chemistry

Covalent molecules
They can exist in two forms (Macromolecules OR Simple Covalent) :

Simple Covalent Compound (example CO2):
They bond by covalent bond, each molecule has 8 electrons in their outermost shell, and they WILL NOT give out electrons, so they are INSULATORS. They have van der waals force between molecules, but they are not as strong as, the ones in graphite, because they have less weight, lower Mr(molecular mass) , so they exists as gas.

Please take a look at the website, don't read the words (they are confusing), just press the buttons and see what is going on.
http://www.avogadro.co.uk/structure/chemstruc/network/g-molecular.htm

Only SOME covalent compound have the structure Macromolecules (E.g. Diamond, Graphite) the majority of other covalent compound exist in simple covalent structures.


Diamond:
- The first picture in the website.
- It is made out of PURELY Covalent bond, they are STRONG, and therefore it is HARD.
- All of the four 4 electrons of carbon is used to bond therefore there are NO FREE ELECTRONS, so diamond is an INSULATOR.

Graphite:
-The second picture in the website.
- Again, all the carbon atoms are bonded together by covalent bonds, BUT ONLY 3 of them are used to bond with each other. WITHIN the molecules (The sheet of hexagons) are bonded by covalent bonds which are strong.
-The only one electron left out unbonded will attract another unbonded electrons, that result in the WEAK VAN DER WAALS FORCE BETWEEN the sheets of hexagons, therefore they can SLIDE between each other.

*Fun thing to try, it is like three sheets of paper, the middle one can slide out easily. The little fraction between the sheets of paper represents the van der waals force. But you can not really break ONE sheet of paper. This represents the strong covalent bond between the sheets of hexagons in the graphite molecule.

Because of this SLIDING properties, Graphite is used in pencil, as you rub the Graphite rod (macromolecule) across the paper, sheets of hexagons starts to come off the macromolecule and it left on the paper, as you saw the pencil mark on the paper. Graphite can act as lubricant as well, base on this properties
-Due to there are only 3 electrons are used to bond, the un-bond electrons are FREELY MOVING around within the sheet of hexagon, so, Graphite is a CONDUCTOR.

PLEASE COMPLETELY UNDERSTAND THIS:
Covalent::
Share of electrons
Non metal – Non metal

Ionic:
Donating and Accepting electrons.
Non metal – metal

Giant Ionic Compounds: (E.g NaCl)
IONIC Compound that has ionic bond.
Each ion have electrostatic attraction between them, one Na+ ion bond with 4 Cl- ions.

Again those bonds are strong, so giant ion compound is hard and they are usually crystals. There are NO free electrons, so they are INSULATORS, but as they are in MOLTEN State or in AQUEAOUS State, the ions are now free moving, so, they are conductors in that state.