Chemical Bonding and Atomic Structure MCQs for NEET — Chemistry Questions with Answers

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

The NCERT defines it as: 'The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy between the actual structure and the lowest energy resonance structure is called the resonance stabilisation energy or simply the resonance energy.'

The NCERT states, 'Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others.'

The NCERT states, 'Resonance averages the bond characteristics as a whole.'

The statement 'The cannonical forms have no real existence' followed by 'The molecule does not exist for a certain fraction of time in one cannonical form and for other fractions of time in other cannonical forms' implies that canonical forms are not actual structures that the molecule adopts, but rather theoretical representations that collectively describe the true structure (the resonance hybrid).

According to the NCERT text, 'Lewis pictured the atom in terms of a positively charged ‘Kernel’ (the nucleus plus the inner electrons) and the outer shell that could accommodate a maximum of eight electrons.' This highlights his model of valence electrons and the 'Kernel'.

The text states, 'Kössel and Lewis in 1916 developed an important theory of chemical combination between atoms known as electronic theory of chemical bonding. According to this, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as octet rule.' The inertness of noble gases, with their stable octet, was the basis for this theory.

The NCERT text mentions, 'In some compounds, the number of electrons surrounding the central atom is less than eight. This is especially the case with elements having less than four valence electrons. Examples are LiCl, BeH$_2$ and BCl$_3$.' In BCl$_3$, Boron (B) has only 6 valence electrons around it, making it an incomplete octet.

The text states, 'Kössel’s postulations provide the basis for the modern concepts regarding ion-formation by electron transfer and the formation of ionic crystalline compounds.' It also mentions 'the electrovalence is thus equal to the number of unit charge(s) on the ion.' Electrovalence is directly associated with the concept of ionic bonding where electrons are transferred to form ions.

The text explicitly states, 'In the formation of a molecule, only the outer shell electrons take part in chemical combination and they are known as valence electrons.'