Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Electronegativity is how strongly an atom in a bond attracts the shared electrons. Pauling scale, no unit; fluorine is highest (3.98).
- Across a period it increases: larger effective nuclear charge, same shell. Down a group it decreases: extra shell, bonding electrons further away and more shielded.
- Nonpolar covalent: electrons shared between atoms of equal or similar electronegativity (H₂, Cl₂; C–H counts as nonpolar).
- Polar covalent: unequal sharing. The more electronegative atom is δ−, the other δ+. For single bonds, a bigger difference means a bigger bond dipole.
- Every polar bond has some ionic character. Ionic and covalent are ends of a continuum, not separate boxes.
- Ionic: usually a metal with a nonmetal. Covalent: usually two nonmetals. Properties of the compound give the best evidence.
- Metallic: metal cations with delocalized valence electrons that belong to no single atom.
Key relationships
| Situation | Bond type | Example |
|---|---|---|
| Same element, or very small difference | nonpolar covalent | Cl–Cl; C–H (0.35) |
| Two nonmetals, clear difference | polar covalent | H–Cl (0.96); O–H (1.24) |
| Metal + nonmetal, large difference | ionic | NaCl (2.23) |
| Metal atoms only | metallic | Cu, brass |
| Property evidence | Points to |
|---|---|
| Conducts as a solid, malleable | metallic |
| Conducts only when molten or dissolved, brittle, high melting point | ionic |
| Low melting point, does not conduct | covalent molecules |
Assumptions
- Electronegativity values describe atoms in bonds and are averages; they guide, not decide.
- “Bigger difference, bigger dipole” is used for single bonds.
Mistakes to avoid
- Using a fixed cut-off (such as 1.7) to call a bond ionic. HF (1.78) is covalent.
- Writing full charges (Cl⁻) on atoms in a polar covalent bond. Use δ+ and δ−.
- Putting δ− on the wrong atom. It goes on the more electronegative atom, even if both are nonmetals (in O–F, F is δ−).
- Calling C–H polar.
- Describing metals as sharing fixed electron pairs. Say “delocalized”.
- Ignoring the properties when the question gives them.
Quick self-check
- Which atom is δ− in a Si–H bond (Si 1.90, H 2.20)? (H)
- Is the bonding in MgO more likely ionic or covalent? (Ionic: metal + nonmetal, difference 2.13.)
- A solid conducts only when molten. Which bonding? (Ionic.)
Next: practice questions.