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Nitrogen normally has only three bonds. In a nitro group, it is considered positively charged. Then one of the oxygen is negatively charged, and the other has a double bond. In reality, the best way to represent the nitro group is to consider it to have delocalized electrons. Then all three atoms are sp2 hybridised.
Nitro compounds are very explosive. This is because, at high temperature, they can decompose and release nitrogen gas. This reaction is very exothermic (gives out a lot of energy). Many explosives have nitro groups, for example trinitrotoluene (TNT).