-melting solids that conduct electricity only when molten or dissolved. Cambridge questions expect you to describe electron transfer for named examples, draw the resulting lattice, and explain — using ionic charge and radius — why some ionic compounds bind more strongly than others.
Key ideas — Defining ionic bonding
Ionic bonding is the electrostatic attraction between oppositely charged ions. It forms when one or more electrons transfer from an atom of low electronegativity (typically a metal) to an atom of high electronegativity (typically a non-metal), producing a positive cation and a negative anion that attract each other strongly.
🧑🏫 Teacher's explanation
Imagine two powerful magnets with opposite poles. If you bring them close, they snap together with a strong force. Ionic bonding is essentially that, but instead of magnetism, it is the pull between positive and negative electrical charges. When a metal atom gives up an electron to a non-metal, they become ions. Because they are now oppositely charged, they are irresistibly drawn to one another.
📖 Definitions
🔍 Worked example — Easy
Calculate the formula of the ionic compound formed between Magnesium (Mg^2+) and Chlorine (Cl^-).
🔍 Worked example — Standard
Explain why the lattice energy of Magnesium Oxide (MgO) is higher than that of Sodium Chloride (NaCl).
🔍 Worked example — Exam-style
Calculate the mass of Calcium Fluoride (CaF_2) in 0.50 moles.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Ionic bonding in sodium chloride, magnesium oxide and calcium fluoride
In sodium chloride, each sodium atom, Na\ (2,8,1), loses its single outer electron to a chlorine atom, Cl\ (2,8,7), forming Na^+\ (2,8) and Cl^-\ (2,8,8), both with stable noble gas electron configurations.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where people are wearing bright red or bright blue vests. If a person in a red vest moves to a group of people in blue vests, they become 'positive' (losing something) and the blue group becomes 'negative' (gaining something). Because they are different colors, they are drawn together by a strong magnetic-like force. In chemistry, this 'magnetic force' is the electrostatic attraction between positive and negative ions.
📖 Definitions
🔍 Worked example — Easy
Calculate the total charge of a unit cell in magnesium oxide (MgO) if it contains one Mg^2+ ion and one O^2- ion.
🔍 Worked example — Standard
Predict whether MgO or NaCl has a higher melting point based on ionic charge, assuming ionic radii are similar.
🔍 Worked example — Exam-style
A sample of calcium fluoride (CaF_2) contains 0.50 moles of Ca^2+ ions. Calculate the number of moles of F^- ions present.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — The giant ionic lattice
In the solid state, ions do not exist as isolated pairs. Each cation is surrounded by several anions, and each anion by several cations, packed in a regular, repeating three-dimensional arrangement called a giant ionic lattice. Every ion is held in place by electrostatic attraction to all of its oppositely charged neighbours, not just one, so the lattice as a whole is a single giant structure rather than a collection of discrete ion pairs or molecules. This is why ionic solids have high melting and boiling points: melting requires breaking a very large number of strong electrostatic attractions throughout the lattice, not just one bond.
🧑🏫 Teacher's explanation
Imagine a massive, perfectly organized crowd of people in a dance hall. Instead of people dancing in pairs, every person is holding hands with several people around them—some to their left, some to their right, some in front, and some behind. If you try to pull one person away, you can't just break one hand-hold; you have to break dozens of connections simultaneously to move them. This is exactly how a giant ionic lattice works.
📖 Definitions
🔍 Worked example — Easy
Explain why sodium chloride, NaCl, has a higher melting point than a hypothetical molecule of NaCl consisting of a single Na^+ and Cl^- pair.
🔍 Worked example — Standard
Predict whether magnesium oxide (MgO) or sodium chloride (NaCl) has a higher melting point based on ionic charge.
🔍 Worked example — Exam-style
Compare the lattice energy (strength of bonding) of CaF_2 and MgO. (Assume ionic radii are similar).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Effect of ionic charge and ionic radius on bonding strength
The strength of the electrostatic attraction between ions, and hence the strength of the ionic bonding, depends on two factors:
🧑🏫 Teacher's explanation
Imagine two magnets. If you have two very strong magnets, they pull on each other with a lot of force. If you have two weak magnets, the pull is much lighter. In chemistry, the 'strength' of the magnet is the charge on the ion. A 2+ ion pulls much harder than a 1+ ion.
📖 Definitions
🔍 Worked example — Easy
Compare the relative strength of the ionic bonding in NaCl and MgO.
🔍 Worked example — Standard
Predict whether CaO or MgO has a higher melting point based on ionic radius.
🔍 Worked example — Exam-style
Calculate the ratio of the force between ions if the distance between them is halved, assuming charge remains constant.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. The melting points of three ionic oxides are: Na_2O, 1132 °C; MgO, 2852 °C; CaO, 2613 °C. Explain, in terms of ionic charge and ionic radius, why MgO has a higher melting point than both Na_2O and CaO. [4]
🔍 Worked example — Easy
Calculate the formula of the ionic compound formed between Al^3+ and O^2- ions.
🔍 Worked example — Standard
Compare the lattice energy (strength of bonding) of NaCl and MgO.
🔍 Worked example — Exam-style
Explain why LiF has a higher melting point than CsI.
⚠️ Common errors
Common mistakes
Practice
Answers