NEET Practice Questions (MCQs) with Answers & Solutions

The NCERT notes, 'Further , the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene...'

The NCERT states, 'However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. ...Therefore, the presence of nitro group at meta- position does not stabilise the negative charge and no effect on reactivity is observed by the presence of –NO2 group at meta-position.'

One of the reasons given in NCERT for less reactivity towards nucleophilic substitution is, 'Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.'

The NCERT states, 'Haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.' Also, 'In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, the carbon atom attached to halogen is sp 2-hybridised.'

According to the context, 'when the atoms come together to form a solid they are close to each other. So the outer orbits of electrons from neighbouring atoms would come very close or could even overlap. This would make the nature of electron motion in a solid very different from that in an isolated atom. ... Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands.'

The context states: 'One can have a metal either when the conduction band is partially filled and the balanced band is partially empty or when the conduction and valance bands overlap. When there is overlap electrons from valence band can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction. ... Therefore, the resistance of such materials is low or the conductivity is high.'

The text explicitly states for insulators, 'a large band gap $E_g$ exists ($E_g > 3 \text{ eV}$). There are no electrons in the conduction band, and therefore no electrical conduction is possible.'

Figure 14.1 description notes: 'The lower band, called the valence band, consists of closely spaced completely filled energy states.' Also, earlier text mentions 'With no external energy, all the valence electrons will reside in the valence band.'

For semiconductors, the text states: 'Here a finite but small band gap ($E_g < 3 \text{ eV}$) exists. Because of the small band gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band. These electrons (though small in numbers) can move in the conduction band.'

The context mentions: 'In the energy band diagram of n-type Si semiconductor, the donor energy level $E_D$ is slightly below the bottom $E_C$ of the conduction band and electrons from this level move into the conduction band with very small supply of energy.'