Covalent bonding (AQA GCSE Combined Science: Trilogy): Revision Note

Formation of covalent bonds

What is covalent bonding?

• Non-metal atoms can share electrons with other non-metal atoms to obtain a full outer shell of electrons

• When two atoms share pairs of electrons, they form covalent bonds

• Covalent bonds form due to the electrostatic attraction between the shared pair of electrons and the positive nuclei of both atoms

• Covalent bonds between atoms are very strong

• When two or more atoms are covalently bonded together, they form ‘molecules’

• Covalently bonded substances may consist of small molecules or giant molecules

• Weak intermolecular forces exist between individual molecules

  • For example, in methane, CH₄:

  • Each molecule consists of four hydrogen atoms with strong covalent bonds to a central carbon atom

  • In between individual methane molecules there are weak intermolecular forces

• 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

• Simple covalent molecules do not conduct electricity as they do not contain free electrons

Covalent bonding

Simple covalent molecules

• Covalent substances tend to have small molecular structures, such as:

  • Cl₂

  • H₂O

  • CO₂

• These small molecules are known as simple molecules

Covalent dot and cross diagrams

• Small covalent molecules can be represented by 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, H₂

Dot & cross representation of a molecule of hydrogen

Chlorine, Cl₂

Dot & cross representation of a molecule of chlorine

Oxygen, O₂

Dot & cross representation of a molecule of oxygen

Nitrogen, N₂

Dot & cross representation of a molecule of nitrogen

Hydrogen chloride, HCl

Dot & cross representation of a molecule of hydrogen chloride

Water, H₂O

Dot & cross representation of a molecule of water

Ammonia, NH₃

Dot & cross representation of a molecule of ammonia

Methane, CH₄

Dot & cross representation of a molecule of methane

Empirical formula

• The empirical formula of a compound shows the simplest whole number ratio of atoms of each element present

• It can be deduced from a dot-and-cross diagram, a structural formula, or any representation showing the atoms in a molecule

• To find the empirical formula:

  1. Count the atoms of each element shown in the diagram or model

  2. Write the ratio of those atoms

  3. Simplify the ratio to the smallest whole numbers

• For example:

  • Ethene is shown with 2 carbon atoms and 4 hydrogen atoms

    • Ratio C:H = 2:4

    • This simplifies to 1:2

    • So, the empirical formula is CH₂

  • Methane is shown with 1 carbon atom and 4 hydrogen atoms

    • Ratio C:H = 1:4

    • This is already the simplest possible ratio

    • So, the empirical formula is CH₄

• The empirical formula and the molecular formula are the same when the ratio cannot be simplified further

  • Examples include:

    • Water H₂O

    • Methane, CH₄

    • Ammonia, NH₃

Polymers & giant structures

Polymers

• Not all covalent molecules are small; covalent molecules can also be very large

• Polymers are very large molecules made from many smaller repeating units joined together by covalent bonds

• Common polymers include:

  • Polythene, used extensively in plastic bags

  • Polyvinyl chloride (PVC), widely used in the production of water pipes and window frames

Formation of polyethene

You should be able to represent the covalent bonds in a variety of molecules, including simple molecules (ethene) and polymers (polyethene)

Giant structures

• Some covalently bonded substances have giant covalent structures, such as graphite, diamond, and silicon dioxide

• These substances form giant crystal structures made from many atoms held together by covalent bonds

Limitations of models

Dot and Cross Diagrams

• Advantages:

  • Useful for illustrating the sharing (or transfer) of electrons

  • Indicates from which atom the bonding electrons come from

• Disadvantages:

  • Fails to illustrate the 3D arrangements of the atoms and electron shells

  • Doesn’t indicate the relative sizes of the atoms

Ball and Stick Model

• Advantages:

  • Useful for illustrating the arrangement of atoms in 3D space

  • Especially useful for visualizing the shape of a molecule

• Disadvantages:

  • 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

2D Representations of Molecules

• Advantages:

  • 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

• Disadvantages:

  • 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

2D representation of ammonia