[panzer]

In the First part it says ''Put a methyl group on the number 2 carbon atom''

My Question is, can you put it on the third or fourth?

In the second part i would like to know that how can you find out how many Hydrogen to put in.

I have attached a picture so you can clearly see what i don't understand!

Image from: http://www.chemguide.co.uk/basicorg/conventions/names.html

[Wald_ron]

Carbon forms four bonds

 So for C-C-C-C-C

The first Carbon already has one bond to another Carbon , therefore it can have three additional bonds to hydrogen,

the second Carbon has two bonds to two other Carbons, and can thus have two hydrogens.

Simple Rules that will help you to do your naming
-The name of the substitution is counted from the shortest side
ex.  C-C-C-C-C   Is 2 methyl pentane and not 4 methyl pentane
          I
          C

-These are not rigid structures, so

                        C                        C
                        I                         I
C-C-C-C-C  =  C-C-C-C-C =  C-C-C-C-C = C-C-C-C-C
    I                                                               I
    C                                                              C

are all the same thing chemically

-The longest Chain is the base of the name

ex. C-C-C-C-C-C  3 methyl heptane not 2 ethyl hexane
                  I
                  C
                  I
                  C

-methyl- One Carbon added
ethyl - 2 carbons added


if from the example you gave

C-C-C-C-C
    I
    C

is called 2 methyl pentane , what would

C-C-C-C-C
       I
       C

be called which is definitely possible,