Structures - (CCEA)

• Structure describes how atoms are arranged once they have bonded together.

There are 4 main categories of structure:

• Ionic substances have a giant ionic lattice structure, with a large repeating pattern of oppositely charged ions.

• Covalent substances have two different structures: molecular covalent and giant covalent.

• Metallic substances have a metallic structure, with regular layers of positive ions surrounded by delocalised electrons.

In chemistry, ‘bonding’ describes how atoms are held together.

‘Structure’ describes how atoms are arranged once they are bonded together.

The type of structure depends on the type of bonding.

There are two different structures for covalent bonding.

When ionic bonding forms between a metal and a non-metal, lots of ions group together to form a giant ionic lattice.

A giant ionic lattice is a three-dimensional structure of oppositely charged ions held together by ionic bonds.

A physical property describes how a substance behaves.

There are four main physical properties of giant ionic lattices.

Covalent bonding forms substances with two types of structures:

• Molecular covalent structures. Simple molecules held together by weak forces of attraction. Examples include iodine, methane and carbon dioxide. This is the most common type of covalent structure.

• Giant covalent structures. Many atoms joined together by strong covalent bonds. At GCSE the examples are diamond, graphite and graphene.

The atoms in a molecular covalent structure are held together as molecules by strong covalent bonds.

There are weak intermolecular forces between different molecules.

These weak forces are called van der Waals' forces and can be broken with a small amount of energy.

A giant covalent structure is a three-dimensional structure of atoms that are joined together by lots of strong covalent bonds.

Carbon is a non-metal element that can form three different giant covalent structures: diamond, graphite and graphene.

A carbon atom can form up to four covalent bonds.

Diamond, graphite and graphene are examples of allotropes.

Allotropes are different forms of the same element, in the same physical state.

Diamond is a giant covalent structure in which each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral, three-dimensional structure.

Diamond’s properties include:

Diamond is used in cutting tools for drilling through hard materials like rock and glass.

Graphite has a giant covalent structure in which:

• each carbon atom forms three covalent bonds with other carbon atoms
• the carbon atoms form layers of hexagonal rings
• there are weak forces of attraction between the layers
• there is one, non-bonded delocalised electron for each carbon atom

Graphite’s properties include:

Graphite is used as a lubricant and in pencils.

In pencils, the layers of carbon atoms can slide over each other and onto the paper.

Graphene is a single-atom thick layer of graphite with strong covalent bonds between each carbon atom.

The atoms are arranged in hexagons.

Graphene’s properties include:

Graphene is used in batteries and solar cells due to its good electrical conductivity.

Metallic structure consists of regular layers of positive metal ions surrounded by delocalised electrons.

The delocalised electrons are free to move throughout the structure.

An alloy is a mixture of two or more elements, at least one of which is a metal.

The mixture has metallic properties.

Alloys are therefore harder than pure metals.

They have metallic structures, but having different sized atoms in the structure makes it more difficult for the layers of ions to slide over each other.

Chemistry exam questions often provide data about a range of substances and ask students to classify them as an ionic lattice, molecular covalent, giant covalent or metallic structures.