Chemistry · Structure of atom · chapter 2 of 9

Class 11 Chemistry Chapter 2: Structure of Atom MCQs with answers

10 multiple-choice questions on Structure of Atom, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
In a discharge tube the gas is changed from hydrogen to neon, everything else kept the same. Which of these changes?
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Answer: (b) the charge to mass ratio of the canal rays

Cathode rays (electrons) do not depend on the gas or the electrodes. Canal rays are positive ions of the gas itself, so their mass, and hence their charge to mass ratio, depends on the gas.

Question 2
In the gold foil experiment, the fact that about one α-particle in 20,000 bounced back nearly along its own path showed that
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Answer: (c) the positive charge of the atom is concentrated in a very small volume

Only an enormous repulsive force from a tiny region holding all the positive charge can turn an α-particle back. That most passed straight through shows empty space, which is a different observation.

Question 3
Which pair of atoms are isobars?
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Answer: (d) 4018Ar and 4020Ca

Isobars have the same mass number and different atomic numbers: Ar and Ca both have A = 40. The chlorine and carbon pairs are isotopes; ¹⁴C and ¹⁶O have the same number of neutrons (8) but different A.

Question 4
Which species has as many neutrons as 2311Na⁺ has electrons?
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Answer: (a) 199F⁻

Na⁺ has 11 − 1 = 10 electrons. Neutrons = A − Z: F⁻ 19 − 9 = 10; O 8; Mg²⁺ 12 (the charge does not change the neutrons); N 7.

Question 5
Photons of which of these radiations carry the most energy?
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Answer: (b) ultraviolet in sunlight

E = hν. Of the four regions ultraviolet has the highest frequency (about 10¹⁶ Hz), so its photons have the most energy.

Question 6
An absorption line in a spectrum is recorded at a wavenumber of 2000 cm⁻¹. Its wavelength is
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Answer: (c) 5.0 µm

λ = 1/ν̄ = 1/2000 cm = 5.0 × 10⁻⁴ cm = 5.0 × 10⁻⁶ m = 5.0 µm. 50 µm and 0.50 µm slip the cm to µm conversion by ten; 20 µm inverts the wrong quantity.

Question 7
Light of wavelength 480 nm (photon energy 2.59 eV) falls in turn on Li, Na, K, Mg, Cu and Ag, whose work functions are given in Table 2.2 of NCERT (Li 2.42, Na 2.3, K 2.25, Mg 3.7, Cu 4.8, Ag 4.3 eV). Photoelectrons are ejected from
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Answer: (d) Li, Na and K only

An electron is ejected only when hν exceeds the work function W₀. 2.59 eV is more than 2.42, 2.3 and 2.25 eV but less than 3.7, 4.8 and 4.3 eV, so Li, Na and K emit and the other three do not.

Question 8
Ultraviolet light of wavelength 248 nm ejects photoelectrons with a maximum kinetic energy of 0.70 eV from a metal listed in Table 2.2 of NCERT (Li 2.42, Na 2.3, K 2.25, Mg 3.7, Cu 4.8, Ag 4.3 eV). Taking h = 6.626 × 10⁻³⁴ J s, c = 3.0 × 10⁸ m s⁻¹ and 1 eV = 1.602 × 10⁻¹⁹ J, the metal is
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Answer: (a) Ag

E = hc/λ = (6.626 × 10⁻³⁴ × 3.0 × 10⁸)/(248 × 10⁻⁹) = 8.02 × 10⁻¹⁹ J = 5.00 eV. W₀ = hν − KE = 5.00 − 0.70 = 4.30 eV, silver. Cu (4.8 eV) would need KE 0.20 eV; Mg would give 1.30 eV.

Question 9
Light of a fixed frequency above the threshold frequency falls on a metal. If its intensity is doubled,
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Answer: (c) more photoelectrons are ejected, with the same maximum kinetic energy

A brighter beam carries more photons, each of the same energy hν. More electrons are struck and ejected, but the energy given to each, and so its kinetic energy, is unchanged.

Question 10
The lines of the hydrogen spectrum that lie in the visible region all arise from transitions that end at
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Answer: (d) n = 2

The visible lines form the Balmer series, n₁ = 2. Lines ending at n = 1 (Lyman) are ultraviolet and at n = 3 (Paschen) infrared.

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