Formation of Covalent Bonds

  • 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.

Covalent bonding in non-metals, IGCSE & GCSE Chemistry revision notes

Diagram showing covalent bonding in a molecule of carbon dioxide

Simple Molecules, Polymers & Giant Structures

  • 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-Hydrogen_, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of hydrogen

Dot-&-cross-Chlorine_, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of chlorine

Dot & Cross Oxygen, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of oxygen

Dot & cross Nitrogen, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of nitrogen

Dot & cross Hydrochloric-Acid, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of hydrogen chloride

Dot & cross Water, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of water

Dot & cross Ammonia, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of ammonia

Dot & cross Methane, IGCSE & GCSE Chemistry revision notes

Dot & cross representation of a molecule of methane

Limitations of Models

  • 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

  • Advantages:
    • Useful for illustrating the 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.

Electron Dot & Cross Ammonia, IGCSE & GCSE Chemistry revision notes

Dot and cross model of ammonia showing how the electrons are shared between the N and H atoms

Electron-Dot-&-Cross-Ammonia, IGCSE & GCSE Chemistry revision notes

Alternative dot and cross which shows the electrons only

Ball and Stick Model

  • Advantages:
    • Useful for illustrating the arrangement of atoms in 3D space.
    • Very 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 Ammonia, IGCSE & GCSE Chemistry revision notes

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

  • 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.

Ammonia-Displayed-Formula, IGCSE & GCSE Chemistry revision notes

Displayed formula of ammonia

3D Representations of Ionic Solids

  • Advantages:
    • 3D drawings and models depict the arrangement in space of the ions.
    • Also show the repeating pattern in giant lattice structures.
  • Disadvantages:
    • Only illustrate the outermost layer of the compound.
    • These diagrams are difficult and time consuming to draw.

Ionic Lattice Silver Fluoride, IGCSE & GCSE Chemistry revision notes

3D representation of the ionic lattice structure of silver fluoride

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Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.