Because it has one pair of electrons on it, NH2 can release electrons.
The NO2 group contains Nitrogen, which has already shared its single pair with an Oxygen-atom. Therefore, it won’t donate any electrons, but it will extract electrons, as I will show you.
The No2 group can create a vacant orbital for Nitrogen by bonding electrons to Oxygen.
This Nitrogen with Vacant orbital can now accept electrons. That is why the No2 group is electron withdrawing.
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