Formed by sharing electrons. The strengths: Single < Double < Triple. The number of bonds is called the bond order. Higher bond order, smaller bond length, more energy. Coordinate bonds are bonds where both electrons originate from the same atom.
Lewis structure
Useful formula for finding number of bonds: Octet rule: Atoms need 8 bonds to be full. However, there are Octet rule exceptions
Covalent network structures
| Property | Diamond | Graphite | Graphene | Fullerene C60 | Nanotubes |
|---|---|---|---|---|---|
| Network Structure | Each carbon is bonded sp³ hybridized to 4 others. Tetrahedral. | Carbon is sp² hybridized, bonded to 3 others. Trigonal planar. Layers held by broken dispersion. Remaining valence electrons are delocalized. | 2D layer of graphite. | Sphere of 60 carbon atoms, 20 hexagons, and 12 pentagons. sp² hybridized, bonds with 3 others. | Almost cylindrical, with strong covalent bonds and delocalized electrons. |
| Electrical Conductivity | No | Yes | Yes | No, although delocalized, there is no movement across molecules. | Yes. |
| Thermal Conductivity | Better than metals | No, unless parallel to heat flow. | Thermal conductivity is very good. | Low | Good |
| Appearance | Transparent, lustrous crystal. | Grey crystalline solid. | Almost transparent. | Black or purple. | Cylindrical |
| Physical Properties | Hardest known natural substance. Brittle. High melting point. | Soft and slippery due to layering. Good lubricant. Good conductor. | Thinnest material. Also strongest. | Light and strong. Can melt; pliable. Can react with K to form superconductors. | Lightweight. |
| Uses | Jewelry, cutting other metals, glass tools. | Pencil, dry lubricants, electrode rods in electrolysis. | Microscopes, applications being developed. | Nanotubes, lubricants, medical uses. | Sensors, electrodes. |
Molecules are discrete. Networks are continuous, covalently bonded chains. They have many names. Macromolecule, network, giant molecules. Allotropes are different structural forms of the same element.