Valency.

Ok to understand about valences  you must know about compounds. So a compound is made up of two or more elements when they fuse together. How they fuse depends on their valency. So what the hell is this valency?
Well valency is the measure of the friendliness of add of different atoms. It is the combining capacity of atoms ok?
Are you with me?
If yes then ok else reread.
The valency of atoms is determined by how many electrons there  in the last shell of atoms which we talked about earlier.
OK here i give the valencys of the elements till 20
Also atoms like to be in octet state (don't get worried by this it just means they like to have 8 or 2 electrons in their last(remaining) shell so atoms which already have 8 or 2 don't combine.
P.S some elements have more than one valency depending on what they are combining with.
1)  Hydrogen 1
2) Helium 0
3) Lithium 1
4) Beryllium 2
5) Boron 3
6) Carbon 4
7) Nitrogen 3,5
8)Oxygen 2
9) Fluorine 1
10)Neon 0
11) Sodium 1
12) Magnesium 2
13) Aluminium 3
14) Silicon 4
15) Phosphorous 3
16) Sulphur 2,4,6(see a pattern yet?)
17)Chlorine 1
18) Argon 0
19) Potassium 1
20) Calcium 2
So look for a pattern ok? There is one! Comment if you find it.
P.S memorise this!
Next class: types of changes.