Ionic and metallic materials are not collections of isolated particles: their characteristic properties emerge from giant structures and strong electrostatic attraction. This chapter connects electron transfer, lattice arrangement and delocalisation to conductivity, melting point and chemical behaviour.

A large electronegativity difference favours substantial electron transfer and ionic bonding, but there is no single magic boundary. Real compounds can have both ionic and covalent character because electron density responds to the charges and sizes of the ions involved.

Electronegativity describes attraction for a shared pair in a bond; it is not the same as electron affinity or ionisation energy. Use it to predict bond polarity and the likely direction of electron density shift, then use structure to explain the material's properties.

Ionic solids contain oppositely charged ions in a lattice; metallic solids contain positive ion cores and delocalised electrons. Strong electrostatic attractions give many ionic compounds high melting points, but they conduct only when ions can move, such as when molten or dissolved. Metals conduct in solid form through mobile electrons and are malleable because layers can slide without destroying non-directional metallic bonding. Real properties also depend on ion charge, size and lattice structure.

Metals tend to lose outer electrons to form cations and non-metals tend to gain them to form anions. Formulae reflect the simplest whole-number ratio that gives overall zero charge: $\mathrm{M}{\mathrm{g}}^{2+}$ and $\mathrm{C}{\mathrm{l}}^{-}$ give $\mathrm{MgC}{\mathrm{l}}_2$, not a molecule of one magnesium atom tightly paired with two chlorine atoms.

A crystal is a giant lattice in which each ion attracts many oppositely charged neighbours. Dot-and-cross diagrams show electron transfer and outer-shell arrangements, but they do not show the full three-dimensional lattice. Name both the ion charges and the electrostatic attraction in a high-quality explanation.

Ionic solids have high melting temperatures because large amounts of energy are needed to overcome attractions throughout the lattice. They conduct when molten or dissolved because ions can move and carry charge; they do not conduct as solids because ions are fixed in position.

Higher ionic charge and smaller ionic radius usually strengthen electrostatic attraction, giving higher melting temperature and more negative lattice-energy values. Avoid saying that a compound has a high melting point merely because it has strong bonds: specify the strong attraction between oppositely charged ions throughout a lattice.

A small, highly charged cation has high charge density and can distort a neighbouring anion's electron cloud. A large anion is easier to distort. Greater polarisation adds covalent character because electron density becomes less completely localised on the anion.

This model explains why some compounds deviate from a simple ionic prediction. Use it comparatively: $\mathrm{A}{\mathrm{l}}^{3+}$ is more polarising than $\mathrm{N}{\mathrm{a}}^{+}$, and ${\mathrm{I}}^{-}$ is more polarisable than ${\mathrm{F}}^{-}$.

A metal is a giant lattice of positive ions surrounded by delocalised electrons. The attraction between these ions and mobile electrons is metallic bonding. It is non-directional, so layers of ions can slide while attraction is retained; this explains malleability and ductility.

Metals conduct electricity and heat because delocalised electrons move through the structure and transfer charge or energy. Stronger metallic bonding is associated with higher ionic charge, more delocalised electrons and smaller metal-ion radius, although measured melting points also depend on structure.

In an ionic solid, the important structure is a repeating three-dimensional arrangement, not a collection of separate NaCl molecules. Each ion is surrounded by oppositely charged ions, and the lattice energy reflects the cumulative electrostatic attraction. Small ions and larger ionic charges bring charge centres closer or increase the attraction, so they usually produce higher melting temperatures.

Metallic bonding is also a lattice model. Positive metal ions occupy ordered positions while valence electrons are delocalised throughout the structure. These electrons carry charge when a potential difference is applied and can move when layers of ions slide, which explains electrical conductivity and malleability without requiring the metallic bond to break completely.

A conductivity comparison can distinguish mobile ions from fixed ions and delocalised electrons. The test is meaningful only when the apparatus, concentration and electrode contact are controlled.

A property explanation is strongest when it moves from structure, to mobile particles or attraction, to the observed macroscopic result.