Start learning →

CBSE Class 9 Science — practice by topic

Browse 757 questions across 12 Science topics. Choose what you want to revise, then open a question to check its answer and worked solution.

Start learning CBSE Class 9 with Quill →

Science topics

Diversity in Living Organisms (132)The Fundamental Unit of Life (72)Force and Laws of Motion (66)Tissues: Plant and Animal Tissues (66)Work and Energy (63)Matter in Our Surroundings (57)Motion (51)Atoms and Molecules (50)Gravitation (50)Is Matter Around Us Pure? (50)Sound (50)Structure of the Atom (50)

Try a Science question

According to the Bohr model, the ground-state energy of a hydrogen atom is:Read the answer and worked solution →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 →

Worked practice questions

According to the Bohr model, the ground-state energy of a hydrogen atom is:

  1. $-13.6$ eV
  2. $-6.8$ eV
  3. $-27.2$ eV
  4. $-3.4$ eV
Show answer and solution

Correct answer: A. $-13.6$ eV

Bohr's energy formula: $E_n = -\dfrac{13.6}{n^2}$ eV for hydrogen. For the ground state $n = 1$: $E_1 = -13.6$ eV. - Option A ($-3.4$ eV) is $E_2$. - Option D ($-27.2$ eV) is the magnitude of *twice* the ground-state energy (a common confusion with the virial-theorem relation $E = -\frac{1}{2}|U|$).

Open this question →

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. $2n^2$
Show answer and solution

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.

Open this question →

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. $n, l, m, s$
Show answer and solution

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.

Open this question →

How to practise

  1. Choose a Science question and try to work out your answer.
  2. Open the question to check the correct answer and read its worked solution.
  3. Continue in the app for bite-sized lessons, explanations and review with Quill. You can start without creating an account.