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1: Structure and Bonding 15-16

1
...

The
particles only vibrate, they
can not change position, as
they do not have enough
energy to overcome the
strong forces of attraction
holding them together
...

 Incompressible – the particles are in close
contact, and cannot be forced any closer
...


As the solid is heated, its temperature rises, as the particles gain kinetic energy,
and vibrate more rapidly
...
The temperature stays constant (at the
melting point) until all of the solid has melted, as the heat energy is going into
overcoming the forces of attraction, and not into increasing the kinetic energy of the
particles
...
They are able to
move around, but cannot
separate fully from each
other, as they do not have
enough energy to overcome
completely the forces of
attraction
holding
them
together
...

 Incompressible – the particles are still in close
contact, and cannot be forced any closer
...


As the liquid is heated, its temperature rises, as the particles gain kinetic energy,
and move around more rapidly
...
The temperature stays constant (at the boiling
point) until all of the liquid has boiled, as the heat energy is going into overcoming
the forces of attraction, and not into increasing the kinetic energy of the particles
...
They are in rapid,
constant, random motion,
as they have sufficient
energy to have overcome
completely the forces of
attraction between them
...

 Easily compressible – the particles are far apart,
and can easily be forced closer together
...


1: Structure and Bonding 15-16

EVIDENCE
Although atoms and molecules are too small ever to be observed with even the most powerful
microscope, their effects may still be seen under appropriate conditions
...
The purple colour slowly spreads out from the crystal, as the particles (permanganate
ions) move around randomly and spread out through the water molecules
...

The same effect can be seen if a drop of bromine is placed at the bottom of a covered gas jar
...
This is much more rapid than diffusion of a coloured solution, since
the particles in a gas are much further apart, and move more rapidly
...
The structure of a
substance describes how the particles are arranged, and should not be confused with its bonding,
which describes the nature of the forces which the atoms together in a compound
...
This typically gives them low melting and boiling points; although
there are strong bonds within the molecules, the forces between the molecules are weak, and do not
require much heat energy to overcome them
...
This typically gives them high melting and
boiling points, as a lot of heat energy is needed to overcome this strong bonding
...


Strong bonding
occurs throughout the
entire structure
...


Weak forces
between the
molecules
(intermolecular
forces)

BONDING
There are three different types of bonding which hold together the atoms in substances
...

ATOMIC STRUCTURE
Atoms consist of a central nucleus, of positively charged protons and uncharged neutrons, of equal
mass
...
Atoms have no overall charge, because the total number of
electrons is equal to the total number of protons, which can be found on the periodic table as the atomic
number, usually written at the bottom left of the symbol
...
The number of
electrons in the outermost shell of an element is equal to its group number
...

When dissolved in water, acids also form ions
...
The atoms thereby achieve a
full outer shell of electrons, and so become stable
...
This can be represented by a dot-and-cross

diagram:

e
...
magnesium chloride
1
...
Draw the correct number of each type of atom, as given by the formula, showing their outer shell
electrons only
...

Chlorine is in group VII, and so the chlorine
atoms have 7 electrons in their outer shells
...
Show the transfer of all of the outer electrons from the metal to the outer shell of the non-metal,
such that each metal atom loses all of its outer electrons, and each non-metal atom achieves a
full outer shell of 8 electrons:

Electrons are transferred from the metal,
magnesium, to the non-metal, chlorine, as
shown by the arrows
...
Draw the final result, placing the ions in square brackets, and remembering to show the charge
on each ion
...

Magnesium has lost 2 electrons, so has
become a 2+ ion
...

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1: Structure and Bonding 15-16

PROPERTIES OF IONIC COMPOUNDS
Remember:

METALS IONS ALWAYS FORM POSITIVE IONS
...


The ions formed have opposite charges – positive for the metal, negative for the non-metal
...
The ions are generally arranged in an ionic lattice – a giant, highly regular structure, placing
positive ions next to negative, and vice versa, to maximise the total attraction, such as in the sodium
chloride lattice:

Na+ ion

Cl- ion

Ionic compounds typically have the following properties:
PROPERTY

EXPLANATION

High melting and boiling points

The ionic bonding – ionic compounds have a giant structure, with strong
electrostatic attraction between positive and negative ions, requiring a lot of
heat energy to overcome
...


Brittle

To reshape a solid, layers of particles must slide past each other
...
The ions repel each other, and the crystal shatters
...
Polar solvents consist of molecules with slightly positive and
negative ends
...

Ionic compounds are insoluble in non-polar solvents, which do not have
charged ends to the molecules, and so cannot attract the ions
...
When an ionic compound is solid, the ions are held in place in the
lattice – able to vibrate, but not to move around - hence the solid is an
electrical insulator
...
The presence of free-moving charged
particles enables the liquid or solution to conduct electricity, although the
compound is broken down into elements by the current
...
All others have negative charges
...


The size of the charge on the ions can have a considerable effect on the properties of the ionic
compound
...
MgO consists of ions with two units of charge – Mg2+ and O2- - which therefore
attract each other much more strongly than the singly charged Na+ and Cl- ions in NaCl, so much more
heat energy is required to separate them
...

Each metal atom loses all of its outer shell electrons, thereby becoming a positive ion
...
The outer shell electrons which they
have lost are ‘delocalised’ – they are free to move throughout the entire metal
...

e-

e-

ee

ee-

e-

e-

e-

e

e-

-

e-

e
e-

Regular lattice of positive metal ions
...

e-

ee-

PROPERTY

EXPLANATION

High melting and boiling point,
high tensile strength

Metals have a giant structure, with strong electrostatic attraction
between positive metal ions and delocalised electrons, requiring large
amounts of heat energy, or strong forces, to overcome it
...
This can happen without disrupting the metallic
bonding – the metal still consists of a regular lattice of positive metal
ions, surrounded by a cloud of delocalised electrons
...


