IMAT Chemistry · Free notes

Chemical Bonding: Bonds, Shapes and Forces

Chemical Bonding has 24 past questions since 2011, and the ministry papers from 2023 on have asked 5 of them. Almost none of them need a calculation: they ask you to classify a bond, predict a shape, decide whether a molecule is polar, or explain a boiling point. The difficulty sits in chains of reasoning, because polarity depends on shape, and shape depends on lone pairs you have to count first. The commonest wrong answers confuse the forces between molecules with the bonds inside them, so keep those two ideas apart throughout.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

IMAT home All formulas on one page

Subtopic notes

Formula & revision sheet

4 formulas · 11 reference tables · 21 gotchas across all subtopics — the exam-eve cheat-sheet

Ionic, Covalent and Metallic Bonds

Formulas (2)

Electronegativity difference and bond character · Electronegativity difference $\Delta\chi = \left|\chi_A - \chi_B\right|$
Ionic bonds and the formula of an ionic compound · Charge balance in an ionic formula $\mathrm{M}_a\mathrm{X}_b: \quad a \times (\text{charge of M}) + b \times (\text{charge of X}) = 0$

Reference tables (1)

Metallic bonding and the properties it explains5 rows

Property	Explanation
Conducts electricity as a solid and as a liquid	Delocalised electrons move through the lattice when a voltage is applied
Good conductor of heat	Free electrons carry energy quickly through the metal
Malleable and ductile	Layers of ions slide over each other and the electron sea still holds them
Usually high melting point	Strong attraction between the ions and the electron sea
Melting point rises from Na to Mg to Al	More delocalised electrons per atom (1, 2, 3) and smaller, more charged ions

Watch out for (4)

Pure covalent means identical atoms, not just similar ones
→ Electronegativity difference and bond character
A metal and a non-metal are not always ionic
→ Electronegativity difference and bond character
An ionic formula is a ratio, not a molecule
→ Ionic bonds and the formula of an ionic compound
Solid ionic compounds do not conduct; solid metals do
→ Metallic bonding and the properties it explains

Covalent Bonds, Lewis Structures and the Octet Rule

Formulas (1)

Drawing a Lewis structure by counting electrons · Electrons to place in a Lewis structure $N_e = \sum V - q$

Reference tables (2)

Single, double and triple covalent bonds3 rows

Carbon to carbon bond	Shared electrons	Made of	Length (pm)	Bond energy (kJ/mol)
Single, as in ethane	2	1 σ	154	about 348
Double, as in ethene	4	1 σ + 1 π	134	about 614
Triple, as in ethyne	6	1 σ + 2 π	120	about 839

Twice the single bond would be 696 kJ/mol, so the double bond (614) is less than twice as strong.

Exceptions to the octet rule6 rows

Molecule	Kind of exception	Electrons around central atom	Note
$\mathrm{BeCl_2}$ (gas)	Incomplete octet	4	Two bonding pairs, linear
$\mathrm{BF_3}$	Incomplete octet	6	Accepts a lone pair to reach 8
$\mathrm{AlCl_3}$	Incomplete octet	6	Pairs up as $\mathrm{Al_2Cl_6}$ , where each Al has 8
$\mathrm{PCl_5}$	Expanded octet	10	Five bonding pairs
$\mathrm{SF_6}$	Expanded octet	12	Six bonding pairs
NO	Odd electron	11 valence electrons in total	One unpaired electron

Watch out for (5)

A triple bond shares six electrons, not three
→ Single, double and triple covalent bonds
A C=C bond is not twice as strong as a C-C bond
→ Single, double and triple covalent bonds
Count the charge of an ion
→ Drawing a Lewis structure by counting electrons
A dative bond is not weaker or different once formed
→ Dative (coordinate) covalent bonds
Only period 3 and lower can expand the octet
→ Exceptions to the octet rule

VSEPR Shapes, Bond Angles and Polar Molecules

Formulas (1)

Counting electron domains on the central atom (VSEPR) · Domains on the central atom $D = \frac{V + m - q}{2}$

Reference tables (2)

