How many σ bonds and π bonds are there in one molecule of ethene, H2C=CH2?
- There are four C–H single bonds: 4 σ.
- The C=C double bond is 1 σ + 1 π.
- Total σ = 4 + 1 = 5; total π = 1.
Answer: 5 σ bonds and 1 π bond.
Covalent bonding and coordinate (dative covalent) bonding
AS Level Chemistry · Chemical bonding. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
The idea
Covalent bonding is the electrostatic attraction between the nuclei of two atoms and a shared pair of electrons. In a normal covalent bond each atom supplies one electron. In a coordinate bond both electrons come from the same atom, which gives a lone pair to an atom with an empty orbital.
Bonds form when orbitals overlap. End-on overlap gives a σ bond. Sideways overlap of p orbitals gives a π bond, with electron density above and below the line between the nuclei.
Period 3 atoms such as phosphorus and sulfur can hold more than eight outer electrons, as in PCl5 and SF6.
Rules to remember
- Single bond = 1 σ. Double bond = 1 σ + 1 π. Triple bond = 1 σ + 2 π.
- Coordinate bond: a shared pair in which both electrons come from one atom, e.g. NH3 + H+ → NH4+.
- Al2Cl6 has two coordinate bonds, each from a chlorine lone pair to an aluminium atom.
- Hybridisation: sp3 in methane and ethane (109.5°), sp2 in ethene (120°), sp in ethyne and CO2 (180°).
- Bond energy: the energy needed to break one mole of a particular covalent bond in the gaseous state.
- Bond length: the distance between the nuclei of the two bonded atoms.
- More shared pairs give a shorter and stronger bond: C≡C is shorter and stronger than C=C, which is shorter and stronger than C–C.
Common mistake
Students count a double bond as two σ bonds. Any bond between two atoms has only one σ bond; the extra bonds are π bonds.
Worked example
Practice questions
Try each one, then open the answer.
1. How many coordinate bonds are present in the Al2Cl6 molecule, and how do they form?
- Aone; a lone pair on aluminium is donated to chlorine
- Btwo; a lone pair on a chlorine atom of each AlCl3 unit is donated to the electron-deficient aluminium atom of the other
- Ctwo; a lone pair on aluminium is donated to chlorine
- Dsix; all of the Al–Cl bonds are coordinate
Show answer
Answer: B. AlCl3 has only six electrons around Al, so it is electron-deficient. Two chlorine atoms bridge the units, each donating a lone pair into the empty orbital of the other Al: two coordinate bonds.
2. How many bonding pairs and lone pairs of electrons are there in a molecule of nitrogen, N2?
- A1 bonding pair and 3 lone pairs
- B3 bonding pairs and 1 lone pair
- C3 bonding pairs and 2 lone pairs
- D2 bonding pairs and 2 lone pairs
Show answer
Answer: C. Each N atom has five outer electrons. Three from each atom are shared in a triple bond (3 bonding pairs), and each N keeps one lone pair, giving 2 lone pairs in total.
3. How many electrons are there in the outer shell of the sulfur atom in SF6?
- A6
- B8
- C10
- D12
Show answer
Answer: D. Sulfur forms six single bonds, each sharing one electron from S and one from F, so it has 12 electrons in its outer shell. Period 3 elements can expand the octet because they have empty 3d orbitals available.
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