Topic 3: Bonding and Structure for the Edexcel IAL AS Chemistry specification

Topic 3: Bonding & Structure

3A. Ionic Bonding (3.1 - 3.9)

Ionic bonding is the strong electrostatic attraction between oppositely charged ions. It leads to the formation of giant ionic lattices.

Evidence for Ions

• Electron Density Maps: Show discrete islands of electron density with zero density between ions.
• Electrolysis: Evidence includes the physical properties of ionic compounds and the migration of ions, such as colored ions moving toward oppositely charged electrodes.

Trends in Ionic Radii

For isoelectronic ions (e.g., ${\text{N}}^{3-}$ to ${\text{Al}}^{3+}$), the radius decreases as the atomic number increases because the nuclear charge pulls the same number of electrons closer.

Polarisation (Fajans' Rules):
Ionic bonds can have covalent character if the cation distorts the anion's electron cloud.

• High Polarising Power (Cation): Small size, High charge (e.g., ${\text{Al}}^{3+}$).
• High Polarisability (Anion): Large size, High charge (e.g., ${\text{I}}^{-}$).

3B. Covalent Bonding (3.10 - 3.12)

Covalent bonding is the electrostatic attraction between two nuclei and the shared pair of electrons between them.

Dative (Coordinate) Bonding

A bond where both shared electrons come from one atom. Common examples include ${\text{NH}}_4^{+}$ and the dimer ${\text{Al}}_2{\text{Cl}}_6$.

Giant Covalent Structures

• Diamond: Each C bonded to 4 others. Tetrahedral.
• Graphite: C bonded to 3 others. Layers slide (soft).
• Graphene: Single layer of graphite.

3C. Shapes of Molecules (VSEPR) (3.16 - 3.19)

Electron pairs repel each other to be as far apart as possible. Lone pairs repel more than bonding pairs.

Electron Pairs (Bond + Lone)	Shape	Bond Angle	Example
2 BP, 0 LP	Linear	180°	${\text{BeCl}}_2$
3 BP, 0 LP	Trigonal Planar	120°	${\text{BF}}_3$
2 BP, 1 LP	Bent / V-shaped	117.5°	${\text{SO}}_2$
4 BP, 0 LP	Tetrahedral	109.5°	${\text{CH}}_4$
3 BP, 1 LP	Trigonal Pyramidal	107°	${\text{NH}}_3$
2 BP, 2 LP	Bent / V-shaped	104.5°	${\text{H}}_2\text{O}$
5 BP, 0 LP	Trigonal Bipyramidal	120° & 90°	${\text{PCl}}_5$
6 BP, 0 LP	Octahedral	90°	${\text{SF}}_6$

Rule of Thumb: Every lone pair reduces the bond angle by roughly 2.5°.

3D & 3E. Electronegativity & Metallic Bonding

Electronegativity: The power of an atom to attract the shared pair of electrons in a covalent bond.

• Polar Molecules: Non-symmetrical molecules with polar bonds.
• Non-Polar Molecules: Symmetrical molecules where dipoles cancel.

Metallic Bonding: Attraction between a lattice of metal ions in a sea of delocalised electrons. Conductivity depends on the ability of these electrons to move.

[Image of metallic bonding sea of electrons model]

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Practice Zone

Exam-Style Question: Molecular Shapes

Q1: Explain the shape and bond angle of the ${\text{H}}_3{\text{O}}^{+}$ ion.

Click for Worked Solution

Step 1 (Electron Pairs): Oxygen is in Group 6. +1 charge means 1 electron lost. 5 valence electrons + 3 from Hydrogens = 8 electrons total. 4 pairs.

Step 2 (Bonding vs Lone): There are 3 Bonding Pairs (H-O) and 1 Lone Pair.

Step 3 (Conclusion): 4 pairs adopt a tetrahedral arrangement. 1 Lone Pair repels more than bonding pairs, reducing the angle from 109.5° to 107°. Shape is Trigonal Pyramidal.

Quick MCQ: Polarisation

Q2: Which compound is likely to show the most covalent character?

• A) ${\text{MgCl}}_2$
• B) ${\text{AlCl}}_3$
• C) $\text{NaCl}$
• D) ${\text{CaCl}}_2$

Click for Answer

Correct Answer: B. Aluminium (${\text{Al}}^{3+}$) has the highest charge density (high charge, small size), giving it the greatest polarizing power.