Level 1 — RecognitionAtomic Structure (Classical)

Atomic Structure (Classical)

⏱ 20 minutes30 marksprintable — key stays hidden on paper

Level: 1 — Recognition (MCQ + Matching + True/False with justification) Time Limit: 20 minutes Total Marks: 30

Section A — Multiple Choice (1 mark each) [10 marks]

Choose the single best answer.

Q1. Which subatomic particle was discovered by James Chadwick? (a) Electron (b) Proton (c) Neutron (d) Positron

Q2. The cathode ray experiment by J.J. Thomson determined the: (a) charge on the electron (b) charge-to-mass ratio ( $e/m$ ) of the electron (c) mass of the proton (d) radius of the nucleus

Q3. In Rutherford's gold-foil experiment, most $\alpha$ -particles passed straight through because: (a) the atom is mostly empty space (b) $\alpha$ -particles are negatively charged (c) the nucleus is negatively charged (d) gold is very dense

Q4. For a hydrogen-like species, the Bohr radius of the $n$ -th orbit is given by $r_n = 0.529\,\dfrac{n^2}{Z}$ Å. The radius of the 2nd orbit of $He^+$ ( $Z=2$ ) is: (a) 0.529 Å (b) 1.058 Å (c) 2.116 Å (d) 0.264 Å

Q5. The energy of an electron in the $n$ -th orbit of hydrogen is $E_n = -13.6/n^2$ eV. The energy of the ground state ( $n=1$ ) is: (a) $-3.4$ eV (b) $-13.6$ eV (c) $+13.6$ eV (d) $-1.51$ eV

Q6. Atoms of the same element having different mass numbers are called: (a) isobars (b) isotones (c) isotopes (d) isomers

Q7. The spectral series of hydrogen that lies in the visible region is: (a) Lyman (b) Balmer (c) Paschen (d) Pfund

Q8. In the Rydberg formula $\dfrac{1}{\lambda}=R\left(\dfrac{1}{n_1^2}-\dfrac{1}{n_2^2}\right)$ , the Lyman series corresponds to: (a) $n_1=1$ (b) $n_1=2$ (c) $n_1=3$ (d) $n_1=4$

Q9. Which of the following is NOT a postulate of Dalton's atomic theory? (a) Atoms are indivisible (b) Atoms of the same element are identical (c) Atoms contain electrons, protons and neutrons (d) Atoms combine in small whole-number ratios

Q10. A major limitation of the Bohr model is that it: (a) explains the hydrogen spectrum (b) fails to explain spectra of multi-electron atoms (c) uses quantized angular momentum (d) treats the nucleus as positive

Section B — Matching (1 mark each correct pair) [8 marks]

Q11. Match Column I with Column II.

Column I	Column II
(i) Electron discovery	(P) Chadwick
(ii) Proton (canal rays)	(Q) Rutherford
(iii) Neutron	(R) J.J. Thomson
(iv) Nuclear model	(S) Goldstein

Q12. Match the hydrogen spectral series with the lower level $n_1$ .

Column I	Column II
(i) Lyman	(P) $n_1=3$
(ii) Balmer	(Q) $n_1=1$
(iii) Paschen	(R) $n_1=4$
(iv) Brackett	(S) $n_1=2$

Section C — True / False WITH justification (2 marks each) [12 marks]

State True or False (1 mark) and give a one-line justification (1 mark).

Q13. Isobars have the same number of protons but different mass numbers.

Q14. In Thomson's plum-pudding model, electrons are embedded in a uniform sphere of positive charge.

Q15. According to Bohr, angular momentum of an electron is quantized as $mvr = n\dfrac{h}{2\pi}$ .

Q16. ${}^{40}_{18}Ar$ and ${}^{40}_{20}Ca$ are isotopes of each other.

Q17. The radius of a Bohr orbit increases as $n^2$ for a given atom.

Q18. The Bohr model successfully explains the fine structure of spectral lines.

Answer keyMark scheme & solutions

Section A (1 mark each)

Q1. (c) Neutron. Chadwick (1932) discovered the neutral neutron. [1]

Q2. (b) $e/m$ ratio. Thomson measured deflection of cathode rays in fields → charge-to-mass ratio. [1]

Q3. (a) Atom is mostly empty space. Undeflected majority ⇒ tiny dense nucleus, vast empty region. [1]

Q4. (b) 1.058 Å. $r_2 = 0.529\times\dfrac{2^2}{2}=0.529\times 2 = 1.058$ Å. [1]

Q5. (b) $-13.6$ eV. $E_1 = -13.6/1^2 = -13.6$ eV. [1]

Q6. (c) Isotopes. Same $Z$ , different $A$ . [1]

Q7. (b) Balmer. Balmer lines ( $n_1=2$ ) fall in visible region. [1]

Q8. (a) $n_1=1$ . Lyman series terminates at ground level $n_1=1$ . [1]

Q9. (c) Atoms contain electrons, protons and neutrons. This is a modern discovery, contradicting Dalton's "indivisible atom." [1]

Q10. (b) Fails for multi-electron atoms. Bohr works only for one-electron (hydrogen-like) systems. [1]

Section B

Q11. (i)–R, (ii)–S, (iii)–P, (iv)–Q. (1 mark per correct pair, max 4) [4]

Q12. (i)–Q, (ii)–S, (iii)–P, (iv)–R. (1 mark per correct pair, max 4) [4]

Section C (True/False = 1, justification = 1)

Q13. False. Isobars have the same mass number $A$ but different proton numbers $Z$ (e.g. ${}^{40}_{18}Ar$ , ${}^{40}_{20}Ca$ ). [1+1]

Q14. True. Positive charge is spread uniformly; electrons are embedded like plums in a pudding. [1+1]

Q15. True. Bohr's quantization postulate: $mvr = n\dfrac{h}{2\pi}$ , $n = 1,2,3\dots$ [1+1]

Q16. False. They have the same $A=40$ but different $Z$ (18 vs 20) → they are isobars, not isotopes. [1+1]

Q17. True. For fixed $Z$ , $r_n = 0.529\,n^2/Z \propto n^2$ . [1+1]

Q18. False. Bohr's model fails to explain fine structure (splitting of lines); this needs relativistic/quantum corrections. [1+1]

[
  {"claim":"He+ 2nd orbit radius = 1.058 Angstrom (Q4)","code":"r=0.529*(2**2)/2; result = abs(r-1.058)<1e-9"},
  {"claim":"H ground state energy = -13.6 eV (Q5)","code":"E=-13.6/(1**2); result = E==-13.6"},
  {"claim":"Bohr radius scales as n^2 (r4/r1 = 16)","code":"ratio=(0.529*16)/(0.529*1); result = abs(ratio-16)<1e-9"},
  {"claim":"Ar-40 and Ca-40 are isobars: same A, different Z","code":"A_ar,Z_ar=40,18; A_ca,Z_ca=40,20; result = (A_ar==A_ca) and (Z_ar!=Z_ca)"}
]