Chemical Bonding
High SchoolHow atoms join
Atoms bond to reach a lower-energy, more stable electron arrangement — usually a full outer shell (the octet rule). Three principal bond types:
- Ionic bonding — electrons transfer from a metal to a non-metal, forming cations and anions held together by electrostatic attraction in a giant lattice (NaCl).
- Covalent bonding — non-metal atoms share electron pairs. Sharing can be equal (non-polar) or unequal (polar), depending on the electronegativity difference.
- Metallic bonding — metal cations sit in a "sea" of delocalised valence electrons, explaining conductivity, malleability and lustre.
Lewis structures track valence electrons as dots and bonds. VSEPR theory (Valence Shell Electron Pair Repulsion) then predicts molecular shape: electron domains (bonds and lone pairs) arrange to maximise their separation.
| Domains | Bond pairs / lone pairs | Shape | Bond angle | Example |
|---|---|---|---|---|
| 2 | 2 / 0 | Linear | 180° | CO₂ |
| 3 | 3 / 0 | Trigonal planar | 120° | BF₃ |
| 4 | 4 / 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 3 / 1 | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 / 2 | Bent | ~104.5° | H₂O |
Shape decides everything
Why is water bent while carbon dioxide is straight? Both are AB₂, but water's central oxygen has two lone pairs. Lone pairs repel more strongly than bonding pairs (they are held by only one nucleus, so they spread out), squeezing the H–O–H angle down to about 104.5°. Carbon dioxide's carbon has no lone pairs, so its two double bonds point directly opposite each other — linear.
This geometry has enormous consequences. Because water is bent and O–H bonds are polar, the two bond dipoles do not cancel: water is a polar molecule with a net dipole. CO₂'s two polar bonds point oppositely and cancel, so CO₂ is non-polar despite polar bonds. Water's polarity is why it dissolves salts, climbs plant stems, and supports life — all traceable to two lone pairs bending a molecule.
A molecule is polar only if it has polar bonds and an asymmetric shape that prevents the bond dipoles from cancelling. Symmetry can hide polarity.
Ionic bond
- Electrons transferred metal → non-metal
- Oppositely charged ions in a giant lattice
- High melting points; hard and brittle
- Conducts only when molten or dissolved
- e.g. NaCl, MgO
Covalent bond
- Electron pairs shared between non-metals
- Discrete molecules (or giant covalent networks)
- Often lower melting points; many are gases/liquids
- Molecular substances don't conduct (no free ions)
- e.g. Cl₂, H₂O, CH₄
Worked example — Lewis structure and shape of ammonia (NH₃)
- Count total valence electrons: N contributes 5, each H contributes 1 → 5 + 3(1) = 8 electrons (4 pairs).
- Place nitrogen as the central atom (least electronegative, forms most bonds); attach three H atoms.
- Form three N–H single bonds, using 3 pairs (6 electrons).
- Place the remaining 1 pair as a lone pair on nitrogen.
- Count electron domains on N: 3 bonding + 1 lone = 4 domains → tetrahedral electron geometry.
- Because one domain is a lone pair, the molecular shape is trigonal pyramidal, with H–N–H angles ≈ 107°.
The lone pair also makes ammonia a base (it can accept a proton) and gives the molecule a net dipole, so NH₃ is highly soluble in water.
Classify the bonding in MgO, Cl₂, and Cu.
MgO — ionic (metal + non-metal; Mg²⁺ and O²⁻). Cl₂ — covalent, non-polar (two identical non-metals sharing equally). Cu — metallic (metal cations in a sea of delocalised electrons).
Why do ionic compounds conduct electricity when molten or dissolved, but not as solids?
Conduction needs mobile charge carriers. In a solid ionic lattice the ions are locked in fixed positions and cannot move. Melting or dissolving frees the ions to migrate toward electrodes, carrying charge. Covalent solids like sugar produce no ions, so their solutions do not conduct.
Predict the shape and polarity of CH₄ and CH₃Cl.
Both are tetrahedral (4 bonding domains, no lone pairs on C). CH₄ is non-polar: four identical C–H bonds arranged symmetrically, so dipoles cancel. In CH₃Cl one bond is the more polar C–Cl, breaking the symmetry, so the dipoles no longer cancel — CH₃Cl is polar.
What is a coordinate (dative) covalent bond? Give an example.
A covalent bond in which both shared electrons come from the same atom. When NH₃ donates its lone pair to a proton, it forms the ammonium ion NH₄⁺: the fourth N–H bond is dative, yet once formed it is identical to the other three.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Draw the Lewis structure and shape of ammonia, NH3.
- Valence electrons: N has 5, each H has 1, total = 8.
- N bonds to three H atoms (3 pairs) and keeps one lone pair.
- Four electron groups with one lone pair give a trigonal pyramidal shape.
Example 2Is the bond in NaCl ionic or covalent?
- Na is a metal, Cl is a nonmetal.
- The large electronegativity difference means Na transfers an electron to Cl.
- Transfer of electrons produces Na+ and Cl- ions.
Now you try
Work each one out first, then tap to reveal the worked answer.