Chemistry MCQs for NEET — Practice Questions with Answers

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.'

The NCERT text describes expanded octets: 'Elements in and beyond the third period of the periodic table have, apart from 3s and 3p orbitals, 3d orbitals also available for bonding. In a number of compounds of these elements there are more than eight valence electrons around the central atom. This is termed as the expanded octet. ... Some of the examples of such compounds are: PF$_5$, SF$_6$, H$_2$SO$_4$'. In SF$_6$, sulfur has 12 valence electrons around it.

The text mentions, 'Langmuir (1919) refined the Lewis postulations by abandoning the idea of the stationary cubical arrangement of the octet, and by introducing the term covalent bond.'

Under 'Limitations of the Octet Rule', the text states, 'In molecules with an odd number of electrons like nitric oxide, NO and nitrogen dioxide, NO$_2$, the octet rule is not satisfied for all the atoms'.

The text explains expanded octets are found in elements 'in and beyond the third period of the periodic table' because they have available 3d orbitals. Sulfur (S) is in the third period, while Carbon (C), Nitrogen (N), and Oxygen (O) are in the second period and generally observe the octet rule (except for odd-electron and incomplete octet exceptions specifically for elements like B, Be, Li).

The text mentions, 'Kössel’s postulations provide the basis for the modern concepts regarding ion-formation by electron transfer and the formation of ionic crystalline compounds.' It distinguishes this from sharing of electrons, which is characteristic of the covalent bond according to Lewis and Langmuir.