Models in science help you get the idea of what something looks like that's why your teacher may ask you to draw a diagram to help you remember what the object looks like.
Hope this helps.
You just need the number of protons and number of neutrons as the mass of eelctrons is negligible
Structure of a-chloro-b-methylbutyric acid (2-chloro-3-methylbutyric acid is attached below.
The structure was drawn using following information.
1) First, the parent chain was identified. The parent chain is containing four carbon atoms, also, this chain belongs to carboxylic acid as the compound name ends with Butyric acid i.e. butyr (Butane) -ic acid (carboxylic acid).
2) Secondly, the positions of substituents were assigned by using the rule which sates that the numbering in carboxylic acid must satrt from the carbonyl carbon. Therefore, the carbon which is attached to carbonyl carbon is named as alpha (2nd position) and carbon next to alpha carbon is named as beta carbon (3rd position). Hence, we add chlorine atom on 2nd carbon next to carbonyl group and methyl group at 3rd carbon with respect to carbonyl carbon.