Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT states, 'The concept of resonance was introduced to deal with the type of difficulty experienced in the depiction of accurate structures of molecules like O3. According to the concept of resonance, whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons are taken as the canonical structures of the hybrid which describes the molecule accurately.'

The NCERT states that resonance structures must have '(i) the same positions of nuclei and (ii) the same number of unpaired electrons.' It also mentions that canonical structures are 'with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons'. Therefore, having different arrangements of atoms is incorrect; the skeletal structure remains the same.

The NCERT text explicitly states, 'Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single cannonical structure'.

For the carbonate ion ($CO_3^{2-}$), the NCERT states, 'According to the experimental findings, all carbon to oxygen bonds in CO3^2– are equivalent. Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms I, II, and III shown below,' implying bonds intermediate in character.

The NCERT notes, 'The carbon-oxygen bond lengths in CO2 (115 pm) lie between the values for C=O and C≡O. Obviously, a single Lewis structure cannot depict this position and it becomes necessary to write more than one Lewis structures and to consider that the structure of CO2 is best described as a hybrid of the canonical or resonance forms I, II and III.'

The NCERT text explicitly states, 'Resonance is represented by a double headed arrow.'

For ozone ($O_3$), the NCERT states, 'Experimentally determined oxygen-oxygen bond lengths in the O3 molecule are same (128 pm). Thus the oxygen-oxygen bonds in the O3 molecule are intermediate between a double and a single bond.'

The NCERT states, 'Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single cannonical structure'.

The VSEPR theory postulates: 'A multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.' It also mentions that 'Where two or more resonance structures can represent a molecule, the VSEPR model is applicable to any such structure.'

The NCERT explicitly states, 'The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons.'