Does 2-chloro-1,3-dinitrobenzene show resonance? I am preparing for my exam, and then I stuck up here. The answer to this question given by my teacher was 


  No, this doesn't show resonance because there are two bulky groups alongside chlorine, due to this chlorine will leave the plane and as we know resonance is a planar phenomenon, it won't show resonance


My question to you is:

Why did chlorine leave the plane on the first place? And what is the whole concept behind it? Please help me with it. 

My guess to this is that since these two groups are really big and bulky, and are present near Chlorine, they take up almost all the space around which causes chlorine to leave the plane.
2-chloro-1,3-dinitrobenzene is shown below:



I believe your question is referring to the lack of conjugation between the nitro groups and the aromatic ring. this is best shown in an X-ray crystal structure such as this one:



https://www.ccdc.cam.ac.uk/structures/Search?Ccdcid=722415

Due to the sterics between the nitro groups and the chloro substituent, the nitro groups cannot adopt a conformation planar with the ring. This prevents the $\pi$ system of the nitro group from achieving substantial overlap with the $\pi$ system of the ring and prevents full conjugation of the electrons. The sterics are better seen in a spacefilling view:



So, it is not the chloro group that is out of the plane, but rather the nitro groups.