Ionic Bonding and Properties of Ionic Compounds
Ionic bonding at IGCSE goes beyond simple 1:1 compounds such as sodium chloride: it covers working out and writing the formulae of ionic compounds containing ions of different charges, including common polyatomic ions such as sulfate, carbonate, nitrate and hydroxide, explaining why giant ionic lattices are brittle rather than simply hard, and constructing dot-and-cross diagrams for compounds where more than one ion of one type is needed to balance the charges, such as magnesium chloride or aluminium oxide.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the charges of common ions from their group number (Group 1 ions are 1+, Group 2 ions are 2+, Group 3 ions are 3+, Group 6 ions are 2-, Group 7 ions are 1-), and learn the charges and formulae of common polyatomic ions by heart: hydroxide OH-, nitrate NO3-, carbonate CO3^2-, sulfate SO4^2-, and ammonium NH4+.
- To construct a formula, use the criss-cross method: swap the numerical value of each ion's charge (ignoring the sign) to become the subscript of the other ion, then simplify the ratio to its lowest whole-number terms - for example Mg2+ and Cl- gives MgCl2, and Al3+ and O2- gives Al2O3.
- When a polyatomic ion needs more than one unit in a formula, put brackets around it and write the multiplying subscript outside the brackets - for example calcium hydroxide is Ca(OH)2, not CaOH2.
- In a dot-and-cross diagram, draw only the outer-shell electrons, use dots for the electrons from one atom and crosses for the electrons from the other so it is clear where each electron came from, show each ion's charge in square brackets, and remember that for a compound like MgCl2 you must draw two separate chloride ions, each having gained one electron from the single magnesium ion (which loses two).
- Explain why giant ionic lattices are hard but brittle: hard, because many strong electrostatic forces of attraction between oppositely charged ions must be broken to separate the lattice; brittle, because a force applied to one layer of ions can make like-charged ions from neighbouring layers line up next to each other, and the resulting repulsion shatters the lattice along that plane.
- Learn the general properties of ionic compounds and link each one to structure: high melting and boiling points (many strong ionic bonds throughout a giant lattice), conducting electricity only when molten or in aqueous solution (ions are then free to move and carry charge, but are fixed in a solid lattice), and often being soluble in water (polar water molecules can pull ions away from the lattice).
Worked example
Aluminium ions have a charge of 3+ and oxide ions have a charge of 2-. Deduce the formula of aluminium oxide using the criss-cross method.
- Write the ion charges: Al3+ and O2-.
- Criss-cross the numerical charges, ignoring the signs, so each becomes the other ion's subscript: aluminium gets a subscript of 2 (from oxide's charge), oxygen gets a subscript of 3 (from aluminium's charge).
- Write the resulting formula: Al2O3.
- Check the ratio cannot be simplified further, since 2:3 has no common factor other than 1.
- Final answer: the formula of aluminium oxide is Al2O3.
Practice questions
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Q1Give the formula and charge of the sulfate ion.Show answer
Answer: SO4^2- (a charge of 2-).
Q2Deduce the formula of calcium hydroxide, formed from Ca2+ ions and OH- ions.Show answer
Answer: Ca(OH)2
Q3Deduce the formula of sodium carbonate, formed from Na+ ions and CO3^2- ions.Show answer
Answer: Na2CO3
Q4State the charge on an ion formed by a Group 6 element gaining electrons to obtain a full outer shell.Show answer
Answer: 2-
Q5Explain, in terms of ions, why solid magnesium oxide does not conduct electricity but molten magnesium oxide does.Show answer
Answer: In the solid, the ions are held in fixed positions in the giant lattice and cannot move to carry charge; when molten, the lattice breaks down and the ions become free to move, so they can carry electrical charge.
Q6Explain why ionic compounds shatter (are brittle) when struck, rather than bending.Show answer
Answer: A force can shift one layer of ions relative to the layer next to it, so that ions of the same charge become aligned next to each other; the resulting repulsion between like charges forces the layers apart, shattering the lattice.
Q7Deduce the formula of ammonium sulfate, formed from NH4+ ions and SO4^2- ions.Show answer
Answer: (NH4)2SO4
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
Deduce the formulae of (a) iron(III) chloride, formed from Fe3+ ions and Cl- ions, (b) potassium nitrate, formed from K+ ions and NO3- ions, and (c) magnesium hydroxide, formed from Mg2+ ions and OH- ions.
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Describe, in words, a dot-and-cross diagram for magnesium chloride, MgCl2, formed from one magnesium atom and two chlorine atoms.
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Sodium chloride has a melting point of 801 degrees C and does not conduct electricity as a solid, but does conduct electricity when molten or dissolved in water. Explain, in terms of structure and bonding, both of these properties of sodium chloride.
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Free printable worksheet
Want more practice on paper? Download the ionic bonding and properties of ionic compounds worksheet pack - 5 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 14 of IGCSE Science Workbook, the whole course as one free printable PDF.
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