Q.What are the three most common states of matter?

Diamond is a covalent (network) crystal.

In diamond every carbon atom is sp3 hybridised and is covalently bonded to four neighbouring carbon atoms, forming a continuous three-dimensional network. This gives an extremely hard, high-melting, non-conducting solid.

• Ionic crystals contain ions; molecular crystals contain discrete molecules; metallic crystals have a sea of electrons — none describes diamond.

Diamond is a giant covalent network solid; iron and copper are metallic, NaCl is ionic.

Classifying the options:

• Iron and Copper are metallic solids (metal atoms held by a sea of delocalised electrons).

• Sodium chloride (NaCl) is an ionic solid (Na+ and Cl- held by electrostatic forces).

• Diamond is a covalent (network) solid: every carbon atom is sp3 hybridised and covalently bonded to four neighbouring carbons, forming a continuous three-dimensional network. This makes it extremely hard with a very high melting point.

Dry ice (solid CO2) is a molecular solid — CO2 molecules sit at the lattice points, held together only by weak van der Waals forces.

Solids are classified by what occupies the lattice points and the forces holding those particles together:

• Ionic solids: cations and anions held by strong electrostatic (Coulombic) attraction, e.g. NaCl.
• Molecular solids: discrete covalent molecules held together by weak van der Waals/dispersion forces (sometimes hydrogen bonds), e.g. dry ice (CO2), ice, iodine.
• Covalent (network) solids: atoms linked in a continuous covalent-bond network, e.g. diamond, SiO2.
• Metallic solids: metal cations sitting in a sea of delocalised electrons, e.g. Cu, Fe.

Dry ice is solid carbon dioxide. Each CO2 unit (O=C=O) is a small, non-polar, fully covalently-bonded molecule. In the solid, these individual molecules pack together and are held in place only by weak van der Waals (London dispersion) forces — there is no ionic bonding, no continuous covalent network linking all the atoms, and no metallic bonding. This weak intermolecular attraction is why dry ice is soft and sublimes directly to gas at a low temperature (about -78.5 degC at 1 atm) instead of melting like an ionic or metallic solid.

Graphite is a covalent (network) solid — carbon atoms held in layers by strong covalent bonds.

Why: In graphite each carbon atom is $sp^2$ hybridised and is covalently linked to three neighbouring carbon atoms, forming flat hexagonal sheets. Because the whole crystal is a continuous network of covalent bonds (not ions, not a sea of electrons over a metal lattice, and not discrete molecules held by weak forces), graphite is classed as a covalent / network solid (like diamond and $SiO_2$). The one delocalised electron per carbon makes it a good conductor, but the bonding within the solid is covalent.

A network solid is held together by a continuous framework of covalent bonds rather than discrete molecules, giving it extreme hardness and a very high melting point.

In a network (covalent) solid, atoms are linked to their neighbours by strong covalent bonds that extend in a giant three-dimensional (or two-dimensional) network throughout the crystal - there are no separate molecules, the entire crystal is essentially one giant molecule.

Diamond is the classic example: each carbon atom is sp3 hybridised and forms four covalent bonds to four other carbon atoms in a rigid tetrahedral network. This is why diamond is exceptionally hard, has a very high melting point, and does not conduct electricity (no free electrons).

Other examples include silicon carbide (SiC, carborundum) and quartz/silica (SiO2).

Iodine ($I_2$) is a non-polar molecular solid.

Iodine exists as diatomic $I_2$ molecules. Since the two iodine atoms are identical, the molecule has no permanent dipole moment (non-polar). In the solid state, these non-polar $I_2$ molecules are held in the crystal lattice only by weak van der Waals (London dispersion) forces, which is characteristic of a non-polar molecular solid — soft, low melting point, and volatile (iodine sublimes readily).