NEET Practice Questions (MCQs) with Answers & Solutions

Dalton's atomic theory regarded the atom as the ultimate particle of matter and did not include any internal structure. The detailed understanding of the electronic structure of atoms, including the distribution and relative energies of electrons, developed with later atomic models like Bohr's and the quantum mechanical model. The question 'why two or more atoms combine to form molecules, why some elements are metals while others are non-metals... find simple explanation from the electronic configuration. These questions have no answer in the Daltonian model of atom.' (NCERT, Chapter: STRUCTURE_OF_ATOM)

The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1808. (NCERT, Chapter: STRUCTURE_OF_ATOM)

According to the NCERT text, 'Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge.'

The NCERT text states, 'As we go down the group in the periodic table, the size of halogen atom increases. Fluorine atom is the smallest and iodine atom is the largest. Consequently the carbon-halogen bond length also increases from C—F to C—I.' This implies that C-F has the shortest bond length and C-I has the longest bond length.

The context mentions, 'C— Cl bond acquires a partial double bond character due to resonance' where 'lone pair of electrons on halogen atom are in conjugation with p -electrons of the ring'.

The NCERT text states, 'C— Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.' Additionally, it mentions, 'C— Cl bond length in haloalkane is 177pm while in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.'

The NCERT text defines haloalkanes as compounds containing 'halogen atom(s) attached to the sp$^3$ hybridised carbon atom of an alkyl group'.

Vinylic halides are defined as compounds 'in which the halogen atom is bonded to a sp$^2$-hybridised carbon atom of a carbon-carbon double bond (C = C)'.

The NCERT text explains, 'The sp$^2$ hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp$^3$-hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is 177pm while in haloarene is 169 pm.'

The context states, 'C— Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.'