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1.3.2Physical Chemistry

Nature of Covalent and Dative Covalent Bonds — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 63

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 5Covalent Bonds

1.3.2 Nature of Covalent and Dative Covalent Bonds
Shared pair
  • A covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
Who forms them
  • Covalent bonds typically form between non-metal atoms that each need electrons to complete their outer shell.
Single bond
  • One shared pair, as in Cl–Cl or H–Cl.
Double bond
  • Two shared pairs, as in O=O or C=C.
Triple bond
  • Three shared pairs, as in N≡N.

Part 2 of 5Bond Length and Strength

Bond Length and Strength
Bond length
  • The distance between the nuclei of two bonded atoms, which falls as the number of shared pairs rises.
Bond strength
  • The energy needed to break a bond, which increases from single to double to triple bonds between the same atoms.
Lone pairs
  • Pairs of outer electrons not used in bonding, such as the one pair on nitrogen in NH₃.
Dot-and-cross
  • Use dots and crosses to show where electrons originate, with overlapping circles showing the shared pairs.

Part 3 of 5Dative Covalent Bonds

Definition
  • A dative (coordinate) covalent bond is a shared pair of electrons in which both electrons come from one atom.
Requirements
  • One atom needs a lone pair to donate and the other needs an empty orbital to accept it.
Ammonium ion
  • NH₃ donates its lone pair to an H⁺ ion to form NH₄⁺, in which all four N–H bonds are identical.
Other examples
  • The oxonium ion H₃O⁺ and the compound NH₃BF₃ are also formed by dative bonding.
Notation
  • A dative bond is drawn as an arrow pointing from the donor atom to the acceptor atom.

Part 4 of 5Drawing Dot-and-Cross Diagrams

1
Work out how many outer electrons each atom has from its group number.
2
Place the central atom and arrange the others around it.
3
Share electrons so every atom obtains a full outer shell, using crosses for one atom and dots for the other.
4
Show lone pairs as unshared pairs on each atom.
5
Check the total number of electrons, remembering to add or remove electrons for ions.

Part 5 of 5Exam Checklist & Tips

1.3.2 – Key Points for the Exam

  • Covalent bond A shared pair of electrons between two nuclei.
  • Dative covalent bond A shared pair in which both electrons are supplied by the same atom.
  • Lone pair A pair of outer-shell electrons not involved in bonding.
  • Bond length The distance between the nuclei of two bonded atoms.

Common Mistakes

  • Saying covalent bonds are attractions between atoms rather than between shared electrons and nuclei.
  • Drawing an NH₄⁺ ion with three bonds of one kind and one of another; all four are identical.
  • Forgetting the lone pairs when drawing dot-and-cross diagrams.
  • Missing the charge of an ion in the final diagram.

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