Giant Structures, Simple Molecules and Types of Solid

Classify a solid by its particles and what holds them:

• Giant ionic: ions in a lattice, ionic bonds. High melting point, brittle, conducts only when molten or dissolved.
• Giant covalent (covalent network): atoms joined by covalent bonds throughout. Very high melting point, hard, insoluble, usually does not conduct.
• Metallic: positive ions in a sea of electrons. Conducts as a solid, malleable.
• Simple molecular: separate molecules held by intermolecular forces. Low melting point, soft, does not conduct.
• An amorphous solid (glass, many plastics) has no regular long-range order. That describes the arrangement, not the bonding: glass is a disordered covalent network.

Know these four giant structures by heart:

• Diamond: each C bonded to four others in a tetrahedron (109.5°), a 3D network. Hardest natural substance, very high melting point, no free electrons, so no conduction.
• Graphite: each C bonded to three others in flat hexagonal layers; the fourth electron is delocalised along the layer. Conducts along the layers; layers held by weak forces slide, so it is soft and a lubricant.
• Silicon dioxide (quartz, sand): each Si bonded to four O, each O to two Si. Formula $\mathrm{SiO_2}$ is a ratio, not a molecule.
• Sodium chloride: a cubic lattice in which each $\mathrm{Na^+}$ is surrounded by six $\mathrm{Cl^-}$ and each $\mathrm{Cl^-}$ by six $\mathrm{Na^+}$.
• Diamond and graphite are allotropes: different structural forms of the same element (fullerenes such as $\mathrm{C_{60}}$ are a third, molecular form).

• An element contains one kind of atom; a compound contains two or more elements chemically bonded.
• A molecule is a group of atoms covalently bonded into a separate unit. Elements can be molecular ($\mathrm{O_2}$, $\mathrm{N_2}$, $\mathrm{Cl_2}$, $\mathrm{P_4}$, $\mathrm{S_8}$); so can compounds ($\mathrm{H_2O}$, $\mathrm{CO_2}$).
• Noble gases exist as single, separate atoms, not molecules.
• Ionic compounds and giant covalent substances contain no molecules: NaCl, $\mathrm{SiO_2}$, diamond.
• The seven diatomic elements: $\mathrm{H_2}$, $\mathrm{N_2}$, $\mathrm{O_2}$, $\mathrm{F_2}$, $\mathrm{Cl_2}$, $\mathrm{Br_2}$, $\mathrm{I_2}$.