Bonding and structure: Atoms, molecules and lattices
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Ionic bonding and ionic lattices
Ionic bonding occurs when one atom transfers one or more electrons to another atom, producing positively charged cations and negatively charged anions. Opposite charges attract and form a regular arrangement called a giant ionic lattice. Strong electrostatic forces between ions produce high melting and boiling points and make ionic compounds hard and brittle. Ions can move when molten or dissolved, which produces electrical conductivity in the liquid or solution state but not in the solid state.
Covalent bonding between atoms
Covalent bonding occurs when two atoms share one or more pairs of electrons to achieve a full outer shell. Shared electron pairs form strong directional bonds that hold atoms together in molecules or networks. Simple covalent molecules consist of a limited number of atoms and have low melting and boiling points because weak intermolecular forces act between molecules. Giant covalent structures extend the covalent network across many atoms, producing very high melting points and hardness due to a continuous network of strong covalent bonds.
Metallic bonding and properties of metals
Metallic bonding occurs in metals when atoms lose outer electrons that become delocalised across a lattice of positive metal ions. The sea of delocalised electrons binds the ions together and allows electrons to flow, producing electrical conductivity in solids. Metallic bonding causes malleability and ductility because layers of metal ions can slide over each other while the delocalised electrons continue to hold the lattice together. Strong metallic bonding produces high melting points and high density in many metals.
Shapes of molecules and electron pair repulsion
Molecular shape results from the arrangement of bonded atoms and lone electron pairs around a central atom. Electron pairs repel each other and adopt positions that minimise repulsion, producing predictable geometries such as linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral. Lone pairs exert greater repulsion than bonded pairs, which slightly alters bond angles and molecular polarity. Molecular shape and polarity influence intermolecular forces, solubility, melting and boiling points, and reactivity.
Giant structures and links to behaviour
Giant structures include ionic lattices, giant covalent networks and metallic lattices. Arrangement and bonding type determine macroscopic properties: ionic lattices show high melting points and usually dissolve in polar solvents; giant covalent networks are very hard with extremely high melting points and do not conduct electricity (except graphite); metallic lattices conduct electricity and heat and are malleable. The type of bonding restricts or allows particle movement, which causes differences in conductivity, melting point, solubility and mechanical strength.
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