Summary — What This Chapter Covers
- Electronegativity is the power of an atom to attract electrons to itself; it rises across a period and falls down a group.
- Ionic bonding: electrostatic attraction between oppositely charged ions.
- Metallic bonding: electrostatic attraction between positive metal ions and delocalised electrons.
- Covalent bonding: electrostatic attraction between the nuclei of two atoms and a shared pair of electrons. Coordinate bonds: both electrons come from one atom.
- σ bonds form by direct (end-on) overlap; π bonds by sideways overlap of p orbitals. Hybrid orbitals: sp³, sp², sp.
- VSEPR: electron pairs repel to get as far apart as possible; lone pairs repel more than bonding pairs.
- Van der Waals' forces: id-id (London) forces and pd-pd forces, including hydrogen bonding.
- Ionic, covalent and metallic bonds are generally stronger than intermolecular forces.
1. Electronegativity and Bonding
Electronegativity
What it is and what changes it
Electronegativity is the power of an atom to attract electrons to itself (in a covalent bond).
It depends on:
- Nuclear charge: more protons, higher electronegativity.
- Atomic radius: bonding electrons closer to the nucleus are attracted more strongly.
- Shielding by inner shells and sub-shells: more shielding, lower electronegativity.
Across a period electronegativity increases: nuclear charge rises, the radius falls and shielding is similar. Down a group it decreases: more shells, larger radius and more shielding outweigh the extra protons. Fluorine is the most electronegative element (Pauling value 4.0).
Predicting bond type from Pauling values
| Electronegativity difference | Bond type | Example |
|---|
| 0 | Non-polar covalent | Cl–Cl (3.0 − 3.0) |
| Small (up to about 1.8) | Polar covalent | H–Cl (3.0 − 2.1 = 0.9) |
| Large (above about 1.8) | Ionic | NaCl (3.0 − 0.9 = 2.1) |
Practice Question
Using Pauling values Mg 1.2, O 3.5, C 2.5, H 2.1, predict the bonding in MgO and in CH₄.
2. Ionic Bonding
Ionic bonding
Oppositely charged ions in a lattice
Ionic bonding is the electrostatic attraction between oppositely charged ions (positively charged cations and negatively charged anions).
Metal atoms lose electrons to form cations; non-metal atoms gain electrons to form anions, usually reaching a noble-gas configuration. The ions are held in a giant lattice, with attractions acting in all directions.
| Compound | Electron transfer | Ions |
|---|
| Sodium chloride, NaCl | Na loses 1 e⁻ to Cl | Na⁺ [2,8] and Cl⁻ [2,8,8] |
| Magnesium oxide, MgO | Mg loses 2 e⁻ to O | Mg²⁺ [2,8] and O²⁻ [2,8] |
| Calcium fluoride, CaF₂ | Ca loses 1 e⁻ to each of two F atoms | Ca²⁺ [2,8,8] and 2 F⁻ [2,8] |
MgO has a much higher melting point than NaCl (2852 °C vs 801 °C) because its ions carry larger charges (2+ and 2−) and are smaller, so the attraction between them is stronger.