The VSEPR shapes and their bond angles11 rows

Domains	Lone pairs	Shape	Bond angle	Examples
2	0	Linear	180°	$\mathrm{BeCl_2}$ , $\mathrm{CO_2}$ , $\mathrm{C_2H_2}$
3	0	Trigonal planar	120°	$\mathrm{BF_3}$ , $\mathrm{SO_3}$ , $\mathrm{CO_3^{2-}}$
3	1	Bent	just under 120°	$\mathrm{SO_2}$ , $\mathrm{O_3}$
4	0	Tetrahedral	109.5°	$\mathrm{CH_4}$ , $\mathrm{NH_4^+}$ , $\mathrm{SO_4^{2-}}$
4	1	Trigonal pyramidal	about 107°	$\mathrm{NH_3}$ , $\mathrm{PCl_3}$ , $\mathrm{H_3O^+}$
4	2	Bent	about 104.5°	$\mathrm{H_2O}$ , $\mathrm{H_2S}$
5	0	Trigonal bipyramidal	120° and 90°	$\mathrm{PCl_5}$
5	1	Seesaw	below 120° and 90°	$\mathrm{SF_4}$
5	2	T-shaped	about 90°	$\mathrm{ClF_3}$
6	0	Octahedral	90°	$\mathrm{SF_6}$
6	2	Square planar	90°	$\mathrm{XeF_4}$

Polar and non-polar molecules: when bond dipoles cancel8 rows

Molecule	Shape	Bond dipoles cancel?	Polar molecule?
$\mathrm{CO_2}$	Linear	Yes	No
$\mathrm{H_2O}$	Bent	No	Yes
$\mathrm{BF_3}$	Trigonal planar	Yes	No
$\mathrm{NH_3}$ , $\mathrm{PCl_3}$	Trigonal pyramidal	No	Yes
$\mathrm{CCl_4}$	Tetrahedral, four identical atoms	Yes	No
$\mathrm{CHCl_3}$ , $\mathrm{CH_3Cl}$	Tetrahedral, mixed atoms	No	Yes
$\mathrm{PCl_5}$ , $\mathrm{SF_6}$	Trigonal bipyramidal, octahedral	Yes	No
HCl, HBr	Linear, two different atoms	Only one bond	Yes

Watch out for (4)

Forgetting the charge changes the shape
→ Counting electron domains on the central atom (VSEPR)
Ammonia is pyramidal, not tetrahedral
→ The VSEPR shapes and their bond angles
Polar bonds can still give a non-polar molecule
→ Polar and non-polar molecules: when bond dipoles cancel
A tetrahedral molecule can be polar
→ Polar and non-polar molecules: when bond dipoles cancel

Intermolecular Forces, Boiling Points and Solubility

Reference tables (3)

The three intermolecular forces and where each acts4 rows

Force	Acts between	Cause	Relative strength
London (dispersion)	All molecules and noble gas atoms	Temporary dipole inducing a dipole in a neighbour	Weakest for small molecules; grows with size
Dipole-dipole	Polar molecules	Permanent $\delta^+$ and $\delta^-$ ends attracting	Stronger than London for molecules of similar size
Hydrogen bond	Molecules with H bonded to N, O or F	Very $\delta^+$ H attracted to a lone pair on N, O or F	Strongest intermolecular force
Ion-dipole	An ion and a polar molecule	Ion charge attracting the opposite end of the dipole	Strong; explains salts dissolving in water

Hydrogen bonding: which substances have it9 rows

Substance	H bonded to N, O or F?	Hydrogen bonds between its molecules?
Water, $\mathrm{H_2O}$	Yes, O to H	Yes
Ammonia, $\mathrm{NH_3}$	Yes, N to H	Yes
Hydrogen fluoride, HF	Yes, F to H	Yes
Ethanol, $\mathrm{C_2H_5OH}$	Yes, O to H	Yes
Methylamine, $\mathrm{CH_3NH_2}$	Yes, N to H	Yes
Methane, $\mathrm{CH_4}$	No	No
Hydrogen sulfide, $\mathrm{H_2S}$	No, H on S	No
Dimethyl ether, $\mathrm{CH_3OCH_3}$	No, every H is on C	No
Propanone, $\mathrm{CH_3COCH_3}$	No, every H is on C	No

Solubility: like dissolves like6 rows

Substance	Dissolves well in	Reason
Sodium chloride	Water, not hexane	Ion-dipole attraction to water molecules
Ethanol	Water, in any proportion	Hydrogen bonds with water
Glucose	Water	Several O to H groups hydrogen bond with water
Iodine, $\mathrm{I_2}$	Hexane, only slightly in water	Non-polar, London forces only
Cooking oil	Hexane, not water	Long non-polar chains
Hexan-1-ol	Hexane better than water	The long chain outweighs one O to H group

Watch out for (5)

