Atomic Structure MCQs for NEET — Chemistry Questions with Answers

$$\begin{gathered} \because \frac{1}{\mathrm{\lambda }}=\mathrm{R}\left(\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right); {\mathrm{n}}_1=1, {\mathrm{n}}_2=?; \\ \frac{1}{\mathrm{\lambda }}=\mathrm{R}\left(\frac{1}{1}-\frac{1}{{\mathrm{n}}_2^2}\right) \Rightarrow {\mathrm{n}}_2^2=\frac{\mathrm{R\lambda }}{\mathrm{R\lambda }-1} \\ \Rightarrow {\mathrm{n}}_2=\sqrt{\frac{\mathrm{\lambda R}}{\mathrm{\lambda R}-1}} \end{gathered}$$

$$\begin{gathered} \mathrm{E}={\mathrm{E}}_1+{\mathrm{E}}_2; \frac{\mathrm{hc}}{\mathrm{\lambda }}=\frac{\mathrm{hc}}{{\mathrm{\lambda }}_1}+\frac{\mathrm{hc}}{{\mathrm{\lambda }}_2} \\ \Rightarrow \frac{\mathrm{hc}}{\mathrm{\lambda }}\mathrm{hc} \left(\frac{{\mathrm{\lambda }}_2+{\mathrm{\lambda }}_1}{{\mathrm{\lambda }}_1{\mathrm{\lambda }}_2}\right); \mathrm{\lambda }=2\frac{{\mathrm{\lambda }}_1{\mathrm{\lambda }}_2}{{\mathrm{\lambda }}_1+{\mathrm{\lambda }}_2} \end{gathered}$$

The context states the general order of filling as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s... After 3p, 4s is filled next. This is also consistent with the (n+l) rule where (n+l) for 3p is (3+1)=4 and for 4s is (4+0)=4. When (n+l) values are equal, the orbital with the lower 'n' value has lower energy. However, in the given sequence, 4s (n=4, l=0) has higher n but is filled before 3d (n=3, l=2). The general order provided in the NCERT text specifies 4s before 3d, making 4s the correct answer. (Reference: 'However, following order of energies of the orbitals is extremely useful: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s...')

The Pauli Exclusion Principle states that 'only two electrons may exist in the same orbital and these electrons must have opposite spin.' Therefore, the statement that electrons in the same orbital must have the same spin is incorrect. (Reference: 'According to this principle : no two electrons in an atom can have the same set of four quantum numbers. Pauli exclusion principle can also be stated as : “ only two electrons may exist in the same orbital and these electrons must have opposite spin.”')

The context states: 'pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.' It further clarifies, 'pairing of electrons will start in the p, d and f orbitals with the entry of 4th, 6th and 8th electron, respectively.' Since d-subshell has 5 degenerate orbitals, each orbital will receive one electron (total 5 electrons) before pairing starts. The 6th electron will be the first one to pair. (Reference: 'pairing of electrons will start in the p, d and f orbitals with the entry of 4th, 6th and 8th electron, respectively.')

The text explicitly states: 'unlike orbitals of hydrogen or hydrogen like species, whose energies depend only on the quantum number n, the energies of the orbitals in multi-electron atoms depend on quantum numbers n and l.' (Reference: 'the energies of the orbitals in multi-electron atoms depend on quantum numbers n and l.')

The Aufbau principle states: 'in the ground state of the atoms, the orbitals are filled in order of their increasing energies. In other words, electrons first occupy the lowest energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled.' (Reference: 'Aufbau Principle: The principle states : in the ground state of the atoms, the orbitals are filled in order of their increasing energies.')

The text states: 'The restriction imposed by Pauli’s exclusion principle on the number of electrons in an orbital helps in calculating the capacity of electrons to be present in any subshell. For example, subshell 1s comprises one orbital and thus the maximum number of electrons present in 1s subshell can be two, in p and d subshells, the maximum number of electrons can be 6 and 10 and so on. This can be summed up as : the maximum number of electrons in the shell with principal quantum number n is equal to $2n^2$.' (Reference: 'the maximum number of electrons in the shell with principal quantum number n is equal to $2n^2$.')

The text provides the useful order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p... It specifically states: 'For example, valence electron in potassium must choose between 3d and 4s orbitals and as predicted by this sequence, it is found in 4s orbital.' (Reference: 'For example, valence electron in potassium must choose between 3d and 4s orbitals and as predicted by this sequence, it is found in 4s orbital.')

The context on molecular orbitals states: 'The bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital.' (Reference: 'The bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital.')