[Miriiii]

So I stupidly enrolled in biochemistry this semester despite having not been in a chemistry class since I was 15. I have a lot of basic questions.

I am trying to memorise the structure of proline. Noticed that its side chain is 3 CH2 groups. 2 Are CH2, one is H2C, what's the difference. Why would you write them differently?

Similarly, I've noticed that NH2 groups are written as H2N a lot of the time, why? When do you flip this around?

Cheers! And respect to you chem people, trying to wrap my head around this is doing me in.

[chenbeier]

You have a molecule H₂N-R-X  another one is Y-R-NH₂. At least it doesnt matter how you write it.
Or you have H₂C=CH₂ which show the doublebond between the carbon atoms. It is not wrong to write CH₂CH₂.

[Borek]

These are the same things, just seen from different sides.

[Corribus]

If you wrote CH₂=O for, e.g., formaldehyde, it could be mistaken which nucleus is attached to the oxygen. With H₂C=O, it is unambiguous that the carbon is attached to the oxygen. Likewise for O=CH₂. This is really most important when you are drawing bonds.