GR 11 BONDING, MOLECULE SHAPES, IMFs by Lizelle Swanepoel

CHEMICAL BONDING NOTES

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BOND FORMATION

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COVALENT BONDING

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MOLECULE SHAPES AND VSEPR THEORY

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The shapes of molecules are determined using valence shell electron pair repulsion theory (VSEPR) - it looks at the effects of repulsion between electrons and electron pairs and can be summarised into a few principles:

Molecular shape is determined by minimising repulsion between negative electron pairs.
Electron pairs will always occupy the geometrical shape that corresponds to the lowest energy configuration.
Bond pairs (BP) of electrons repel LESS than lone pairs (LP) due to being localised:
LPLP > BPLP > BPBP

MOLECULAR GEOMETRY

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Molecule shapes

INTERMOLECULAR FORCES - ADDITIONAL NOTES

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EXERCISE: IS THIS H-BONDING?

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INTERPARTICLE FORCES IN SOLIDS - SUMMARY

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Generally referred to as "intermolecular" forces, although:

network solids have interatomic forces between atoms forming a network
ionic solids (salts) have interionic forces between ions forming a crystal structure
metallic solids have electrostatic forces between cations & delocalised electron in a metal crystal structure

BONDING - ADDITIONAL NOTES

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Covalent vs ionic vs metallic bonding

DATIVE COVALENT BONDING

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A dative covalent bond is a bond that is formed by the overlapping of orbitals and the sharing of an electron pair both of which belong to one atom.
The atom that contributes the electron pair (a lone pair) is electron rich - it is called the donor atom.
The atom that accepts the electron pair is electron deficient - it is called the electron acceptor.

Examples:
(a) ammonium ion (NH₄⁺)
(b) hydronium ion (H₃O⁺) - example of oxonium ion (oxygen cation with 3 bonds)

octahedral shape

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MOLECULE SHAPES

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HYDRONIUM ION

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Once a dative covalent bond is formed, there is no difference between it and an ordinary covalent bond. The electron pair involved is usually represented by an arrow.
Formation of the NH₄⁺ion shown below:

1. LONDON FORCES

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London forces occur between non-polar molecules.

Electrons are mobile in the orbitals of a molecule so that at any given moment it is possible that there is an uneven distribution of electrons in the molecule giving it a temporary dipole.
This momentary dipole can induce a temporary dipole in a neighbouring molecule resulting in a temporary weak attraction known as a London force.

The strength of London forces between molecules increases with:

increased number of electrons (which leads to greater polarizability)
increased size of electron cloud (“increased molecular mass” as generic statement results in a larger electron cloud that is more easily polarized)
increased surface area (linear isomers have stronger London forces than branched isomers due to greater contact between molecules)

INTERMOLECULAR FORCES - EXIST BETWEEN MOLECULES

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A short summary of some of the different types of IMF's:

INTERMOLECULAR FORCES AND PROPERTIES DUE TO IMF's

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2021 notes

2. DIPOLE-DIPOLE INTERMOLECULAR FORCES

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This exists between polar molecules (molecules which are dipoles), e.g. between HCl molecules:

3. HYDROGEN BONDING

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Hydrogen bonding is a special type of dipole-dipole attraction and are only found when:

i) hydrogen is covalently bonded to a small and highly electronegative nitrogen,

oxygen or fluorine atom.

ii) there is at least one lone pair on the N, O or F.

e.g. HF, H₂O, NH₃, (plus all alcohols and carboxylic acids)

POLARITY: POLAR VS NON-POLAR VS. IONIC BONDS

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IMF's ANIMATION

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ID THE TYPE OF BONDS

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Determine the difference in electronegativity values then decide the type of bond in the following list:

K₂S

PCl₃

I₂

Answer key:

K₂S:

∆EN = 2.5-0.8 = 1.7
An ionic bond with polar covalent character

PCl₃∆EN = I₂∆EN =HI ∆EN =

SHAPES OF SIMPLE MOLECULES

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PRACTICE PROBLEMS

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MOLECULE SHAPES - NOTES 2021

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The table below shows you how to determine the shape of a molecule by considering the:

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· number of lone pair electrons

· number of electron pairs (electron cloud regions)
You are expected to know the following molecule shapes: linear, angular, trigonal planar, trigonal pyramidal, tetrahedral, trigonal bipyramidal and octahedral.

EXERCISE ON POLARITY, BOND ANGLE AND MOLECULE SHAPE

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Do these questions

CHEMICAL BONDS VS. INTERMOLECULAR FORCES

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There is a distinction between chemical bonds and intermolecular forces.

Intramolecular forces = the forces that exist within a molecule (holding atoms together), these are proper chemical bonds.
Intermolecular forces = the forces that exist between molecules.

Intermolecular forces (IMF's) determine the melting and boiling points of substances.

