1.7 Types of substance

Question 1

Simple molecular bonding

Answer 1

A simple consists of just a few atoms, joined to each other by strong covalent bonds. Simple molecular substances can be:
Non-metal elements, such as H2, O2, Cl2
Compounds of non-metals, such as HCL, H2O, CH4

Simple molecular substances usually have:
Low melting points
Low boiling points

They are usually in the gas or liquid state room temperature because of this

There are weak attractive forces between molecules, called intermolecular forces.

When a simple molecular substance such as oxygen, O2 or O=O, melts or boils:
intermolecular forces are overcome
covalent bonds do not break

Question 2

Non-conductors of electricity and solubility in water (simple molecular)

Answer 2

Simple molecular substances do not conduct electricity when solid, liquid or gas. This is because their molecules:
Are not electrically charged, and do not contain electrons that are free to move

Solubility in water
Many simple molecular substances are insoluble in water. The intermolecular forces between water and these substances are weaker than those between:
water molecules
molecules of the substance itself

Simple molecular substances dissolve in water if they can form strong enough intermolecular forces with water molecules:
Hydrogen and oxygen are sparingly soluble
Chlorine, carbon dioxide, sulfur dioxide and ammonia are soluble in water
Ethanol and ethanoic acid are soluble
Sugar is soluble in water

Question 3

Giant molecular bonding and structure

Answer 3

A giant molecule consists of many atoms. In giant molecules, the atoms are:
Joined by strong covalent bonds
arranged in a regular lattice structure

Giant molecular substances can be:
Non-metal elements, such as carbon
Compounds such as silica

Giant molecular substances usually have:
High melting points
High boiling points

They are solids at room temperature. A lot of energy must be transferred to break the many strong covalent bonds during melting and boiling. Giant molecular substances are insoluble in water.

Question 4

Modelling giant molecules (diamond and graphite)

Answer 4

There are many atoms in an entire giant molecule. You cannot represent an entire giant molecule using displayed formulae or dot-and-cross diagrams.

Diamond
Each atom is bonded to four others
Strong covalent bonds between atoms

Graphite
Each atom is bonded to three others
Weak intermolecular forces between layers
Strong covalent bonds between atoms in a layer

Diamond and graphite are both forms of carbon. They are giant molecular substances.

The rigid lattice structure and strong bonds of diamond make it very hard. This is why it is useful for cutting tools.

In graphite, the weak intermolecular forces let the layers slide over each other. This is why it is slippery and useful as a lubricant.

Question 5

Properties of fullerenes and graphene

Answer 5

Graphene
Graphene is a giant molecular substance. Its structure resembles a single layer of graphite:
Each carbon atom is covalently bonded to three other carbon atoms
It has a regular lattice structure

Fullerenes
Fullerenes resemble a sheet of graphene rolled to form:
Hollow balls, often called buckyballs

Buckminsterfullerene, C60, has carbon atoms arranged in pentagons as well as hexagons. Materials made from buckyballs:
Conduct electricity because they have delocalised electrons
Are soft when in the solid state because they have weak intermolecular forces

Hollow tubes, called carbon nanotubes.
Nanotubes have closed ends or open ends.
They can be several mm long. Nanotubes:
Conduct electricity because they have delocalised electrons
Are very strong because the structure has many strong covalent bonds.

Question 6

Poly(ethene) and simple polymers

Answer 6

Polymers are large molecules made from many smaller molecules, called monomers, joined together.

Poly(ethene) is not a fullereneL
It is a hydrocarbon (a compound of carbon and hydrogen)

Polymer molecules are described as macromolecules rather than giant covalent molecules.

Question 7

Properties of metals

Answer 7

Metals
Appearance = shiny
Electrical conduction = good
Density = high
Melting/boiling point = high

Non-metals
Appearance = dull
Electrical conduction = poor
Density = low
Melting/boiling point = low

Metals are malleable - they can be pressed into shape without shattering
Non-metals are brittle in the solid state - they shatter when bent or hit.

Question 8

Metallic structure and bonding

Answer 8

A metal:
Consists of a giant lattice of positively charged metal ions
Has a ‘sea of delocalised electrons

The delocalised electrons come from the outer shells of the atoms.

Metallic bonds are strong electrostatic forces of attraction between positive metal ions and delocalised electrons.

Question 9

Limitations of models

Answer 9

Ball-and-stick models
You can draw ball-and-stick models. You can also make them using plastic modelling kits.
These models show:
How each atom is bonded to other atoms
The molecule’s three-dimensional shape

They do not show the bonding and non-bonding electrons or each element’s chemical symbol.

Dot-cross diagrams don’t express the relative attraction of shared electrons due to electronegativity

2D diagrams don’t show the 3D arrangement of atoms, and 3D diagrams don’t show the share or transfer of electrons.