Structure of Atoms MCQs with Answer

Structure of Atoms MCQs
Multiple Choice Questions for NEET, IIT-JEE and CUET

1. Which of the following statement is wrong about cathode rays?

• (a) They produce heating effect
• (b) They carry negative charge
• (c) They produce X-rays when striking material with high atomic masses
• (d) None of the above

View Answer & Explanation

Correct Answer: (d) None of the above

All the listed properties are true characteristics of cathode rays, hence none of them is wrong.

2. The correct statement about proton is:

• (a) It is a nucleus of deuterium
• (b) It is an ionised hydrogen atom
• (c) It is an ionised hydrogen molecule
• (d) It is an α-particle

View Answer & Explanation

Correct Answer: (b) It is an ionised hydrogen atom

A proton is essentially a hydrogen nucleus (H⁺), formed when hydrogen loses its single electron.

3. The absolute value of the charge on electron was determined by:

• (a) J.J. Thomson
• (b) R.A. Millikan
• (c) Rutherford
• (d) Chadwick

View Answer & Explanation

Correct Answer: (b) R.A. Millikan

Millikan’s oil-drop experiment provided the first accurate measurement of the electron’s charge.

4. The highest value of e/m of anode rays has been observed when the discharge tube is filled with:

• (a) N₂
• (b) O₂
• (c) H₂
• (d) He

View Answer & Explanation

Correct Answer: (c) H₂

Hydrogen ions have the smallest mass, giving the highest charge-to-mass (e/m) ratio among anode rays.

5. Which of the following statements is incorrect?

• (a) The charge on electron and proton are equal and opposite
• (b) Neutrons have no charge
• (c) The mass of proton and electron are nearly the same
• (d) None of the above is incorrect

View Answer & Explanation

Correct Answer: (c) The mass of proton and electron are nearly the same

The mass of a proton is about 1836 times greater than that of an electron, so they are not nearly the same.

6. When a gold sheet is bombarded by a beam of α-particles, only a few of them get deflected, whereas most go straight, undeflected. This is because:

• (a) The force of attraction exerted on α-particle by electrons is insufficient
• (b) The volume of nucleus is smaller than atom
• (c) The force of repulsion acting on fast moving α-particle is very small
• (d) The neutrons have no effect on α-particle

View Answer & Explanation

Correct Answer: (b) The volume of nucleus is smaller than atom

Most α-particles pass through undeflected because the nucleus occupies a very small volume compared to the atom.

7. The Rutherford experiment led to the conclusion that:

• (a) Mass and energy are related
• (b) Mass and positive charge of an atom are concentrated in its centre as nucleus
• (c) Neutrons are present in the nucleus
• (d) None of the above

View Answer & Explanation

Correct Answer: (b) Mass and positive charge of an atom are concentrated in its centre as nucleus

Rutherford’s scattering experiment demonstrated that atoms have a dense, positively charged nucleus at the centre.

8. Which of the following is true for Thomson's model of atom?

• (a) The radius of an electron can be calculated by using this model
• (b) In an undisturbed atom, the electron will be at their equilibrium position, where the attraction between the cloud of positive charge and the electrons balances their mutual repulsion
• (c) It can explain the existence of protons
• (d) None of the above

View Answer & Explanation

Correct Answer: (b) In an undisturbed atom, the electron will be at their equilibrium position...

Thomson’s “plum pudding” model suggested electrons embedded in a positively charged sphere, balanced by electrostatic forces.

9. Rutherford scattering formula fails for very small scattering angles because:

• (a) The kinetic energy of α-particles is larger
• (b) The gold foil is very thin
• (c) The full nuclear charge of the target atom is partially screened by its electrons
• (d) All of the above

View Answer & Explanation

Correct Answer: (c) The full nuclear charge of the target atom is partially screened by its electrons

At small angles, electron screening reduces the effective nuclear charge experienced by α-particles, causing deviation from Rutherford’s formula.

