Types of chemical bond. Crystal lattices
A chemical bond arises from the interaction of atoms and is described by bond energy, bond length, bond angle and bond order; a higher bond order gives a shorter and stronger bond. A covalent bond forms through shared electron pairs: when the atoms have the same electronegativity it is nonpolar (Cl₂, O₂), when they differ a little it is polar (HCl, H₂O) and the shared pair shifts toward the more electronegative atom. In the donor–acceptor mechanism the bond is formed from one atom’s lone pair and another atom’s empty orbital (NH₃ + H⁺ → NH₄⁺). When the electronegativity difference is very large (roughly above 1.7) the electron is transferred completely and the electrostatic attraction between oppositely charged ions is an ionic bond (NaCl, CaO); in metals there is a metallic bond between positive ions and free electrons. A bond lying along the line joining the nuclei is a σ bond and a sideways overlap is a π bond; a single bond is only σ, a double bond is σ + π and a triple bond is σ + 2π. A hydrogen bond arises between the H atom of one molecule and an O, N or F atom of another and explains the high boiling point of water. The nodes of a crystal lattice hold molecules (molecular lattice, melts easily), atoms (atomic lattice, very hard and hard to melt), ions (ionic lattice, brittle with a high melting point) or metal ions (metallic lattice, conducts electricity).
Modelling: build models of H₂O, CO₂ and NH₃ from ready-made molecular model kits (or modelling clay and short sticks); label each bond as σ or π and count the bonds.