115 practice questions covering Structure of Atom, organised into 2 topics.
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Assertion (A): Bohr's model cannot account for the Zeeman effect, the splitting of spectral lines in a magnetic field.
Reason (R): Bohr's model assumes that the angular momentum of the orbiting electron is quantised in units of $h/2\pi$.
A(a) Both A and R are true, and R is the correct explanation of A.
B(b) Both A and R are true, but R is not the correct explanation of A.
C(c) A is true, but R is false.
D(d) A is false, but R is true.
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Correct answer: B - (b) Both A and R are true, but R is not the correct explanation of A.
Both statements are true. Splitting in a magnetic field needs states that differ in their orientation in space, an idea the Bohr model has no room for, since it allows only flat circular orbits labelled by a single number. R states one of Bohr's postulates correctly, but that postulate fixes the allowed radii and energies and says nothing about behaviour in a magnetic field, so R does not explain A.
Q2
easy
In Thomson's model of the atom, how are the electrons and the positive charge arranged?
AAll the positive charge sits in a tiny central core while electrons fill the rest of the volume
BElectrons and positive charge occupy separate hemispheres of the atom
CElectrons revolve in fixed circular orbits around a dense central positive nucleus
DElectrons are embedded in a uniform sphere of positive charge, like seeds in a watermelon
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Correct answer: D - Electrons are embedded in a uniform sphere of positive charge, like seeds in a watermelon
Thomson pictured the atom as a sphere of positive charge of atomic size with the electrons embedded in it, so that the atom as a whole is neutral. Electrons revolving around a central nucleus, and a tiny core holding all the positive charge, are both features of Rutherford's later model rather than Thomson's. Separate hemispheres of charge was never proposed and would not give a neutral, stable arrangement.
Q3
easy
How many electrons can be accommodated in the 4p subshell?
A6
B8
C10
D4
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Correct answer: A - 6
The 4p subshell has 3 orbitals (m_l = -1, 0, +1), each holding 2 electrons, totaling 6. Options 2, 3, and 4 assume incorrect number of orbitals or electron capacity.
Q4
medium
Which of the following transitions in a hydrogen atom produces a line belonging to the Paschen series?
A$n = 5 \to n = 2$
B$n = 3 \to n = 2$
C$n = 4 \to n = 1$
D$n = 5 \to n = 3$
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Correct answer: D - $n = 5 \to n = 3$
The Paschen series is made up of every transition that ends on $n = 3$, and its lines fall in the infrared. A jump ending on $n = 2$ belongs to the Balmer series in the visible region, and one ending on $n = 1$ belongs to the Lyman series in the ultraviolet. Only the jump from $n = 5$ to $n = 3$ terminates on the third level, so only it is a Paschen line.
Q5
easy
In the photoelectric effect, which statement is correct about the threshold frequency?
AIt is the highest frequency that can cause the emission of electrons.
BIt is the minimum frequency of light required to eject electrons from a metal surface.
CIt is the frequency above which the intensity of light no longer affects how many electrons are emitted.
DIt is the frequency at which all incident photons are absorbed by electrons.
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Correct answer: B - It is the minimum frequency of light required to eject electrons from a metal surface.
Threshold frequency is the minimum frequency of incident light that can eject electrons from a given metal; below it no photoelectrons appear however intense the light. It is a minimum, not a maximum, so calling it the highest frequency that causes emission is wrong. Above the threshold, intensity still matters because it fixes the number of photoelectrons emitted, and the threshold has nothing to do with all incident photons being absorbed.
Q6
easy
According to Hund's Rule, how many unpaired electrons are present in a neutral nitrogen atom?
A1
B2
C3
D4
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Correct answer: C - 3
Nitrogen has the electronic configuration 1s^2 2s^2 2p^3. According to Hund's Rule, the three electrons in the 2p orbitals remain unpaired, leading to three unpaired electrons. Options assuming pairing or incorrect electron counts are misleading.
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