Chemical bonds — AQA GCSE Chemistry

Strong chemical bonds hold atoms or ions together in compounds and elements. There are three types: ionic, covalent and metallic. In ionic bonding, electrons are transferred from a metal atom to a non-metal atom, producing positively charged cations and negatively charged anions. The oppositely charged ions attract each other electrostatically in a giant ionic lattice. For example, sodium transfers one electron to chlorine, forming Na⁺ and Cl⁻, which arrange in a regular lattice of alternating charges. The strength of the bond comes from the electrostatic attraction between these oppositely charged particles. You should be able to describe the transfer of electrons, write the resulting ion charges using superscript signs, and explain that the particles involved are ions rather than atoms or molecules.

Covalent bonding occurs between non-metal atoms that share pairs of electrons to achieve stable electron arrangements. Each shared pair is attracted to both nuclei, holding the atoms together. For example, in H₂ each hydrogen atom contributes one electron to a shared pair, and in H₂O oxygen shares two pairs, one with each hydrogen. Metallic bonding occurs in metals, where positive metal ions are surrounded by a sea of delocalised electrons. The particles in metallic bonding are often described as positive ions, but the specification phrase 'atoms which share delocalised electrons' emphasises that the outer electrons are not attached to any single atom. You should be able to contrast the two: covalent bonding involves localised shared pairs between specific atoms, while metallic bonding involves delocalised electrons shared across the whole structure.

Ionic bonding is found in compounds made from a metal and a non-metal. The metal atoms lose electrons to form positive ions, and the non-metal atoms gain electrons to form negative ions. The oppositely charged ions are held together by strong electrostatic attraction in a giant ionic lattice. For example, magnesium reacts with oxygen to form magnesium oxide, MgO, containing Mg²⁺ and O²⁻ ions. Sodium chloride, NaCl, contains Na⁺ and Cl⁻ ions. You should be able to identify from a formula or a reaction whether ionic bonding is present by checking that a metal is combined with a non-metal. You should also be able to explain why the compound is neutral overall: the total positive charge equals the total negative charge. This rule helps you work out formulas such as MgCl₂, where one Mg²⁺ balances two Cl⁻ ions.

Covalent bonding is the mechanism by which non-metal atoms achieve a full outer electron shell by sharing pairs of electrons. This type of bonding is found in most non-metallic elements, such as diatomic oxygen or chlorine, and in compounds formed entirely from non-metals, like water or carbon dioxide. Because non-metals have high electronegativities, neither atom readily loses electrons, so sharing is the only way to achieve stability. The shared electrons are held in place by the electrostatic attraction between the positively charged nuclei of the bonded atoms and the negatively charged shared electron pair, creating a strong covalent bond.

Metallic bonding is the strong electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons. This type of bonding is characteristic of all metallic elements, such as copper and iron, as well as alloys, which are mixtures of a metal with at least one other element, like steel or brass. In a solid metal, the outer shell electrons detach from the atoms, leaving behind positively charged ions arranged in a regular, repeating three-dimensional pattern. The delocalised electrons are free to move throughout this giant structure, which explains key metallic properties such as electrical conductivity and malleability.

All chemical bonding is fundamentally driven by electrostatic forces, which are the attractions between positive and negative charges. In ionic bonding, electrons are transferred from a metal to a non-metal, creating oppositely charged ions (e.g., Na⁺ and Cl⁻) that are held together by strong electrostatic attraction. In covalent bonding, non-metal atoms share pairs of electrons; the electrostatic attraction occurs between the positive nuclei of the atoms and the negative shared electrons. In metallic bonding, the attraction is between a lattice of positive metal ions and a sea of negative delocalised electrons. Understanding these electrostatic interactions is key to explaining bond strength.