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Structure of Atom — NEET & JEE MCQs with answers
Unit 2 traces the experimental discoveries that revealed atomic substructure — cathode rays (electron), canal rays (proton), and Chadwick's neutron — then develops successive atomic models: Thomson, Rutherford, and Bohr. The unit closes with the quantum mechanical model, introducing wave-particle duality, the Heisenberg uncertainty principle, the Schrödinger equation, quantum numbers, orbital shapes and energies, and the rules for filling electrons into orbitals.
Topics in this chapter
- 2.1 Discovery of Sub-atomic Particles
- 2.2 Atomic Models
- 2.3 Developments Leading to the Bohr Model
- 2.4 Bohr's Model for Hydrogen Atom
- 2.5 Towards Quantum Mechanical Model of the Atom
- 2.6 Quantum Mechanical Model of Atom
Free practice questions
The mass of the electron in unified atomic mass units (u), as listed in the table, is approximately:
- A. 1.00727 u
- B. 1.00867 u
- C. 0.00054 u
- D. 1 u
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Answer: C — In Table 2.1, the mass of the electron is 0.00054 u (approximate mass 0 u).
NCERT Class 11 Chemistry · Ch 2 · 2.1 Discovery of Sub-atomic Particles
The de Broglie wavelength of an electron is $1.2 \times 10^{-10}\,\text{m}$. The momentum of the electron is: (Take $h = 6.626 \times 10^{-34}\,\text{J s}$)
- A. $1.2 \times 10^{-10}\,\text{kg m s}^{-1}$
- B. $5.52 \times 10^{-44}\,\text{kg m s}^{-1}$
- C. $7.95 \times 10^{-44}\,\text{kg m s}^{-1}$
- D. $5.52 \times 10^{-24}\,\text{kg m s}^{-1}$
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Answer: D — $p = h/\lambda = (6.626 \times 10^{-34})/(1.2 \times 10^{-10}) = 5.52 \times 10^{-24}\,\text{kg m s}^{-1}$.
NCERT Class 11 Chemistry · Ch 2 · 2.5.1 Dual Behaviour of Matter
The approximate thickness of the gold foil used in Rutherford's scattering experiment was:
- A. About 100 nm
- B. About 1 mm
- C. About 10⁻¹⁵ m
- D. About 1 cm
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Answer: A — A stream of high energy α-particles was directed at a thin foil of gold metal of thickness approximately 100 nm.
NCERT Class 11 Chemistry · Ch 2 · 2.2 Atomic Models
According to Pauli's principle, two electrons in the same orbital can have the same n, l and ml but must differ in:
- A. the azimuthal quantum number
- B. the magnetic quantum number
- C. the principal quantum number
- D. the spin quantum number
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Answer: D — NCERT states two electrons can have the same value of three quantum numbers n, l and ml but must have the opposite spin quantum number.
NCERT Class 11 Chemistry · Ch 2 · 2.6 Quantum Mechanical Model of Atom
Using $\nu = 3.29\times10^{15}\left(\dfrac{1}{n_1^2}-\dfrac{1}{n_2^2}\right)$ Hz, the frequency of the photon for the $n_2=4\to n_1=3$ transition in hydrogen is about:
- A. $3.2 \times 10^{14}$ Hz
- B. $-1.6 \times 10^{14}$ Hz
- C. $1.6 \times 10^{14}$ Hz
- D. $5.71 \times 10^{14}$ Hz
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Answer: C — $\nu = 3.29\times10^{15}\left(\frac{1}{3^2}-\frac{1}{4^2}\right) = 1.6 \times 10^{14}$ Hz.
NCERT Class 11 Chemistry · Ch 2 · 2.4 Bohr's Model for Hydrogen Atom
Which series of the hydrogen spectrum appears in the visible region?
- A. Brackett series
- B. Balmer series
- C. Lyman series
- D. Paschen series
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Answer: B — The Balmer series ($n_1 = 2$) is the only series of the hydrogen spectrum that appears in the visible region of the electromagnetic spectrum.
NCERT Class 11 Chemistry · Ch 2 · 2.3 Developments Leading to the Bohr Model
Cathode rays travel in a cathode ray tube from the:
- A. Cathode (negative electrode) to the anode (positive electrode)
- B. Cathode back to the cathode
- C. Anode to the cathode
- D. Anode out through the glass walls
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Answer: A — When sufficiently high voltage is applied, current flows as a stream of particles moving from the negative electrode (cathode) to the positive electrode (anode). These are called cathode rays.
NCERT Class 11 Chemistry · Ch 2 · 2.1 Discovery of Sub-atomic Particles
The chlorine isotopes ³⁵₁₇Cl and ³⁷₁₇Cl contain, besides 17 protons, the following numbers of neutrons respectively:
- A. 18 and 20
- B. 17 and 17
- C. 35 and 37
- D. 20 and 18 — but in reverse order to the masses
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Answer: A — Number of neutrons = A − Z. For ³⁵₁₇Cl, 35 − 17 = 18 neutrons; for ³⁷₁₇Cl, 37 − 17 = 20 neutrons. The NCERT text states chlorine atoms contain 18 and 20 neutrons besides 17 protons.
NCERT Class 11 Chemistry · Ch 2 · 2.2 Atomic Models
Wavenumber $\bar{\nu}$ is defined as:
- A. the energy per unit wavelength
- B. the number of wavelengths per unit length, $\bar{\nu} = 1/\lambda$
- C. the number of waves per unit time, $\bar{\nu} = \nu/c$
- D. the product of frequency and wavelength
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Answer: B — Wavenumber is defined as the number of wavelengths per unit length, i.e. $\bar{\nu} = 1/\lambda$. Its units are the reciprocal of the wavelength unit ($\text{m}^{-1}$ or commonly $\text{cm}^{-1}$).
NCERT Class 11 Chemistry · Ch 2 · 2.3 Developments Leading to the Bohr Model
The net positive charge experienced by the outer electrons after partial screening by inner shell electrons is the:
- A. effective nuclear charge
- B. shielding constant
- C. total nuclear charge
- D. ionic charge
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Answer: A — NCERT states the net positive charge experienced by the outer electrons is known as effective nuclear charge.
NCERT Class 11 Chemistry · Ch 2 · 2.6 Quantum Mechanical Model of Atom
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