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Atoms — NEET & JEE MCQs with answers
This chapter traces the development of atomic models from Thomson's plum-pudding model through Rutherford's nuclear model to Bohr's model of the hydrogen atom. Bohr's postulates explain the line spectra of hydrogen and predict its ionisation energy, and de Broglie's matter-wave hypothesis later provided an explanation for Bohr's second postulate, the quantisation of angular momentum.
Topics in this chapter
- 12.1 Introduction
- 12.2 Alpha-Particle Scattering and Rutherford's Nuclear Model of the Atom
- 12.3 Atomic Spectra
- 12.4 Bohr Model of the Hydrogen Atom
- 12.5 The Line Spectra of the Hydrogen Atom
- 12.6 De Broglie's Explanation of Bohr's Second Postulate of Quantisation
Free practice questions
The energy released when a hydrogen electron settles from the $n=4$ level into the $n=3$ level is.
- A. $1.13\ \text{eV}$
- B. $1.51\ \text{eV}$
- C. $0.661\ \text{eV}$
- D. $-0.661\ \text{eV}$
Show answer
Answer: C — $E=13.6\left(\dfrac{1}{9}-\dfrac{1}{16}\right)=0.661\ \text{eV}$.
NCERT Class 12 Physics Ch 12 ("the difference of energy is carried away by a photon of frequency")
The energy difference between the $n=4$ and $n=3$ levels of hydrogen is.
- A. $1.13\ \text{eV}$
- B. $1.32\ \text{eV}$
- C. $0.66\ \text{eV}$
- D. $1.51\ \text{eV}$
Show answer
Answer: C — $\Delta E=13.6\left(\dfrac{1}{9}-\dfrac{1}{16}\right)=0.66\ \text{eV}$.
NCERT Class 12 Physics Ch 12 ("come closer and closer together as")
What was the thickness of the gold foil used in the Geiger–Marsden experiment?
- A. $2.1 \times 10^{-7}$ m
- B. $1.2 \times 10^{-7}$ m
- C. $2.1 \times 10^{-6}$ m
- D. $2.1 \times 10^{-8}$ m
Show answer
Answer: A — NCERT specifies that the beam fell on a thin foil of gold of thickness $2.1 \times 10^{-7}$ m.
NCERT Class 12 Physics Ch 12 ("a thin foil of gold of thickness 2.1 × 10-7 m")
What does a line spectrum, per NCERT, provide that is useful?
- A. It provides only decorative information about light sources
- B. It provides useful information about the atomic structure
- C. It provides no information of any scientific value
- D. It provides information about the temperature of condensed matter only
Show answer
Answer: B — NCERT states the line spectrum (a set of isolated parallel lines) provides useful information about the atomic structure.
NCERT Class 12 Physics Ch 12 ("a set of isolated parallel lines termed as line spectrum")
In de Broglie's model, an allowed Bohr orbit must contain.
- A. any fractional number of wavelengths
- B. a whole number of de Broglie wavelengths
- C. a quarter wavelength
- D. half a wavelength
Show answer
Answer: B — The circumference must hold a whole number of de Broglie wavelengths.
NCERT Class 12 Physics Ch 12 ("2p rn = nl")
According to Rutherford's nuclear model, where are the entire positive charge and most of the mass of the atom concentrated?
- A. A uniformly spread cloud around the atom
- B. A small volume called the nucleus
- C. The empty space between electrons
- D. The outermost electron shell
Show answer
Answer: B — Rutherford's nuclear model states that the entire positive charge and most of the mass of the atom is concentrated in a small volume called the nucleus.
NCERT Class 12 Physics Ch 12 ("the entire positive charge and most of the mass of the atom is concentrated in a small volume called the nucleus")
According to Eq. (12.7), $r_n=\left(\dfrac{n^2}{m}\right)\left(\dfrac{h}{2\pi}\right)^2\dfrac{4\pi\varepsilon_0}{e^2}$, which statement about the radius of the nth orbit is correct?
- A. $r_n$ is directly proportional to $e^2$
- B. $r_n$ does not depend on Planck's constant h
- C. $r_n$ is directly proportional to the electron mass m
- D. $r_n$ is inversely proportional to the electron mass m
Show answer
Answer: D — p.299, Eq. (12.7): m and $e^2$ appear in the denominator, so $r_n\propto1/m$ and $r_n\propto1/e^2$; $r_n$ depends on h as $h^2$.
NCERT Class 12 Physics · Ch 12 · 12.4 Bohr Model of the Hydrogen Atom
The energy carried away by the photon in the $n=11\to n=2$ transition of hydrogen is.
- A. $-3.29\ \text{eV}$
- B. $5.56\ \text{eV}$
- C. $3.4\ \text{eV}$
- D. $3.29\ \text{eV}$
Show answer
Answer: D — $E=13.6\left(\dfrac{1}{4}-\dfrac{1}{121}\right)=3.29\ \text{eV}$.
NCERT Class 12 Physics Ch 12 ("the difference of energy is carried away by a photon of frequency")
How did subsequent studies describe the actual distribution of electrons and positive charge compared with Thomson's proposal?
- A. Exactly as Thomson had proposed
- B. Impossible to determine experimentally
- C. Identical for every element
- D. Very different from Thomson's proposal
Show answer
Answer: D — Subsequent studies (leading to Rutherford's work) showed that the distribution of the electrons and positive charges are very different from Thomson's proposal.
NCERT Class 12 Physics Ch 12 ("the distribution of the electrons and positive charges are very different")
How many de Broglie wavelengths of the electron fit into the circumference of the $n=2$ Bohr orbit?
- A. $4$
- B. $2$
- C. $1$
- D. $3$
Show answer
Answer: B — From $2\pi r_n=n\lambda$, exactly $n=2$ wavelengths fit.
NCERT Class 12 Physics Ch 12 ("2p rn = nl")
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