Covalent and dative covalent bondingAQA A-Level Chemistry: Revision notes
Section 1
The covalent bond
A single covalent bond contains a shared pair of electrons between two atoms. Each atom normally supplies one electron to the pair, and the positively charged nuclei are held together by their attraction to the shared electrons.
Examples: H–H, H–Cl, and the four C–H bonds in methane, CH₄. Each atom in a molecule is usually surrounded by a full outer shell of electrons (including the shared pairs and any lone pairs).
Section 2
Multiple bonds
Atoms can share more than one pair of electrons:
- double bond: two shared pairs (four electrons), as in O=O, CO₂ (O=C=O) and C=C in ethene
- triple bond: three shared pairs (six electrons), as in N≡N
In a displayed formula, a single bond is drawn as one line, a double bond as two lines and a triple bond as three lines. Each line is one shared pair.
In ethene, H₂C=CH₂, there are four C–H single bonds (4 pairs) and one C=C double bond (2 pairs), a total of six shared pairs.
Count bonds as pairs: one line = one pair = two electrons. A double bond has four electrons, a triple bond six.
Section 3
Dative covalent (co-ordinate) bonds
A dative covalent bond (also called a co-ordinate bond) is a shared pair of electrons in which both electrons are supplied by one atom. Once formed, it is a covalent bond like any other.
It needs:
- a donor atom with a lone pair
- an acceptor with an empty orbital that can take the pair (for example H⁺ or an atom with fewer than eight outer electrons)
It is drawn as an arrow pointing from the donor to the acceptor, whereas an ordinary covalent bond is drawn as a line.
Drawing the arrow from the acceptor to the donor. The arrow points away from the atom supplying the lone pair, towards the atom receiving it.
Section 4
Examples of dative bonding
- Ammonium ion: the lone pair on N in NH₃ is donated to H⁺, giving NH₄⁺.
- Oxonium ion: a lone pair on O in H₂O is donated to H⁺, giving H₃O⁺.
- Carbon monoxide: C≡O has two ordinary covalent bonds and one dative bond, with both electrons from oxygen (arrow from O to C).
- Boron trifluoride and ammonia: BF₃ has only six electrons around boron, so it accepts the lone pair from NH₃ to form H₃N→BF₃, and boron then has four bonding pairs.
In each case the donor has a lone pair, and the acceptor is short of electrons.
Must Know
- Covalent bond = shared pair of electrons; a line in a displayed formula
- Double bond = 2 pairs; triple bond = 3 pairs
- Dative covalent (co-ordinate) bond = shared pair with both electrons from one atom
- Drawn as an arrow from donor to acceptor
- Donor has a lone pair; acceptor has an empty orbital (H⁺, BF₃)
- NH₄⁺, H₃O⁺, CO and H₃N→BF₃ are examples
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Covalent and dative covalent bonding
- Nitrogen gas from the air is used to make ammonia. Ammonia is then converted into fertilisers that contain the ammonium ion, NH₄⁺.Explain how an ammonia molecule forms an ammonium ion with a hydrogen ion, H⁺.2 marks
- Carbon monoxide, CO, is a toxic gas formed by the incomplete combustion of carbon-containing fuels. A carbon atom has four outer-shell electrons and an oxygen atom has six. In carbon monoxide the carbon and oxygen atoms are joined by a triple bond, and each atom has one lone pair of electrons.Explain why one of the bonds in carbon monoxide is a dative covalent bond, and state how this bond is represented in a displayed formula.2 marks
- Boron trifluoride, BF₃, is a gas in which the boron atom has only six electrons in its outer shell. It reacts with ammonia, NH₃, to form a solid compound with the formula H₃N→BF₃.Explain how the bond between the nitrogen atom and the boron atom forms.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).