Compound -- Two or more elements chemically combined
Elements are chemically combined by forming bonds. Bonds involve electrons being lost, gained, or shared....Two types: Ionic and Covalent.
Octet rule -- A useful rule to predict the bonding between elements but there are a lot of exceptions.

'Atoms on reaction tend to reach an electron arrangement with eight electrons in the outermost energy level'

Exceptions:
Beryllium and boron atoms have too few electrons to be able to reach eight in the outer level.
The d-block elements (between groups II and III) do not usually obey the octet rule.
The rule works in some cases for sulfur and phosphorous, but nor inn others.
Hydrogen and lithium atoms tend to reach the electronic structure of helium, but they are unable to gain the large number of electrons required to obtain an octet.

4. The valency of an element is the amount of bonds it forms in a compound.

e.g. oxygen = 2 (needs two more electrons to have a full outer shell)

carbon = 4

hydrogen = 1