Covalent & dative bonds
Covalent & dative bonds
Covalent bonds and dot-and-cross diagrams
🔓 CRACK IT
In a covalent bond, two atoms share a pair of electrons. The shared electrons are attracted to both positive nuclei, holding the atoms together.
A single bond contains one shared pair, a double bond two and a triple bond three. In a dot-and-cross diagram, put every shared pair in the overlap between the atoms.
A lone pair is not shared in a bond. Show these electrons outside the overlap, and use dots and crosses consistently to show their atom of origin. Count all the outer-shell electrons before finishing.
- Single covalent bond
- A shared pair of electrons attracted to both positive nuclei.
| Bond | Shared pairs | Line example |
|---|---|---|
| Single | 1 | H–H |
| Double | 2 | O=O |
| Triple | 3 | N≡N |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
- Define a single covalent bond. [2]
- State the number of shared pairs in Cl2, O2 and N2 and give the line representation of each molecule. [3]
- State two pieces of information shown by a dot-and-cross diagram. [2]
Total: 7 marks
Show answer & marking points
- A shared pair of electrons [1], attracted to both positive nuclei [1].
- Cl2: one shared pair, Cl–Cl [1]; O2: two shared pairs, O=O [1]; N2: three shared pairs, N≡N [1].
- The outer-shell electrons [1] and the atom from which each electron originated [1].
- Draw a dot-and-cross diagram for Cl2, showing outer-shell electrons only. [3]
- Draw a dot-and-cross diagram for O2, showing outer-shell electrons only. [3]
Total: 6 marks
Show answer & marking points
- One shared pair containing one dot and one cross [1]; three lone pairs on each chlorine [1]; consistent electron-origin symbols [1].
- Two shared pairs, each containing one dot and one cross [1]; two lone pairs on each oxygen [1]; consistent electron-origin symbols [1].
Dative bonds: donor to acceptor
🔓 CRACK IT
A covalent bond contains a shared pair of electrons. Usually, each atom supplies one electron. In a dative covalent bond, one atom supplies both.
The atom supplying the pair is the donor. The species receiving it is the acceptor. Draw the arrow from the donor's lone pair to the acceptor.
For NH₃ reacting with H⁺, nitrogen supplies its lone pair to the hydrogen ion. Once NH₄⁺ has formed, all four N-H bonds are equivalent. The origin of the electrons does not make one of those bonds different afterwards.
- Dative covalent bond
- A shared pair of electrons with both electrons supplied by one atom.
| Role | NH3 + H+ → NH4+ |
|---|---|
| Donor | Nitrogen in NH3 has an available lone pair |
| Acceptor | H+ accepts the pair |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.
- Define a dative covalent bond. [2]
- For NH3 + H+ → NH4+, identify the donor and acceptor, state the arrow direction and state how the four N–H bonds compare after formation. [4]
- In F- + BF3 → BF4-, identify the electron-pair donor and state the dative-arrow direction. [2]
Total: 8 marks
Show answer & marking points
- A shared pair of electrons [1] with both electrons supplied by one atom [1].
- NH3, through nitrogen, is the donor [1]; H+ is the acceptor [1]; the arrow starts at the nitrogen lone pair and points to H+ [1]; all four N–H bonds become equivalent [1].
- F- is the donor [1]; the arrow points from a fluoride lone pair towards boron [1].
Ammonia forms a dative covalent bond with AlCl3. The electron-deficient aluminium atom accepts a lone pair.
- Identify the electron-pair donor and acceptor. [2]
- Draw the dative arrow on H3N and AlCl3. [2]
- Explain why ammonia can act as the donor. [1]
- Explain why an arrow from aluminium towards nitrogen would be incorrect. [2]
Total: 7 marks
Show answer & marking points
- NH3 through N is the donor [1]; AlCl3 through Al is the acceptor [1].
- The arrow starts at the N lone pair [1] and points towards Al: H3N → AlCl3 [1].
- N has an available lone pair [1].
- N supplies both electrons, not Al [1]; an Al → N arrow reverses donor-to-acceptor direction [1].
Fluoride ions form BF4- by donating a lone pair to BF3. A salt contains Mg2+ and BF4- ions.
- Identify the dative donor and acceptor and describe the arrow fully. [4]
- Write the salt’s formula. [1]
- State the bonding within BF4- and between Mg2+ and BF4- ions in the solid. [2]
- Explain why the solid does not conduct but the molten salt does. [3]
- Explain why a high melting point is expected. [3]
Total: 13 marks
Show answer & marking points
- F- donor [1]; B in BF3 acceptor [1]; arrow starts at fluoride lone pair [1] and points to B [1].
- Mg(BF4)2 [1].
- Covalent within BF4- [1]; ionic between Mg2+ and BF4- [1].
- Solid ions are fixed [1]; molten ions are mobile [1] and carry charge [1].
- Giant ionic lattice [1]; strong electrostatic attractions between opposite ions [1] require much energy to overcome [1].