Giant Covalent

When beginning to learn the important concepts and knowledge required to engage with the DP Curriculum it is important to have a basic understanding and experience of some of the ideas in each topic. At AIS the majority of students follow the iGCSE Coordinated Science curriculum as a precursor to studying DP chemistry and this site will use some of the objectives from the iGCSE to provide an area of prior learning - allowing those students who would like to refresh their knowledge or for new students to engage wth matter that has become prior to starting this course.

To this end students are expected to have a basic understanding of the C3 Atoms, elements and Compounds with the direct aim of constructing formula for common elements and compounds and balancing symbol equations.

Giant Covalent Structures

From the previous page on Simple Covalent Molecules you should now understand the nature of covalent bonds

However under certain circumstances atoms can covalently bond to make Giant Structures. For this area we will study Allotropes of Carbon.

An Allotrope is 'A different form of the same element'

Diamond

Diamond is an Allotrope of Carbon

It consists of a Central Carbon atom Covalently Bonded to 4 other carbon atoms in a TETRAHEDRAL STRUCTURE.

Diamond is one of the Hardest Naturally Occurring Substances and this is because each Carbon atom is Covalently Bonded to 4 other atoms. Covalent Bonds are the strongest bonds and so take immense amounts of energy to break. This makes them very hard.

Diamonds are also used for Cutting. They are sharp because, at their peaks, diamonds are only one atom thick. Thus when focusing a Force through the diamond it all concentrates on one atom.

Diamonds have Very large Melting and Boiling Points because you DO NEED TO BREAK STRONG COVALENT BONDS IN ORDER TO SEPARATE THE ATOMS. In Diamond this is 4 Covalent bonds per atom!

Silicon Dioxide is another example of Giant Covalent Bonding with a Tetrahedral Structure that you need to know. This is very similar to diamond in structure however each layer of Carbon atoms is separated by a layer of Oxygen atoms that form two bonds. (See below)

Graphite

Graphite is another allotrope of Carbon. It's very strange that the same substance that we breathe out, makes up sugars we eat and makes diamonds can also be used in pencils!

Graphite has a Repeating Hexagonal Structure that repeats in 2-Dimensional Layers.

Each Carbon has 3 Covalent Bonds to other Carbon Atoms.

This leaves one unbonded free electron per atom.

These 'free' delocalised electrons can move through the structure and so Graphite CAN CONDUCT ELECTRICITY where Diamond cannot.

There are weak intermolecular forces between the layers and so the graphite layers can slip across each other (this is how graphite pencils work) - this also means that graphite can be used as a lubricant.

Like Diamond - Graphite has a very large melting point - this is because you would need to break 3 strong covalent bonds in order to separate the atoms.