STRUCTURE 2
Models of Bonding
IONIC • COVALENT • METALLIC • VSEPR • ALLOYS
2.1 The Ionic Model
Guiding Principle: Ionic bonding is not just electron transfer; it is the electrostatic stability of a giant crystal lattice.
1. The Ionic Bond
Definition: The electrostatic attraction between oppositely charged ions (cations and anions).
Formation: Occurs when electronegativity difference ($\Delta\chi$) is > 1.8. (Metal loses $e^-$, Non-metal gains $e^-$).
2. The Lattice Structure
Ionic compounds do not exist as discrete molecules. They form a Giant Ionic Lattice—a continuous 3D repeating pattern.
3. Physical Properties (How to argue for marks)
Large energy required to overcome strong electrostatic forces holding the lattice together.
Solid: No (Ions are fixed in position).
Molten/Aqueous: Yes (Lattice broken, ions free to move).
Never say "electrons move" in ionic conduction. In ionic compounds, ions carry the charge.
Force causes layers to slide. Ions of same charge align. Massive repulsion shatters the crystal.
2.2 The Covalent Model
Guiding Principle: Atoms share electrons to achieve a full valence shell because neither atom is strong enough to take electrons from the other.
1. The Covalent Bond
Definition: The electrostatic attraction between a shared pair of electrons and the positively charged nuclei.
Coordinate (Dative) Bonds: A bond where both electrons in the shared pair originate from the same atom (e.g., $NH_4^+$).
When drawing Lewis structures for ions (e.g., $NH_4^+$), you must place the entire structure in square brackets $[ ... ]^+$ with the charge outside.
2. VSEPR Theory (Molecular Geometry)
Valence Shell Electron Pair Repulsion: Domains repel to be as far apart as possible.
Repulsion Hierarchy: Lone Pair-Lone Pair > Lone Pair-Bonding Pair > Bonding Pair-Bonding Pair.
| Domains | Bonding | Lone | Shape Name | Angle | Example |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | $180^\circ$ | $CO_2$ |
| 3 | 3 | 0 | Trigonal Planar | $120^\circ$ | $BF_3$ |
| 3 | 2 | 1 | Bent (V-shaped) | $<120^\circ$ | $SO_2$ |
| 4 | 4 | 0 | Tetrahedral | $109.5^\circ$ | $CH_4$ |
| 4 | 3 | 1 | Trigonal Pyramidal | $107^\circ$ | $NH_3$ |
| 4 | 2 | 2 | Bent (V-shaped) | $104.5^\circ$ | $H_2O$ |
3. Polarity & Intermolecular Forces
4. Giant Covalent Structures
Tetrahedral. Hard. No conduction (no free electrons).
Hexagonal layers. Conducts (delocalized electron). Soft (layers slide).
2.3 & 2.4 Metallic Model
1. Metallic Bonding
Definition: The electrostatic attraction between a lattice of positive metal ions (cations) and a sea of delocalized electrons.
- Conductivity: Delocalized electrons move freely to carry current.
- Malleability: Non-directional bonding allows layers to slide without breaking.
2. Alloys (Materials)
Definition: Homogeneous mixtures of metals (e.g., Brass, Steel).
Why are alloys harder? Different sized atoms distort the lattice, preventing layers from sliding over each other.
Advanced Theory
The following section is for HL Students ONLY.
Advanced Covalent Bonding
1. Formal Charge (The Tie-Breaker)
When multiple valid Lewis structures exist, Formal Charge (FC) determines the stable one.
- Choose the structure where FCs are closest to zero.
- If charges exist, negative FC must be on the most electronegative atom.
2. Resonance, Sigma ($\sigma$) & Pi ($\pi$) Bonds
Resonance: Occurs when a double bond can be in multiple positions (e.g., Benzene). The true structure is a hybrid (average).
Head-on overlap. $e^-$ density between nuclei. Stronger.
Sideways overlap. $e^-$ density above/below plane. Weaker.
3. Hybridization ($sp, sp^2, sp^3$)
Atoms mix orbitals to create new "hybrid" orbitals of equal energy.
| Domains | Hybridization | Geometry | Example |
|---|---|---|---|
| 4 | $sp^3$ | Tetrahedral | Methane ($CH_4$) |
| 3 | $sp^2$ | Trigonal Planar | Ethene ($C_2H_4$) |
| 2 | $sp$ | Linear | Ethyne ($C_2H_2$) |
4. Expanded Octets
Period 3+ elements (P, S, Cl) use d-orbitals to hold >8 electrons.
| Domains | Lone | Shape | Angles | Ex |
|---|---|---|---|---|
| 5 | 0 | Trigonal Bipyramidal | 90, 120, 180 | $PCl_5$ |
| 5 | 1 | Seesaw | <90, <120 | $SF_4$ |
| 5 | 2 | T-shaped | <90 | $ClF_3$ |
| 5 | 3 | Linear | 180 | $I_3^-$ |
| 6 | 0 | Octahedral | 90, 180 | $SF_6$ |
| 6 | 2 | Square Planar | 90 | $XeF_4$ |
The Examiner's Vault
Strictly assessed on Structure 2 content.
Highest charge product (+2/-2) and smallest ionic radius = Strongest attraction.
Central I has 2 bonds + 3 lone pairs = 5 domains. Trigonal Bipyramidal geometry $\rightarrow$ Linear shape.
3 electron domains (Resonance counts as 1 domain). Trigonal Planar = $sp^2$.
Explain why Graphite is used as a lubricant, whereas Diamond is used in cutting tools.
- Graphite has layers held by weak London (dispersion) forces which allow them to slide. [2]
- Diamond has a rigid 3D tetrahedral lattice where every C is bonded to 4 others. [1]
(a) Deduce the number of sigma ($\sigma$) and pi ($\pi$) bonds.
(b) Identify the hybridization of the central carbon atom.
- (a) Sigma: 6 (3 C-H + 2 C-C + 1 C-H). Pi: 2 (in the triple bond). [2]
- (b) $sp$ hybridization (Linear geometry around triple bond). [1]