Bonding and Structure

Intramolecular bonding

Learning objectives
• Describe the trends in the periodic table
• Understand and predict the formation of ionic bonds
• Understand and predict covalent bonds
• Describe electronegativity
• Describe molecular shape, including the Valence Shell Electron Pair
Repulsion (VSEPR) model


Problem statement
• Why is water so important to life?
– To answer this question, we need to delve into
the fundamental aspects of bonding in order to
explain some of the unique properties of water.

• Our body is made up of approximately 80
percent water. Salts are essential in our body to
create the right balance – why is this?


The periodic table
• Consider the periodic table to understand how atoms are bonded to one
another.


Bonding: caring and sharing
• What do the group numbers tell
us?
– The group numbers tell us the
number of electrons in the outer
shell.
– These valence electrons form
bonds with other atoms.
– Atoms combine to become more
stable.

• Bonding occurs in different ways, Click the
depending on the atoms involved: magnifying glass.
– Covalent
– Polar covalent
– Ionic


Bonding: caring and sharing
• What do the group numbers tell
us?
– The group numbers tell us the
number of electrons in the outer
shell.
– These valence electrons form
bonds with other atoms.
– Atoms combine to become more
stable.

• Bonding occurs in different ways, Click the
depending on the atoms involved: magnifying glass
– Covalent
– Polar covalent
– Ionic


Covalent bonding
• A covalent bond is a bond in which the atoms
share electrons.
• They are formed between two non-metals.
• A non-polar covalent bond is a covalent bond in
which the bonding electrons are shared equally by
the bonded atoms, resulting in a balanced
distribution of electrical charge:
– Examples: H2, N2, O2, F2, Cl2, Br2, I2

• A polar covalent bond is formed when electrons
are unequally shared between two atoms.
– Examples: hydrogen-oxygen bond in the water
molecule Consider the
question before
• Do you think methane has polar or non-polar continuing.
covalent bonding?


Ionic bond
• Ionic bonds are usually formed between
metals and non-metals.
• An ionic compound is composed of a positive
and a negative ion that are combined so that
the number of positive and negative charges
are equal.
• Sodium is in group 1A
– It loses an electron to become a cation.

• Chlorine is in group 7A
– It takes an electron to become an anion.
Sodium Chloride


Electronegativity
• Electronegativity is the ability of an atom in a molecule to attract electrons in
the chemical bond towards it.
• Electronegativity is affected by both the atomic weight and the distance of the
valence electrons from the nucleus of the atom.
• The electronegativity provides one way to tell the
difference between a polar and non-polar
covalent bond:
- If the electronegativities are equal, the compound is
non-polar.
- If the electronegativities are not equal, the
compound is polar.
- If the difference in electronegativities are between:
• 1.7 to 4.0, it is an ionic bond
▪ 0.0 to 0.3, it is a non-polar covalent bond Click the
▪ 0.3 to 1.7, it is a polar covalent bond magnifying glass.


Exercises
• Use your mouse to click the correct cell or cells in each column. You will
need to refer to an electronegativity periodic table to complete the first
table.


Answer

PROPERTIES
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The universal solvent
• Consider how salt and water mix.
– We know that water is comprised of a polar covalent
molecule made up of two hydrogen atoms, which
have a partial positive charge, and a single atom of
oxygen, which has a partial negative charge.
– Looking at the positions in the periodic table of Na
and Cl and understanding the electronegativity
difference, we can establish that sodium chloride is
ionic.
– Water molecules collide with NaCl and the polar ends
of the water molecules attract oppositely charged Click the
ions strongly enough to pull them away from their magnifying glass.
ionic crystal lattice.
– When this happens, the forces on the molecules are
stronger than the bond that holds the sodium and
chloride together, so the ions separate, and the
sodium chloride dissolves.


