Bonding, structure and properties of matter

In a nutshell

The melting and boiling point of substances depend on the type of bonding present between molecules and throughout lattice structures. If there are weak intermolecular forces between molecules of a substance, the melting and boiling point will be low. Data can be used to determine the type of substance present.

Melting and boiling point

The melting and boiling point of a substance is influenced by the type of bonding present.

The melting and boiling point of a substance reflects how much energy needs to be supplied to break bonds/interactions.

In a substance where there are strong bonds/interactions between particles, lots of heat energy must be supplied to break them and the substance will have a high melting and boiling point.

Simple covalent molecules

Simple covalent molecules have strong covalent bonds between atoms.

Simple covalent molecules contain weak intermolecular forces between molecules. Little energy is required to break these weak intermolecular forces, therefore they have relatively low melting and boiling points.

Example

Carbon dioxide is a simple covalent substance.

Chemistry; Elements, compounds and mixtures; KS4 Year 10; Bonding, structure and properties of matter

Giant covalent lattices

In giant covalent lattices, strong covalent bonds extend throughout the structure; lots of energy is required to break these bonds therefore giant covalent structures have very high melting and boiling points.

In some cases, giant covalent structures sublime and go straight from solid to gas.

Examples

Buckminster fullerenes and graphite.

Metals

In metals, the positive metal ions are strongly attracted to the sea of delocalised electrons; this means that metallic bonding is strong and most metals have high melting and boiling points.

1.	Positive metal ions
2.	Sea of delocalised electrons

Chemistry; Elements, compounds and mixtures; KS4 Year 10; Bonding, structure and properties of matter

Ionic lattices

In ionic lattices there are strong electrostatic forces of attraction between negatively and positively charged ions, therefore ionic lattices have high melting and boiling points.

Ionic substances can conduct electricity when they are liquid (or in solution) as the ions are free to move about, however they cannot conduct electricity when solid.

Chemistry; Elements, compounds and mixtures; KS4 Year 10; Bonding, structure and properties of matter

Predicting substances using data

Information such as melting points and boiling points can be used to deduce what type of bonding is present in a substance.

Example

SUBSTANCE	Melting point $(^{\circ}C)$	Boiling point ( $^{\circ}C$ )	ELECTRICAL CONDUCTOR
A	$0$	$100$	No
B	$660$	$2467$	Yes
C	$801$	$1413$	When molten
D	$3550$	$4830$	No

Substance A

Substance A has a low melting and boiling point so it is likely to be a simple covalent substance.

Substance A is water. Ice melts at $0^{\circ}C$ and water boils at $100^{\circ}C$ .

$100^{\circ}C$

Substance B

Substance B has a relatively high melting and boiling point so it is unlikely that it is a simple covalent substance. Substance B can conduct electricity therefore it is likely to be a metal.

Substance B is aluminium.

Substance C

Substance C has a relatively high melting and boiling point so it is unlikely that it is a simple covalent substance. Substance C can conduct electricity when molten so it is likely to be a giant ionic lattice.

Substance C is sodium chloride. Sodium chloride is a solid below $801^{\circ}C$ , a gas above $1413^{\circ}C$ and a liquid between these two temperatures.

$1413 ^{\circ}and a liquid between these two temperatures$

$1413 ∘C1413^{\circ}Cand a liquid between these two temperatures.$

Substance D

Substance D has a very high melting and boiling point so cannot be a simple covalent substance. Substance D cannot conduct electricity, not even in the liquid state, therefore it is likely to be a giant covalent lattice.

Substance D is diamond.