10. The conclusion of Rutherford scattering experiment does not include:

• (a) α-particle can come within a distance of the order of 10⁻¹⁴ m of the nucleus
• (b) The radius of the nucleus is less than 10⁻¹⁴ m
• (c) Scattering follows Coulomb's law
• (d) The positively charged parts of an atom move with extremely high velocities

View Answer & Explanation

Correct Answer: (d) The positively charged parts of an atom move with extremely high velocities

Rutherford’s scattering experiment concluded that the nucleus is very small, dense, and positively charged, and that scattering follows Coulomb’s law. However, it did not suggest that the positively charged parts of the atom move with high velocities.

11. In a set of degenerate orbitals, the electrons distribute themselves to retain like spins as far as possible. This statement belongs to:

• (a) Pauli's exclusion principle
• (b) Aufbau principle
• (c) Hund's rule of maximum multiplicity
• (d) Slater's rule

View Answer & Explanation

Correct Answer: (c) Hund's rule of maximum multiplicity

Hund's rule states that electrons occupy degenerate orbitals singly with parallel spins before pairing. This maximizes exchange energy and stabilizes the atom. Pauli's principle restricts identical quantum numbers, Aufbau explains orbital filling order, and Slater's rule estimates effective nuclear charge.

12. The change in orbital angular momentum corresponding to an electron transition inside an atom can be:

• (a) h/4π
• (b) h/π
• (c) h/8π
• (d) None of these

View Answer & Explanation

Correct Answer: (d) None of these

Selection rules dictate that the change in orbital angular momentum quantum number must be ±1, not the fractional values listed.

13. In an X-ray experiment, different metals are used as the target. Which of the following plots will be a straight line?

• (a) ν against Z
• (b) ν against √Z
• (c) √ν against Z
• (d) None of these

View Answer & Explanation

Correct Answer: (c) √ν against Z

Moseley’s law states √ν = a(Z − b), hence a straight line is obtained when √ν is plotted against Z.

14. In Moseley's equation [√ν = a(Z − b)]:

• (a) b depends on metal and a is independent of metal
• (b) Both a and b are metal dependent
• (c) Both a and b are independent of metal
• (d) a is metal dependent while b is not

View Answer & Explanation

Correct Answer: (a)

In Moseley’s law, the constant b depends on the screening effect of inner electrons, while a is universal and independent of the metal.

15. Which of the following is not a basic postulate of Dalton's atomic theory?

• (a) Atoms are neither created nor destroyed in a chemical reaction
• (b) In a compound, the relative number and kinds of atoms are constant
• (c) Atoms of all elements are alike, including their masses
• (d) Each element is composed of extremely small particles called atoms

View Answer & Explanation

Correct Answer: (c)

Dalton assumed atoms of the same element are identical, but modern science shows isotopes exist with different masses.

16. Bohr's theory is applicable to:

• (a) He
• (b) Li²⁺
• (c) He²⁺
• (d) None of these

View Answer & Explanation

Correct Answer: (b) Li²⁺

Bohr’s theory applies only to hydrogen-like species (one electron systems) such as H, He⁺, Li²⁺.

17. The mass of electron moving with the velocity of light is:

• (a) 2m_e
• (b) 3m_e
• (c) Zero
• (d) Can be calculated as m′ = m_e/√(1 − v²/c²)

View Answer & Explanation

Correct Answer: (d)

Relativistic mass increases with velocity and is given by m′ = m_e/√(1 − v²/c²). At v = c, the denominator becomes zero, making mass undefined.

18. Compared to the mass of lightest nuclei, the mass of an electron is approximately:

• (a) 1/80
• (b) 1/800
• (c) 1/1800
• (d) 1/2800

View Answer & Explanation

Correct Answer: (c) 1/1800

The electron’s mass is about 1/1836 of a proton’s mass, often approximated as 1/1800 relative to light nuclei.

19. The probability of finding the electron in the orbital is:

• (a) 100%
• (b) 90–95%
• (c) 70–80%
• (d) 50–60%

View Answer & Explanation

Correct Answer: (b) 90–95%

Orbitals are regions of high probability; about 90–95% of the electron density is contained within the boundary surface of an orbital.