The universal solvent
• Consider how salt and water mix.
– We know that water is comprised of a polar covalent
molecule made up of two hydrogen atoms, which
have a partial positive charge, and a single atom of
oxygen, which has a partial negative charge.
– Looking at the positions in the periodic table of Na
and Cl and understanding the electronegativity
difference, we can establish that sodium chloride is
ionic.
– Water molecules collide with NaCl and the polar ends
of the water molecules attract oppositely charged Click the
ions strongly enough to pull them away from their magnifying glass
ionic crystal lattice.
– When this happens, the forces on the molecules are
stronger than the bond that holds the sodium and
chloride together, so the ions separate, and the
sodium chloride dissolves.


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Intermolecular bonding

Learning objectives
• Predict physical and chemical properties of molecules
• Explain the overall structure of water


Problem statement
• ‘Hot and cold’
• Can we use bonding interactions between
molecules to understand why:
– Solid metals sink in liquid metal, but ice floats in
water?
– Water has a high boiling point?


Why does ice float?
• One interesting property of water is that its solid
form is less dense than its liquid form.
• How does the chemical bonding in water explain
its properties?


Intermolecular bonding in water
• The water molecule has two important
properties that underlie its importance to life.
– It is polar.
– It is highly cohesive.

• Why is the water molecule so cohesive?


Hydrogen bonding
• The most important property of water is the
ability to form hydrogen bonds. How strong or
weak are they, and what effect will they have on
the density of ice?
• Hydrogen bonds are weak attractions between
the partially negative oxygen of one water
molecule and the partially positive hydrogen of
a different water molecule.
• The small size of hydrogen, along with the
shape and polarity of the water molecule, adds
up to a relatively strong attraction between
water molecules.
• Hydrogen bonding is the strongest
intermolecular force, and it plays an important
role in the formation of ice.

Case studies

Learning objective
• Explain how an understanding of intermolecular forces can help us
explain:
– Why oil and water do not mix
– Why drug solubility is critical for activity in the body

Case studies

Problem statements
• How would you clean up an oil spill?
• Some drugs have limited water solubility –
how do they get into your body and act?

Case studies

The Exxon Valdez oil spill
• On 24 March 1989, the Exxon Valdez oil
tanker scraped along a rocky ocean reef
in Alaska, cutting open the hull of the ship.
Thousands of tonnes of oil spilled into the
ocean − enough to fill a football stadium.
• How was this cleaned up?

Case studies

Why don’t oil and water mix?
• Water is held together by hydrogen bonds and can interact
efficiently with anything that has well-developed permanent
charges.
• Oils, however, are made up of C and H atoms, which are
non-ionic and non-polar. These molecules interact by London
or van der Waals dispersion forces.
• Therefore, the interaction between oil and water molecules is
not strong.
– Oil cannot hydrogen bond because it is non-polar.
– Water can't form strong London forces to the oil because it has a
different polarisability.

• A second issue is that water molecules are much smaller
than most other molecules, so to accommodate oil
molecules, many water molecules have to have their
hydrogen bonds broken.

Case studies

Water and oil in biology
• This difference between non-polar ‘hydrophobic’
and polar ‘hydrophilic’ molecules is exploited in
biology.
• Fats (i.e. oils) are used for storage and need to
remain in a cell.
• Sugars, which need to be moved quickly around
the body, are very hydrophilic.
• Cell membranes are formed by amphiphiles, with
one end hydrophilic and the other oily
(hydrophobic).
• Transmembrane proteins are anchored in the cell
membrane by having hydrophobic sidegroups.
Globular proteins have large amounts of
hydrophobic sidegroups that make them fold with
these on the inside, away from the water.

Case studies

Cleaning up the oil spill
• In the case of the Exxon Valdez oil spill, the
first clean-up response was through the use
of a dispersant, a surfactant and solvent
mixture.
• This was applied using a helicopter and was
quite successful, reducing 113,400 litres of oil
to 1,134 litres of removable residue.
• However, there was not enough wave action
to mix the dispersant with the oil in the water,
so this approach was discontinued, and
booms and skimmers were then used to
mechanically clean up the oil – again using
the fact that oil and water do not mix.

Case studies


Bonding and Structure

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