Nature of covalent and dative covalent bonds — AQA A-Level Chemistry
Test yourself on Nature of covalent and dative covalent bonds with AQA A-Level practice questions.
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Nature of covalent and dative covalent bonds explained
A single covalent bond is one pair of electrons shared between two atoms.
Read the full explanation
The bond exists because both positive nuclei are attracted to the same negative pair of electrons sitting between them, an electrostatic attraction that holds the atoms at a fixed separation where attraction and repulsion balance. In a dot-and-cross diagram, the shared pair sits in the overlap of the two circles, with one dot and one cross, and any electrons not involved are drawn as lone pairs. In a displayed formula, each shared pair is drawn as a single straight line. The bond is directional, which is why covalent substances form molecules with definite shapes or giant covalent lattices like diamond and silicon dioxide.
Multiple bonds contain multiple pairs of electrons.
A double bond is two shared pairs, four electrons in all, and a triple bond is three shared pairs, six electrons. More shared electrons mean a stronger attraction to both nuclei, so multiple bonds are shorter and stronger than single bonds between the same two atoms: the carbon-carbon bond shortens from ethane to ethene to ethyne while its bond enthalpy rises. When predicting shape, however, a double or triple bond counts as one region of electron density, not two or three, because both pairs lie between the same two nuclei. That is why carbon dioxide, with two double bonds, is linear at 180° rather than bent.
A co-ordinate (dative covalent) bond contains a shared pair of electrons with both electrons supplied by one atom.
A co-ordinate or dative covalent bond is still a shared pair, but both electrons come from the same atom. One species must supply a lone pair and the other must have a vacant orbital to receive it. Ammonia donates its lone pair to a hydrogen ion to give the ammonium ion, water does the same to give the oxonium ion, and ammonia donates to the empty orbital on boron in boron trifluoride. Ligands bond to transition metal ions in exactly this way. Once formed, the bond is indistinguishable from the others: all four bonds in the ammonium ion have the same length and the same strength, and the ion is a perfect tetrahedron.
Students should be able to represent: a covalent bond using a line a co-ordinate bond using an arrow.
Representation carries its own marks. An ordinary covalent bond is drawn as a straight line between the two atoms, one line for each shared pair. A co-ordinate bond is drawn as an arrow whose head points from the atom supplying the lone pair towards the atom receiving it, so in the ammonium ion the arrow runs from nitrogen to the added hydrogen. Do not confuse this with a curly arrow, which is reserved for showing the movement of an electron pair in a mechanism and must start at a lone pair or a bond. A displayed formula must show every atom and every bond, and an ion needs its charge written outside a square bracket enclosing the whole species.
Your focus
- Draw a dot-and-cross diagram for a simple covalent molecule showing shared pairs and lone pairs separately.
- Explain in terms of nuclei and the shared pair why two atoms are held together in a covalent bond.
- Recognise that covalent bonds are directional, leading to simple molecules or giant covalent lattices.
Show all 12 objectives
- State how many electrons are shared in a double bond and in a triple bond.
- Explain why a carbon-carbon triple bond is shorter and stronger than a carbon-carbon single bond.
- Predict the shape of carbon dioxide by treating each double bond as a single region of electron density.
- Identify the lone-pair donor and the electron-deficient acceptor in the formation of a named dative covalent bond.
- Explain why all four bonds in the ammonium ion are identical despite one forming differently.
- Draw the product of ammonia reacting with boron trifluoride, showing the origin of the shared pair.
- Draw the ammonium ion showing three ordinary bonds as lines and the fourth as an arrow in the correct direction.
- Distinguish, in a diagram, between a dative bond arrow and a curly arrow used in a mechanism.
- Show the charge on a polyatomic ion correctly using a square bracket.
Nature of covalent and dative covalent bonds exam tips
Marking Points
- one mark for a shared pair of electrons between the two atoms
- one mark for electrostatic attraction between the shared pair and both nuclei
- one mark for a dot-and-cross diagram showing one electron from each atom in the overlap
- one mark for representing the shared pair as a single line in a displayed formula
- one mark for a double bond containing two shared pairs of electrons
- one mark for a triple bond containing three shared pairs of electrons
- one mark for multiple bonds being shorter and stronger than a single bond between the same atoms
- one mark for treating a multiple bond as a single region of electron density when predicting shape
- one mark for a shared pair of electrons forming the bond
- one mark for both electrons being supplied by the same atom
- one mark for identifying the atom with the lone pair and the species with a vacant orbital
- one mark for stating that the completed bond is identical in length and strength to an ordinary covalent bond
- one mark for a straight line drawn for each shared pair in an ordinary covalent bond
- one mark for an arrow drawn for the co-ordinate bond
- one mark for the arrowhead pointing from the donor atom towards the acceptor atom
- one mark for the overall charge shown outside a bracket enclosing the whole ion
Examiner Tips
- 💡Say shared pair of electrons, not shared electrons; the pair is what earns the mark.
- 💡Include every outer electron in a dot-and-cross diagram, then check the total against the atoms present.
- 💡Use the word electrostatic when explaining what holds the atoms together.
- 💡Count electrons, not lines, when a question asks how many electrons are in a bond.
- 💡For shape questions, count regions of electron density: each multiple bond is one region.
- 💡Link bond length and bond enthalpy together when comparing single, double and triple bonds.
- 💡Name the lone-pair donor and the electron-deficient acceptor explicitly in every explanation.
- 💡Show the lone pair on the donor in your diagram before the bond forms, so the origin of the pair is visible.
- 💡Put the charge outside a square bracket drawn around the whole ion.
- 💡Read the question: if it says show the co-ordinate bond, a plain line will not score.
- 💡In a displayed formula show all bonds and all atoms, including every hydrogen.
- 💡Keep diagrams unambiguous; a structure open to two readings earns nothing.
Common Mistakes
- describing covalent bonding as electrons being given or transferred
- drawing a line to represent a single electron rather than a shared pair
- saying the bond is an attraction between atoms without naming nuclei and the shared pair
- assuming all covalent substances form simple molecules, forgetting giant covalent lattices
- describing a double bond as two separate single bonds between different atoms
- drawing three lines for a double bond or two for a triple
- counting a double bond as two electron pairs when predicting a bond angle, so calling carbon dioxide bent
- claiming a triple bond is weaker because it is shorter
- saying one electron comes from each atom, which describes an ordinary covalent bond
- claiming the dative bond in the ammonium ion is longer or weaker than the other three
- forgetting to show the overall charge on the ion formed
- identifying the acceptor as the electron donor, reversing the description
- drawing a curly arrow, which means the movement of an electron pair, in place of the straight dative arrow
- pointing the arrow from the acceptor back to the donor
- drawing four plain lines in the ammonium ion so the dative bond cannot be identified
- leaving the charge off the ion or placing it on a single atom rather than outside the bracket