- Only non-metal elements participate in covalent bonding.
- As in ionic bonding, each atom gains a full outer shell of electrons.
- When two or more atoms are chemically bonded together, we describe them as ‘molecules’.
- Covalent bonds between atoms are strong and the bonded unit of atoms is what is called a molecule.
- The positively charged nuclei of the atoms in the bond are attracted to the negatively charged area of space where the bonding electrons are by electrostatic forces.
- The forces that act between the molecules are weak intermolecular forces , they are not chemical bonds.
- The shared electrons are called bonding electrons and occur in pairs.
- Electrons on the outer shell which are not involved in the covalent bond(s) are called non-bonding electrons.
Diagram showing covalent bonding in a molecule of carbon dioxide
- Covalent substances tend to be small molecular structures such as H2O or CO2.
- These small molecules are known as simple molecules and differ from ionic compounds which form larger lattice structures.
- Other covalent molecules can be very large, such as polymers or giant covalent structures.
- Common polymers include polythene which is used extensively in plastic bags and polyvinyl chloride (PVC) which has many industrial applications, most notably in the production of water pipes.
- Giant covalent structures include graphite, diamond and silicon dioxide.
- Small covalent molecules are represented using dot-and-cross diagrams.
- You need to be able to describe and draw the structures of the following molecules using dot-and-cross diagrams: hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane.
- The correct dot and cross diagrams for these molecules are shown below.
Dot & cross representation of a molecule of hydrogen
Dot & cross representation of a molecule of chlorine
Dot & cross representation of a molecule of oxygen
Dot & cross representation of a molecule of nitrogen
Dot & cross representation of a molecule of hydrogen chloride
Dot & cross representation of a molecule of water
Dot & cross representation of a molecule of ammonia
Dot & cross representation of a molecule of methane
- There are different ways of representing atoms and molecules depending on the type of substance in question.
- Each method has its advantages and disadvantages.
- The main limitation is that although it is relatively easy to represent smaller molecules, larger and more complex molecules are difficult to show graphically.
Dot and Cross Diagram
- Useful for illustrating the transfer of electrons.
- Indicates from which atom the bonding electrons come from.
- Fails to illustrate the 3D arrangements of the atoms and electron shells.
- Doesn’t indicate the relative sizes of the atoms.
Dot and cross model of ammonia showing how the electrons are shared between the N and H atoms
Alternative dot and cross which shows the electrons only
Ball and Stick Model
- Useful for illustrating the arrangement of atoms in 3D space.
- Very useful for visualizing the shape of a molecule.
- Fails at indicating the movement of electrons.
- The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller.
Ball and stick model of ammonia which illustrates the 3D arrangement of the atoms in space and the shape of the molecule
2D Representations of Molecules
- Displayed formulae are 2D representations and are basically simpler versions of the ball and stick model.
- Adequately indicate what atoms are in a molecule and how they are connected.
- Fail to illustrate the relative sizes of the atoms and bonds.
- Cannot give you an idea of the shape of a molecule and what it looks like in 3D space.
Displayed formula of ammonia
3D Representations of Ionic Solids
- 3D drawings and models depict the arrangement in space of the ions.
- Also show the repeating pattern in giant lattice structures.
- Only illustrate the outermost layer of the compound.
- These diagrams are difficult and time consuming to draw.
3D representation of the ionic lattice structure of silver fluoride