Bohr's Model and Electron Shells K L M N - UNSOLVED PRACTICE SET

Q7. State the three main postulates of Bohr's model of the atom.

Q8. Draw the electron distribution in the shells for the following atoms: (i) Sodium (atomic number 11), (ii) Chlorine (atomic number 17), (iii) Calcium (atomic number 20). Use the notation 2, 8, 8, 2 etc.

Q9. Your teacher explains electron shells using the analogy of seats in a school auditorium — the front rows (K shell) fill first, then the middle rows (L shell), and so on. Explain how this analogy helps understand why the K shell holds only 2 electrons while the L shell holds 8.

Q10. Why did Bohr propose that electrons can only exist in certain discrete orbits and not in between? What problem of Rutherford's model did this solve?

Q11. Write the electronic configuration of the following elements: (i) Carbon (Z = 6), (ii) Aluminium (Z = 13), (iii) Potassium (Z = 19), (iv) Argon (Z = 18).

Q12. What is the significance of the formula 2n² for electron distribution in shells? Calculate the maximum number of electrons in the first four shells using this formula.

Q13. Explain Bohr's model of the atom in detail. State its main postulates and explain how it solved the problem of atomic stability that Rutherford's model could not explain. Draw the electron distribution in the shells for an atom with atomic number 15. What are the limitations of Bohr's model?

Q14. A student is confused about why the K shell can hold only 2 electrons while the L shell can hold 8. Help the student by:

(a) Explaining the formula 2n² and applying it to the K and L shells.

(b) Explaining the concept of energy levels and why shells closer to the nucleus have lower energy.

(c) Drawing the electron distribution for elements from Z = 1 to Z = 10.

(d) Explaining why the filling of shells follows the order K → L → M → N and not randomly.

Q15. "Bohr's model was a brilliant blend of classical physics and quantum ideas, but it was only the beginning of quantum mechanics." Discuss this statement by explaining: (i) how Bohr's model explained the hydrogen spectrum, (ii) why it failed for multi-electron atoms, (iii) how the concept of electron shells is still used today, and (iv) the transition from Bohr's model to the quantum mechanical model.

Q16. The energy of an electron in the nth orbit of a hydrogen atom according to Bohr's model is given by:

Eₙ = -13.6/n² eV

(a) Calculate the energy of an electron in the first orbit (n = 1).

(b) Calculate the energy of an electron in the second orbit (n = 2).

(c) Calculate the energy difference between the first and second orbits.

(d) When an electron jumps from n = 2 to n = 1, a photon is emitted. Calculate the wavelength of this photon. (1 eV = 1.6 × 10⁻¹⁹ J, h = 6.626 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)

Q17. A school science project requires students to create models of atoms using balls and sticks.

(a) If a student is making a model of oxygen (Z = 8), how many electrons should be placed in each shell?

(b) If the student uses red balls for electrons, and each ball costs ₹5, calculate the total cost for making models of oxygen, neon (Z = 10), and sulphur (Z = 16).

(c) The student wants to represent the different energy levels using different coloured rings. If the K shell ring has a radius of 2 cm, and each successive shell ring has a radius 1.5 times the previous one, calculate the radii of the L, M, and N shell rings.

(d) Why is this model only an approximation of the actual atom?

Q18. The visible lines in the hydrogen spectrum (Balmer series) are produced when electrons jump from higher orbits to the second orbit (n = 2).

(a) If an electron jumps from n = 3 to n = 2, calculate the energy of the emitted photon using Bohr's formula.

(b) Calculate the wavelength of this photon. Is it in the visible range? (Visible range: 400–700 nm)

(c) The red colour in a neon sign is produced by a similar electron transition. Explain how this is related to Bohr's model.

(d) Street lights in many Indian cities use sodium vapour lamps that emit yellow light. If the yellow light has a wavelength of 589 nm, calculate the energy difference between the two energy levels involved in this transition.