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

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.

The text states, 'Obviously ionic bonds will be formed more easily between elements with comparatively low ionization enthalpies and elements with comparatively high negative value of electron gain enthalpy.'

The limitations listed are: '• It is clear that octet rule is based upon the chemical inertness of noble gases. However, some noble gases (for example xenon and krypton) also combine with oxygen and fluorine to form a number of compounds like XeF$_2$, KrF$_2$, XeOF$_2$ etc. • This theory does not account for the shape of molecules. • It does not explain the relative stability of the molecules being totally silent about the energy of a molecule.' The octet rule does fail to explain incomplete octets, so this is a limitation, not a non-limitation. The statement that it suggests noble gases are always inert is incorrect; their inertness is the basis, but exceptions exist, making this a limitation rather than a non-limitation that is 'always true.'

According to the NCERT calculation for ozone (O$_3$): 'The formal charge on: • The central O atom marked 1 = 6 – 2 – 1/2 (6) = +1'. Here, 6 is the valence electrons of O, 2 are lone pair electrons, and 6 are bonding electrons (3 shared pairs).

The text states, 'G.N. Lewis, an American chemist introduced simple notations to represent valence electrons in an atom. These notations are called Lewis symbols.'

The text states: 'The valence bond (VB) approach to covalent bonding is basically concerned with the energetics of covalent bond formation about which the Lewis and VSEPR models are silent. Basically the VB theory discusses bond formation in terms of overlap of orbitals.'