F,Cl,Br and I are the elements of VII A group. In a group atomic radii increases from top to bottom and the bond dissociation energy decreases as

R-F>R-Cl>R-Br>R-I

So, during dehydrohalogenation R-I bond breaks more easily than R-I bond. Hence, order of reactivity will be

R-I>R-Br>R-Cl>R-F

Bond order is given by the formula that the total number of bonds between two atoms / the total number of resonating structures, the bond order = 3/2 = 1.5, we can say that there is a relation between resonance and bond order through this equation.

Nitrenes are obtained by the thermolysis of azides. They are neutral monovalent nitrogen species in which nitrogen atom has 2 and shared pair of electrons with a monovalent atom or group attached and they are as reactive as carbenes.

When all the properties of a molecule cannot be shown by a single structure and two or more structures are required to show all the properties of that molecule, then the structures are called resonating structures are canonical forms and the molecule is referred to as resonance hybrid.

Hyperconjugation involves delocalization of Sigma electrons of a carbon-hydrogen bond of an alkyl group attached directly to an atom of unsaturated system or to an atom with an ancient π orbital. It is calso called no Bond formation or Baker Nathan effect.