ANSWERS TO EXERCISE

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Answer Key:

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Two kinds of forces operate in a molecule—intramolecular vs. intermolecular forces:

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Let's try to understand this difference through the following example:

You have six towels— Three are blue in colour labeled hydrogen, and three are pink in colour labeled chlorine.
You are given a sewing needle and thread to sew one hydrogen towel to one chlorine towel.
After sewing, you have three pairs of towels: hydrogen sewed to chlorine.
The next step is to attach these three pairs of towels to each other with Velcro.
Now, pull this assembly from both ends: what do you think will happen?
Imagine the towels to be real atoms, such as hydrogen and chlorine.

These two atoms are bound to each other through a polar covalent bond—analogous to the thread.
Each hydrogen chloride molecule in turn is bonded to the neighboring hydrogen chloride molecule through an intermolecular attraction—analogous to Velcro.

TYPES OF IMF'S

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Hydrogen Bonding Evidence from Boiling Points of the Hydrides

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Explain the trend for H2O, NH3 and HF:

4. ION-DIPOLE FORCES

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BETWEEN IONS OF A SALT DISSOLVED IN A SOLVENT

5. ION INDUCED-DIPOLE FORCES

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E.G. In blood between iron ion and oxygen

Physical properties depend on the strength of intermolecular forces

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The stronger the forces between the molecules or particles:
(a) the higher the melting point
(b) the higher the boiling point
(c) the lower the vapor pressure
(d) the higher the viscosity
(e) the higher the surface tension

EXERCISE

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List all types of IMFs that would occur in each of the following:
a. CH₃CF₃
b. CCl₄
c. SO₂
d. BrF
e. CO₂
f. NH₃
g. Salt water
h. Iron ions in your blood, transporting oxygen gas

WHAT ARE NETWORK SOLIDS?

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A network solid is a solid in which atoms are bonded by strong covalent bonds in a continuous network extending throughout the material. The entire crystal is considered to be a macromolecule (a giant molecule).
These giant molecules can be:
• Plate-like, e.g. graphite
• Three dimensional, e.g. diamond and silicon dioxide

The strong covalent bonds between the carbon atoms explain the hardness, chemical stability and very high melting points of these network solids.

PROPERTIES OF WATER DUE TO H-BONDING

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IONIC SOLIDS

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EXERCISE ON INTERMOLECULAR FORCES

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Consider the following list of substances.
KBr (s) CCl₄ (l) NH₃ (g) HCl (g) Pb (s) Ar (g) Cl₂ (g)

Select the substance which best matches the following descriptions: 1
1 A monatomic substance with London forces between its particles.
2 An excellent conductor of electricity, except in the solid phase.
3 A good solvent for non-polar solids.
4 A good example of a substance in which hydrogen bonds are found.
5 Contains dipole – dipole forces between its molecules.
6 Are non-polar molecules.
7 Contains delocalized electrons.
8 Ionic bonds hold the crystal lattice together.

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Why do substances dissolve (are miscible)?

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When a solute dissolves, it’s particles proceed towards a state of greater DISORDER.
At the same time the solvent (a pure homogeneous substance) also becomes mixed with particles of a different kind.
There is an overall increase in disorder (called entropy), which is the driving force in the dissolution process.
Despite this increase in disorder, which aids dissolution, not all substances dissolve.
Two substances mix perfectly into solution (i.e. dissolve) if:

INTERMOLECULAR FORCES between the molecules/ions of the substance are OVERCOME.
Therefore, when the forces between the molecules of the solute and solvent are of the same ORDER OF MAGNITUDE, then the molecules of the solvent can take the place of the molecules of the solute.
The result is MIXING.
LIKE DISSOLVES LIKE.
Molecular crystals dissolve in non-polar solvents => solute forms Van der Waals LONDON Forces with solvent.
Ionic compounds dissolve in polar solvents => ions of solute form ION-DIPOLE interactions with solvent.

Properties of water due to hydrogen bonding

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High surface tension
High heat capacity
Capillary action due to cohesion and adhesion forces
Density change due to change in temperature

Silicon dioxide (SiO2)

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Diamond vs Graphite

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Diamond cannot conduct electricity, because it has no free electrons in its giant structure.
Graphite can conduct electricity, because it has free (mobile) electrons in its giant structure.

Bonding and IMF Online lesson

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Watch this video for recap of the topic:

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ALLOTROPES OF CARBON

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6. DIPOLE-INDUCED DIPOLE IMFs

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Formation of a covalent bond - Potential Energy Profile

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Larger molecules (like Br₂ and I₂) have shallower energy dips because their bond dissociation energies are lower than those of smaller ones (H₂ and Cl₂).
Atomic size increases down the group, leading to longer bond lengths and more diffuse valence orbitals.
Orbital overlap is less effective in larger atoms due to greater distance and poorer concentration of electron density in the bonding region.
Weaker bonds result, causing less stabilization and a shallower potential energy minimum on the curve.

Formation of covalent bond in H2

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