Question

Is CH2 an alkyl group?

Answer 1

Alone?

It's generally called a carbene, which is very reactive with alkenes, forming cyclopropane.

In a molecule? I would call it a (1) methylene.

Answer 2

Simply put: no, it isn't.

Explanation:

An alkyl group is generally understood to be a stable substituent functional group, in other words, something that is attached to the carbon backbone of an organic species.

`CH_2` is either part of the carbon backbone (not something attached to it), i.e. `-CH_2-`, or (as the other poster commented) it could be in the form of a carbene, which is a highly reactive intermediate with two unshared valance electrons (`:CH_2`).

So in neither case would the term "alkyl group" apply.