2.2.1Additional lessons
Covalent Bonds — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 22 of 98
Part 1 of 6Sharing Electrons
Unlike ionic bonding, no electrons are transferred; the atoms share them.
Where it occurs
- Covalent bonding usually joins atoms of non-metals, which both attract electrons too strongly for either to lose them completely.
Shared pair
- Each atom contributes one electron and the pair is shared, so each atom gains a more stable outer shell.
Electrostatic origin
- The bond is the attraction between the positively charged nuclei and the negatively charged shared electrons between them.
Molecules
- Atoms joined covalently form discrete molecules such as H₂, Cl₂, H₂O and CH₄.
Part 2 of 6The Octet Rule
The Octet Rule
Main idea
- Many atoms in covalent molecules end up with eight electrons in their outer shell, like a noble gas.
Hydrogen
- Hydrogen is an exception to the octet idea because its first shell is full with just two electrons.
Bonds needed
- Fluorine needs one more electron and forms one bond, oxygen forms two, nitrogen three and carbon four.
Exceptions
- Some molecules, such as BF₃, have fewer than eight electrons on the central atom, and others, such as PCl₅, have more.
Part 3 of 6Bond Length and Strength
1
As two atoms approach, attraction between each nucleus and the other atom's electrons lowers the energy of the system.
2
At very close range the nuclei repel each other, so the energy rises again.
3
The lowest-energy separation of the two nuclei is the bond length.
4
The energy needed to break one mole of a particular bond in the gas phase is the bond enthalpy, a measure of bond strength.
5
Bonds that are shorter are generally stronger because the nuclei are held closer to the shared electron pair.
| Bond | Length / pm | Enthalpy / kJ mol⁻¹ |
|---|---|---|
| C–C | 154 | 346 |
| C=C | 134 | 614 |
| C≡C | 120 | 839 |
Part 4 of 6Covalent Compounds and Representation
Single bond
- A single shared pair is drawn as one line, for example H–Cl in hydrogen chloride.
Molecular formula
- The formula shows the actual number of each atom in a molecule, such as C₂H₆ for ethane.
Electron cloud
- The shared electrons are not fixed between the nuclei but form a region of high electron density between them.
Weak between molecules
- Strong bonds hold atoms inside a molecule, whereas much weaker forces act between separate molecules.
Part 5 of 6Worked Example
Question Explain, using the table of bond data, why the C≡C bond is stronger and shorter than the C–C bond.
- C≡C contains three shared pairs of electrons whereas C–C contains one.
- More shared electrons between the nuclei produce a greater electrostatic attraction and pull the nuclei closer together.
- The data agreeC≡C has length 120 pm and enthalpy 839 kJ mol⁻¹, while C–C has 154 pm and 346 kJ mol⁻¹.
Answer A triple bond has more shared electron pairs, giving stronger attraction, a shorter bond length and a higher bond enthalpy.
Part 6 of 6Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Covalent bond The electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
- Bond length The distance between the nuclei of two bonded atoms at lowest energy.
- Bond enthalpy The energy needed to break one mole of a bond in gaseous molecules.
- Octet rule The tendency of atoms to achieve eight electrons in their outer shell.
- Molecule A discrete group of atoms held together by covalent bonds.
Common Mistakes
- Saying covalent bonds are attractions between atoms rather than between the shared electrons and both nuclei.
- Thinking the bond is broken when a covalent substance melts; usually only intermolecular forces are overcome.
- Stating that all atoms obey the octet rule, forgetting hydrogen, boron and expanded octets.
- Mixing up bond length and strength, since shorter bonds between the same atoms are stronger.
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