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What is a characteristic feature of odd-electron molecules regarding the octet rule?
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'.
Which of the following elements, when acting as a central atom, is most likely to exhibit an expanded octet?
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).
According to Kössel's postulations, the formation of ionic compounds is primarily based on:
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.
Which of the following compounds would predominantly form ionic bonds more easily?
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.'
Which of these is NOT a limitation of the octet rule?
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.'
What is the formal charge on the central oxygen atom (marked 1) in the ozone (O$_3$) molecule?
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).
Lewis symbols are simple notations introduced by G.N. Lewis to represent which of the following?
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.'
According to Thomson's atomic model, what statement accurately describes the atom's mass distribution?
As per the NCERT text, 'An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom.' This is in contrast to Rutherford's model where mass is concentrated in the nucleus.
Which of the following models postulates that electrons always experience a net force in their orbits, making the atom inherently unstable according to classical physics?
The NCERT states, 'A classical atom based on Rutherford’s model is doomed to collapse.' This is because electrons revolving around the nucleus are accelerating and, according to classical electromagnetism, should continuously radiate energy and spiral into the nucleus, thus experiencing a net force.
In Rutherford's atomic model, what constitutes the positively charged part of the atom and where is most of the mass located?
According to the NCERT summary for Rutherford's model: 'most of the mass of the atom and all its positive charge are concentrated in a tiny nucleus'.
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The bank mixes NEET previous year questions (PYQs) with practice questions, each tagged with its exam appearances where applicable.