Chemical Bonding: Ionic, Covalent and Metallic
There are three types of chemical bond, each arising from atoms reaching a full outer shell. Ionic bonding occurs between a metal and a non-metal, which transfers electrons to form oppositely charged ions held together by electrostatic attraction; covalent bonding occurs between non-metals, which share pairs of electrons; and metallic bonding occurs in metals and alloys, held together by the attraction between positive ions and a sea of delocalised electrons.
Before you start
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Method
- Decide the bond type from the elements involved before anything else: metal plus non-metal is ionic, non-metal plus non-metal is covalent, metal only is metallic.
- Work out ion charges from the group: group 1 forms 1+, group 2 forms 2+, group 6 forms 2-, group 7 forms 1-. The formula of an ionic compound is the ratio that makes the total charge zero, so magnesium chloride is MgCl2.
- Draw a dot-and-cross diagram for an ionic compound by showing the electron transferred from the metal to the non-metal, then drawing each ion in square brackets with its charge outside and its full outer shell inside.
- Draw a covalent dot-and-cross diagram by overlapping the outer shells and placing one dot and one cross in each shared pair, checking that each atom ends with a full outer shell (2 for hydrogen, 8 for most others).
- Describe metallic bonding using the exact phrase the mark scheme wants: a giant lattice (or regular arrangement) of positive metal ions surrounded by a sea of delocalised electrons, with strong electrostatic attraction between them.
- Explain any property by naming the bond and then what must be overcome: melting an ionic compound requires breaking many strong ionic bonds between oppositely charged ions, so the melting point is high; melting a simple molecular substance only requires overcoming weak intermolecular forces, not the covalent bonds, so the melting point is low.
Worked example
Magnesium reacts with oxygen to form magnesium oxide. Describe the electron transfer, give the formula, and explain why magnesium oxide has a very high melting point.
- Find the electronic structures: magnesium is 2,8,2 and oxygen is 2,6.
- Transfer the electrons: each magnesium atom loses its two outer electrons to form Mg2+ with the structure 2,8; each oxygen atom gains two electrons to form O2- with the structure 2,8.
- Combine in the ratio that makes the total charge zero: one 2+ ion to one 2- ion, so the formula is MgO.
- Describe the structure: a giant ionic lattice in which each ion is surrounded by ions of opposite charge.
- Explain the melting point: melting requires overcoming the strong electrostatic forces of attraction between the oppositely charged ions throughout the lattice, and there are very many of them, so a large amount of energy is needed.
- Add the comparison mark: the 2+ and 2- charges are larger than the 1+ and 1- in sodium chloride, so the attraction is stronger and the melting point of magnesium oxide is higher still.
Practice questions
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Q1What type of bonding is present in a compound formed between a metal and a non-metal?Show answer
Answer: Ionic bonding.
Q2Define a covalent bond.Show answer
Answer: A shared pair of electrons between two non-metal atoms, attracted to both nuclei.
Q3Give the charge on an ion formed by a group 2 element.Show answer
Answer: 2+, because it loses its two outer electrons.
Q4Write the formula of calcium chloride and explain the ratio.Show answer
Answer: CaCl2. Calcium forms Ca2+ and chlorine forms Cl-, so two chloride ions are needed to balance one calcium ion.
Q5Describe metallic bonding.Show answer
Answer: A giant lattice of positive metal ions surrounded by a sea of delocalised electrons, with strong electrostatic attraction between the ions and the electrons.
Q6Why can metals conduct electricity when solid?Show answer
Answer: The delocalised electrons are free to move through the whole structure and carry charge.
Q7Why can an ionic compound conduct electricity when molten or dissolved but not when solid?Show answer
Answer: The ions are charged and must be free to move to carry charge. In the solid they are held in fixed positions in the lattice; when molten or dissolved they can move.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Chlorine forms a molecule with the formula Cl2. Describe the bonding in a chlorine molecule and explain why chlorine is a gas at room temperature.
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Explain why an alloy such as bronze is harder than the pure copper it is made from.
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Free printable worksheet
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This topic is chapter 18 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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