London forces act in polar molecules too
→ The three intermolecular forces and where each acts
Having O or F in the molecule is not enough
→ Hydrogen bonding: which substances have it
Boiling breaks intermolecular forces, not covalent bonds
→ Ranking boiling points from intermolecular forces
Isomers with the same mass can boil at different temperatures
→ Ranking boiling points from intermolecular forces
A molecule with polar bonds may still be non-polar and insoluble in water
→ Solubility: like dissolves like

Giant Structures, Simple Molecules and Types of Solid

Reference tables (3)

The four types of crystalline solid and their properties4 rows

Type	Particles and what holds them	Typical properties	Examples
Giant ionic	Ions; ionic bonds in all directions	High melting point; brittle; conducts molten or in solution	NaCl, MgO, $\mathrm{CaF_2}$
Giant covalent	Atoms; covalent bonds throughout	Very high melting point; hard; insoluble; insulator (graphite apart)	Diamond, graphite, $\mathrm{SiO_2}$ , silicon
Metallic	Cations in delocalised electrons	Conducts as a solid; malleable; shiny	Cu, Fe, Na
Simple molecular	Molecules; intermolecular forces	Low melting point; soft; insulator	Ice, $\mathrm{I_2}$ , dry ice ( $\mathrm{CO_2}$ ), sugar

Diamond, graphite, silicon dioxide and the sodium chloride lattice4 rows

Structure	Arrangement	Bonding	Key properties
Diamond	Each C joined to 4 C, tetrahedral	Covalent, 3D network	Very hard; very high melting point; does not conduct
Graphite	Each C joined to 3 C in hexagonal layers	Covalent in layers; weak forces between layers	Soft, slippery; conducts along layers; high melting point
Silicon dioxide	Each Si joined to 4 O, each O to 2 Si	Covalent, 3D network	Hard; high melting point; does not conduct
Sodium chloride	Each ion surrounded by 6 of the opposite charge	Ionic	High melting point; brittle; conducts molten or dissolved

Elements, compounds, atoms and molecules: classifying substances7 rows

Substance	Element or compound	Smallest units
Helium, He	Element	Single atoms
Oxygen, $\mathrm{O_2}$ ; ozone, $\mathrm{O_3}$	Element	Molecules
Sulfur, $\mathrm{S_8}$	Element	Molecules
Diamond, C	Element	Giant covalent network (no molecules)
Iron, Fe	Element	Metallic lattice (no molecules)
Ammonia, $\mathrm{NH_3}$	Compound	Molecules
Potassium bromide, KBr	Compound	Ions (no molecules)

Watch out for (3)

Simple molecular does not mean weak covalent bonds
→ The four types of crystalline solid and their properties
Graphite is soft but melts very high
→ Diamond, graphite, silicon dioxide and the sodium chloride lattice
"Molecule" and "compound" are different ideas
→ Elements, compounds, atoms and molecules: classifying substances

PYQ weightage by concept

17 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

Ionic, Covalent and Metallic Bonds0 PYQs · 0%

Concept	PYQs	Share
Electronegativity difference and bond characterfoundation	—	—
Ionic bonds and the formula of an ionic compoundfoundation	—	—
Metallic bonding and the properties it explainsfoundation	—	—

Covalent Bonds, Lewis Structures and the Octet Rule0 PYQs · 0%

Concept	PYQs	Share
Single, double and triple covalent bondsfoundation	—	—
Drawing a Lewis structure by counting electronsfoundation	—	—
Dative (coordinate) covalent bondsfoundation	—	—
Exceptions to the octet rulefoundation	—	—

VSEPR Shapes, Bond Angles and Polar Molecules0 PYQs · 0%

Concept	PYQs	Share
Counting electron domains on the central atom (VSEPR)foundation	—	—
The VSEPR shapes and their bond anglesfoundation	—	—
Polar and non-polar molecules: when bond dipoles cancelfoundation	—	—

Intermolecular Forces, Boiling Points and Solubility0 PYQs · 0%

Concept	PYQs	Share
The three intermolecular forces and where each actsfoundation	—	—
Hydrogen bonding: which substances have itfoundation	—	—
Ranking boiling points from intermolecular forcesfoundation	—	—
Solubility: like dissolves likefoundation	—	—

Giant Structures, Simple Molecules and Types of Solid0 PYQs · 0%

Concept	PYQs	Share
The four types of crystalline solid and their propertiesfoundation	—	—
Diamond, graphite, silicon dioxide and the sodium chloride latticefoundation	—	—
Elements, compounds, atoms and molecules: classifying substancesfoundation	—	—