Chemical Bonding and Molecular Geometry

From atoms to bonds

Atoms can join to form substances with very different properties. A chemical bond holds particles together, but the particles and the bonding are not always the same. In ionic bonding, electrons move from one atom to another. In covalent bonding, atoms share pairs of electrons. In a metal, electrons are shared throughout a giant structure rather than between just two atoms. To predict how a substance behaves, we also need to know how its particles are arranged.

Transferring electrons: ionic bonding

When a metal reacts with a non-metal, a metal atom can lose one or more outer-shell electrons and the non-metal atom can gain them. Losing electrons leaves a positive ion; gaining electrons produces a negative ion. The opposite charges attract. This electrostatic attraction is the ionic bond.

Sodium and chlorine show the sequence clearly. A sodium atom transfers its one outer-shell electron to a chlorine atom. The products are Na⁺ and Cl⁻ ions, each with a full outer electron shell. A dot-and-cross diagram can use dots for one atom's electrons and crosses for the other's, so the transferred electron can be traced.

Sodium chloride is not made of separate NaCl molecules. Its ions form a giant, repeating three-dimensional lattice, with attractions between oppositely charged ions acting in every direction. A great deal of energy is needed to overcome these many strong attractions, so ionic compounds have high melting and boiling points. In a solid lattice the ions cannot move from place to place, so they cannot carry an electric current. When the compound melts or dissolves in water, the ions can move and it can conduct.

Sharing electron pairs: covalent bonding

Non-metal atoms can achieve fuller outer shells by sharing electrons instead of transferring them. A shared pair forms a covalent bond. Two hydrogen atoms, for example, share a pair to make an H₂ molecule. Other small molecules include O₂, HCl and CH₄. Their atoms are strongly bonded within each molecule.

The bonds within a molecule are different from the forces between molecules. Small molecular substances often have low melting and boiling points because the forces between their molecules are relatively weak. Melting or boiling separates molecules from one another; it does not usually split the covalent bonds inside them. Larger molecules generally have stronger intermolecular forces and higher melting and boiling points. Small molecular substances do not normally conduct electricity because they lack mobile charged particles.

A polymer is a very large molecule with atoms joined by covalent bonds. Forces between polymer molecules are relatively strong, helping explain why many polymers are solids at room temperature. In a giant covalent structure, such as diamond or silicon dioxide, covalent bonds extend through a network of atoms rather than making separate small molecules. Many strong bonds must be overcome to melt such a structure, giving it a very high melting point.

Why arrangement and shape matter

A chemical formula tells us which atoms are present, but not the whole structure. Molecules are three-dimensional groups of bonded atoms: their shape describes how those atoms are positioned in space. In methane, CH₄, the four hydrogen atoms are arranged around the carbon atom in a tetrahedral shape, rather than lying in a flat cross. This symmetrical shape means that any small differences in charge across its bonds cancel out: the molecule has no permanently positive end and negative end. Methane molecules therefore cannot attract one another through such permanent charged ends. Only weak forces act between them, helping explain why methane is a gas at room temperature even though its atoms are held together by strong covalent bonds.

For a giant substance, the arrangement of ions or atoms matters instead. This is why bonding and structure must be considered together when explaining behaviour: a small covalent molecule, a polymer and a giant covalent network all contain covalent bonds, yet do not behave alike.

Sharing electrons throughout a metal

A metal has a giant, regular structure of positive metal ions surrounded by delocalised electrons. The attraction between the ions and these shared electrons is metallic bonding. The electrons can move through the structure, carrying electrical charge and transferring thermal energy; metals are therefore good electrical and thermal conductors. Layers of particles in a pure metal can slide over one another, allowing it to be bent and shaped. In an alloy, different-sized atoms disrupt the regular layers, making sliding harder and the material generally harder than the pure metal.