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CBSE Class 9 Structure of the Atom — practice questions

Explore 50 free Structure of the Atom questions in CBSE Class 9 Science. Open each question or exercise to read its content, reference answers and available explanation.

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Structure of the Atom questions and exercises with answers

Showing questions 1–30 of 50.

The maximum number of electrons that can be accommodated in the $n$-th shell of an atom is:Read the answer and worked solution →The four quantum numbers describing an electron in an atom are:Read the answer and worked solution →According to Heisenberg's uncertainty principle, $\Delta x \cdot \Delta p \geq$:Read the answer and worked solution →Two electrons in the same orbital must have:Read the answer and worked solution →Calculate the de Broglie wavelength of an electron moving at $10^6$ m/s. (Take $h = 6.6 \times 10^{-34}$ J·s, $m_e = 9.1 \times 10^{-31}$ kg)Read the answer and worked solution →In Rutherford's atomic model, electrons revolve around the nucleus due to:Read the answer and worked solution →A key failure of Rutherford's classical model was that an accelerating electron should:Read the answer and worked solution →In Bohr's model of the hydrogen atom, electrons can only occupy orbits in which the angular momentum is an integer multiple of:Read the answer and worked solution →The radius of the n-th Bohr orbit for hydrogen is proportional to:Read the answer and worked solution →The Bohr radius a_0 is approximately:Read the answer and worked solution →In Bohr's model, the energy of the n-th level of hydrogen is:Read the answer and worked solution →The ground-state (n = 1) energy of the hydrogen atom is approximately:Read the answer and worked solution →The energy of the first excited state (n = 2) of hydrogen is:Read the answer and worked solution →The ionisation energy of hydrogen from its ground state is approximately:Read the answer and worked solution →When a hydrogen atom makes a transition from n = 2 to n = 1, the energy of the emitted photon is approximately:Read the answer and worked solution →Atomic number Z of an atom equals:Read the answer and worked solution →Isotopes of an element have the same:Read the answer and worked solution →Canal rays were discovered in 1886 by:Read the answer and worked solution →The mass of a proton is about how many times that of an electron?Read the answer and worked solution →The charge on a proton is taken as:Read the answer and worked solution →Who first proposed a model of the atom?Read the answer and worked solution →In Thomson's model the electrons are set in a sphere that is:Read the answer and worked solution →Rutherford directed alpha particles at a thin foil of:Read the answer and worked solution →In the scattering experiment, most alpha particles:Read the answer and worked solution →Roughly how many alpha particles rebounded?Read the answer and worked solution →Rutherford concluded that most of the space inside an atom is:Read the answer and worked solution →The rebounding particles showed that positive charge and mass sit in a:Read the answer and worked solution →The neutron was discovered in 1932 by:Read the answer and worked solution →Neutrons are present in the nucleus of all atoms except:Read the answer and worked solution →The distribution of electrons into shells was suggested by:Read the answer and worked solution →

Worked practice questions

The maximum number of electrons that can be accommodated in the $n$-th shell of an atom is:

  1. $n^2$
  2. $n$
  3. $2n$
  4. $2n^2$
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Correct answer: D. $2n^2$

Bohr-Bury rule. Each shell of principal quantum number $n$ contains $n^2$ orbitals, and each orbital holds at most 2 electrons (Pauli). So shell capacity is $2n^2$. Quick check: K shell ($n=1$) holds 2, L ($n=2$) holds 8, M ($n=3$) holds 18, N ($n=4$) holds 32. All powers-of-two-friendly numbers.

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The four quantum numbers describing an electron in an atom are:

  1. $n, l, m_l, m_s$
  2. $n, l, m_l, m_l$
  3. $n, p, d, f$
  4. $n, l, m, s$
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Correct answer: A. $n, l, m_l, m_s$

**Principal $n$** sets the shell and energy. **Azimuthal $l$** sets the subshell shape (s, p, d, f). **Magnetic $m_l$** sets the orbital's orientation in space. **Spin $m_s$** picks $+\tfrac{1}{2}$ or $-\tfrac{1}{2}$. First three come from the Schrödinger equation the spin comes from electron's intrinsic angular momentum. Option D lists subshell labels (s, p, d, f), not quantum numbers.

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According to Heisenberg's uncertainty principle, $\Delta x \cdot \Delta p \geq$:

  1. $\dfrac{h}{2\pi}$
  2. $h$
  3. $\dfrac{h}{4\pi}$
  4. $\dfrac{h}{4}$
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Correct answer: C. $\dfrac{h}{4\pi}$

$\Delta x \cdot \Delta p \geq \dfrac{h}{4\pi} = \dfrac{\hbar}{2}$, where $\hbar = h/2\pi$ is the reduced Planck constant. **Meaning:** you cannot simultaneously know an electron's position and momentum with arbitrary precision. Trying to pin one down makes the other less knowable. This forces us to talk about probability clouds (orbitals) rather than precise electron trajectories. Option B is often a near miss in memory because $\hbar = h/2\pi$, but the uncertainty bound has the factor of 2 baked into the denominator twice over.

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