Covalent Bonding: Simple Molecules and Giant Structures
Covalent bonding at IGCSE requires dot-and-cross diagrams for molecules more complex than the GCSE staples of water and methane, including molecules with double or triple covalent bonds, such as oxygen (O2), carbon dioxide (CO2) and nitrogen (N2), and molecules with lone pairs that affect their chemistry. It also extends giant covalent structures beyond diamond, graphite and silicon dioxide to graphene, a single layer of graphite, and fullerenes such as buckminsterfullerene (C60), linking each structure's arrangement of atoms directly to its physical properties.
Before you start
Make sure you're comfortable with these topics first:
Method
- To draw a dot-and-cross diagram, count each atom's outer-shell electrons, then share pairs of electrons between atoms, one dot and one cross per shared pair, until every atom (except hydrogen, which needs only 2) has a full outer shell, usually 8 electrons.
- Recognise double bonds (two shared pairs, four electrons) in molecules such as O2 and CO2, and triple bonds (three shared pairs, six electrons) in N2; each extra shared pair adds to the strength of the bond between the two atoms.
- Show unshared (lone) pairs of electrons on the outer atoms where relevant, for example the two lone pairs on the oxygen atom in water, since these are still part of that atom's full outer shell even though they are not shared with another atom.
- For a giant covalent structure, describe the repeating pattern of covalent bonds throughout the whole structure, not separate molecules, and explain that very high melting and boiling points occur because a huge number of strong covalent bonds must be broken to melt or boil the substance, not just weak forces between molecules.
- Compare graphene (a single, one-atom-thick layer of graphite, with each carbon atom covalently bonded to three others in a hexagonal lattice) to graphite: graphene has the same delocalised-electron conduction as graphite, but is not soft or slippery like bulk graphite, because there are no other layers for it to slide over.
- Describe buckminsterfullerene (C60) as a discrete molecule made of 60 carbon atoms arranged in a hollow, sphere-shaped cage of hexagons and pentagons, each carbon covalently bonded to three others; because it exists as separate molecules held together only by weak intermolecular forces, C60 has a much lower melting point than diamond or graphite and can act as a lubricant.
Worked example
Describe, in words, a dot-and-cross diagram for a molecule of carbon dioxide, CO2, showing all outer-shell electrons and any double bonds.
- Count outer-shell electrons: carbon (Group 4) has 4, and each oxygen (Group 6) has 6.
- To complete its octet, carbon needs 4 more electrons, and each oxygen needs 2 more electrons.
- Carbon forms a double bond with each oxygen atom, sharing 2 pairs of electrons (4 electrons) with each oxygen, so carbon shares 4 pairs of electrons in total, with no lone pairs.
- Each oxygen atom keeps 2 unshared (lone) pairs of its own, plus the 2 shared pairs from its double bond with carbon, giving each oxygen 8 electrons overall.
- Final answer: the structure is O=C=O, with two carbon-oxygen double bonds; carbon has no lone pairs and each oxygen has 2 lone pairs, and every atom has a full outer shell of 8 electrons.
Practice questions
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Q1State the number of shared pairs of electrons in a triple covalent bond.Show answer
Answer: 3
Q2Give the number of outer-shell electrons a hydrogen atom needs to have a full outer shell.Show answer
Answer: 2
Q3Name the molecule with the formula N2 that contains a triple covalent bond between its two atoms.Show answer
Answer: Nitrogen.
Q4Give one difference in structure between diamond and graphene.Show answer
Answer: In diamond, each carbon atom is bonded to four other carbon atoms in a rigid 3D giant lattice; in graphene, each carbon atom is bonded to only three others, forming a flat, one-atom-thick hexagonal sheet.
Q5State one property that graphene and graphite have in common, and explain why.Show answer
Answer: Both conduct electricity, because in each structure every carbon atom forms only three covalent bonds, leaving one delocalised electron per atom that is free to move and carry charge.
Q6Explain why buckminsterfullerene (C60) has a much lower melting point than diamond.Show answer
Answer: C60 exists as separate, discrete molecules held together only by weak intermolecular forces, which need little energy to overcome; diamond is one giant covalent structure held together throughout by many strong covalent bonds, which need a huge amount of energy to break.
Q7State the number of lone pairs of electrons on the oxygen atom in a molecule of water, H2O.Show answer
Answer: 2
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
Give two similarities in composition or bonding between diamond and buckminsterfullerene (C60).
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A molecule of ethene, C2H4, contains a carbon-carbon double bond. (a) State the total number of shared pairs of electrons between the two carbon atoms. (b) Describe how the outer-shell electrons of the two hydrogen atoms bonded to one of the carbon atoms are arranged, referring to full outer shells.
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Diamond, graphite and buckminsterfullerene (C60) are all forms of the element carbon, but they have very different physical properties: diamond is extremely hard and does not conduct electricity; graphite is soft and slippery and does conduct electricity; buckminsterfullerene has a low melting point and is used as a lubricant. Explain, in terms of structure and bonding, why these three forms of carbon have such different properties, even though they contain only carbon atoms.
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Free printable worksheet
Want more practice on paper? Download the covalent bonding: simple molecules and giant structures 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 15 of IGCSE Science Workbook, the whole course as one free printable PDF.
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