Bonding: The Structure of matter

Atoms can join together to form units called molecules. This process is called chemical bonding.

There are three main types of chemical bond: ionic, covalent, and metallic.
An ionic bond is the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). This type of bond forms when one atom transfers one or more electrons to another atom.
- Examples: Sodium chloride (NaCl), Magnesium oxide (MgO)
A covalent bond is the sharing of a pair of electrons between two atoms. This type of bond creates a strong link between atoms forming a molecule. Covalent bonds can be single, double or triple depending on the number of electron pairs shared.
- Examples: Oxygen (O2), Methane (CH4)
A metallic bond is the electrostatic attraction between positive metal ions and delocalised electrons. This creates a ‘sea’ of electrons that move freely and contributes to properties such as electrical conductivity and ductility.
- Examples: Copper (Cu), Aluminium (Al)

Lewis structures are diagrams used to represent the bonding between atoms of a molecule and any lone pairs of electrons that may exist in the molecule.
Each dot in a Lewis Dot structure represents a valence electron, while each line represents a bond.
Note that Lewis structures do not reveal the molecule’s shape.

Some covalent molecules, such as water (H2O), have polar bonds. This means the electrons are not shared equally between the atoms. These polar molecules have a slight positive charge on one side and a negative charge on the other side.
Intermolecular forces are attractive forces between neighbouring molecules. They include London dispersion forces, dipole-dipole interactions and hydrogen bonding.
These forces play a vital role in determining the physical properties (melting and boiling points) of substances.

Substances with a molecular structure, like water or carbon dioxide, consist of distinct molecules that are held together by relatively weak intermolecular forces. These substances tend to have low boiling points and may exist as liquids or gases at room temperature.
In contrast, substances with an ionic structure, like sodium chloride, consist of a repeating pattern of ions. These substances tend to have high melting points and conduct electricity when dissolved in water or molten.