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1: Structure and Bonding 15-16

COVALENT BONDING
Covalent bonding occurs in substances formed only from non-metals – if a metal is present in a
compound, the bonding will be ionic
...

A COVALENT BOND IS A SHARED PAIR OF ELECTRONS
When the atoms come together, their outer shells overlap
...
As with ionic bonding, it is important to
remember that the number of electrons in the outer shell of any atom is equal to the group number of
the element
...
As
with ionic bonding, covalent bonding can be illustrated in a dot-and-cross diagram:

e
...
oxygen
1
...
Draw the correct number of each type of atom, as given by the formula, showing the correct
number of electrons in the outer shell of each:

Oxygen is in group VI, so there are six
electrons in the outer shell of each atom
...
To show the bonding in the molecule, consider the valency of each atom – this is the number of
covalent bonds which it must form
...
Each line in this diagram represents a covalent bond – a pair of
electrons, one from each atom
...
This
corresponds to two shared
pairs of electrons between the
atoms
...
Consider how many outer shell electrons each atom had initially
...
Every atoms should now have a full outer shell
...
Two are accounted for in the covalent
bond, leaving 4 electrons (2 pairs) on each oxygen
as non-bonding pairs
...

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1: Structure and Bonding 15-16

EXAMPLES

Hydrogen

H

Cl

H

Each chlorine atom needs
one more electron to fill its
outer shell, so they share one
electron each
...
The first shell can only
hold 2 electrons, so is full
...
Each nitrogen atom
has a non-bonding pair of
electrons
...


Cl

Carbon
dioxide

O

H

C

O

H

+

Ammonia

N

+

+

H

H

H

N

H

Carbon has valency 4, and
oxygen valency 2
...

The nitrogen shares one
electron each with three
hydrogen atoms, leaving a
pair of non-bonded electrons
in its outer shell
...
The oxygen
also has 2 non-bonding pairs,
giving 8 electrons in the outer
shell
...
The carbon
atom has a full outer (2nd)
shell of 8 electrons
...

They still have a full outer
shell of 8 electrons
...


PROPERTIES
Most covalently bonded compounds have a simple molecular structure – they form small, discrete
molecules, in which the atoms are held together by the covalent bonds
...


Soft

To break apart a simple molecular solid, it is not necessary to break
the strong covalent bonds which hold the atoms together to form
molecules
...


Electrical insulators

There are no ions present in covalent compounds, and all electrons
are localised on particular atoms, or in the covalent bonds, so there are
no free-moving charged particles to carry an electric current
...


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1: Structure and Bonding 15-16

GIANT COVALENT SUBSTANCES
Not all covalently bonded substances have simple molecular structures
...

This type of substance is best illustrated using two structural forms (allotropes) of carbon
...


Layer lattice, with each carbon atom forming three
covalent bonds to three other carbon atoms, giving
hexagonal layers of atoms
...
This results in single carbon atoms with
no bonds between them, so the solid turns directly to a
gas
...
This results in single
carbon atoms with no bonds between them, so the
solid turns directly to a gas
...


Soft – although the covalent bonds within the layers
are difficult to break, each layer is held to the next only
by weak intermolecular forces
...


Insoluble in all solvents – to dissolve the substance
would require the strong covalent bonds between the
atoms, and no solvent is powerful enough to do this
...


Electrical insulator – all electrons are localised in
covalent bonds, so there are no free-moving charged
particles to carry an electric current
...

These electrons are delocalised – they are free to
move throughout the structure, and can therefore
carry an electric current
...


Uses: in lubricating oils, and as pencil ‘lead’ – the weak
forces between the hexagonal layers allow the layers
to slide over each other easily, or to rub off onto
paper
...
Why do ionic compounds conduct when molten, but not when solid?
Incorrect:

When ionic solids are melted, the electrons become free to move around, and are
able to carry an electric current
...
When molten, or in solution, the ions
are no longer held in place in a lattice, and are free to move and carry an electric
current
...
Why does water have a much lower melting point that sodium chloride?
Incorrect:

The ionic bonding in sodium chloride is strong, but the covalent bonds in water
are weak, so do not require much heat energy to overcome
...
Water has a simple molecular structure – although the
covalent bonds holding the hydrogen and oxygen atoms together are very strong,
these are not broken on melting – it is the WEAK INTERMOLECULAR FORCES
between the molecules which need to be overcome, and this requires little heat
energy
...


SIMPLE
MOLECULAR

GIANT
COVALENT

Non-metals

Non-metals

Giant

Giant

Simple

Giant

BONDING

DESCRIPTION

TYPICAL
PROPERTIES

EXAMPLES

Metallic

Giant lattice of positive metal ions
held together by strong
electrostatic attraction to a ‘sea’ of
delocalised outer-shell electrons
...


Hard but brittle; high
melting and boiling
points; electrical
insulators as solids,
but conductors when
molten or in solution

Sodium chloride
(salt);
magnesium
oxide

Covalent

Small, separate molecules,
consisting of clusters of atoms
held together by strong covalent
bonds (shared pairs of electrons),
but with only weak intermolecular
forces between the molecules
...


METALLIC

STRUCTURE

Sublimes at very high
temperatures; very
hard; electrical
insulator; insoluble

Diamond

Layer lattice of atoms held
together by strong covalent bonds,
producing giant molecular layers
attracted to each other by weak
intermolecular forces

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