20. A Mo atom in its ground state has a 4d⁵5s¹ configuration and a Ag atom 4d¹⁰5s¹ configuration. This is because a shell which is half filled or completely filled is particularly:

• (a) Strongly exchange destabilised
• (b) Weakly exchange stabilised
• (c) Weakly exchange destabilised
• (d) Strongly exchange stabilised

View Answer & Explanation

Correct Answer: (d) Strongly exchange stabilised

Half-filled (like Mo: 4d⁵) and fully filled (like Ag: 4d¹⁰) subshells gain extra stability due to exchange energy. This arises from parallel spin arrangements that maximize exchange interactions, making these configurations energetically favorable.

21. An f-shell containing 6 unpaired electrons can exchange:

• (a) 6 electrons
• (b) 9 electrons
• (c) 12 electrons
• (d) 15 electrons

View Answer & Explanation

Correct Answer: (d) 15 electrons

The number of possible exchanges is given by n(n−1)/2 for n unpaired electrons. For 6 electrons, this equals 15.

22. The statement that does not belong to Bohr's model of atom is:

• (a) Energy of the electrons in the orbit is quantised
• (b) The electron in the orbit nearest to the nucleus is in lowest energy state
• (c) Electrons revolve in different orbits around the nucleus
• (d) The electrons emit energy during revolution due to Coulombic forces of attraction

View Answer & Explanation

Correct Answer: (d)

Bohr’s model states electrons do not emit energy while in stationary orbits; energy is emitted only during transitions.

23. The argument which favours the particle nature of cathode rays is:

• (a) They produce fluorescence
• (b) They travel through vacuum
• (c) They get deflected by electric and magnetic fields
• (d) They cast shadows of objects present in their way

View Answer & Explanation

Correct Answer: (c)

Deflection by electric and magnetic fields demonstrates that cathode rays consist of charged particles (electrons).

24. The total number of electrons in 92 g of an oxide of nitrogen having vapour density 46 will be:

• (a) 23 × N
• (b) 46 only
• (c) 46 × N
• (d) 23 only

View Answer & Explanation

Correct Answer: (c) 46 × N

Vapour density 46 corresponds to molecular mass 92. The molecule has 46 electrons, so in 92 g (1 mole), total electrons = 46 × Avogadro’s number.

25. Which of the following pairs have identical values of e/m?

• (a) A proton and a neutron
• (b) A proton and deuterium
• (c) Deuterium and an α-particle
• (d) An electron and γ-rays

View Answer & Explanation

Correct Answer: (b)

A proton and deuterium nucleus (one proton + one neutron) have the same charge but different mass, so only proton has identical e/m with hydrogen nucleus.

26. Choose the arrangement which shows the increasing value of e/m for e, p, n and α-particle:

• (a) n < α < p < e
• (b) e < p < α < n
• (c) n < p < e < α
• (d) p < n < α < e

View Answer & Explanation

Correct Answer: (a)

Neutron has no charge (e/m = 0), α-particle has low e/m, proton higher, and electron highest due to its very small mass.

28. The electronic configuration with maximum exchange energy will be:

• (a) 3d¹_xy 3d¹_yz 3d¹_xz 4s¹
• (b) 3d¹_xy 3d¹_yz 3d¹_xz 3d¹_x²−y² 3d¹_z² 4s¹
• (c) 3d²_xy 3d²_yz 3d²_xz 3d²_x²−y² 3d¹_z² 4s¹
• (d) 3d²_xy 3d²_yz 3d²_xz 3d²_x²−y² 3d²_z² 4s¹

View Answer & Explanation

Correct Answer: (b)

Maximum exchange energy occurs when all five d-orbitals are singly occupied (half-filled configuration).

29. The number of waves in an orbit are:

• (a) n²
• (b) n
• (c) n−1
• (d) n−2

View Answer & Explanation

Correct Answer: (b) n

According to de Broglie’s condition, the circumference of the orbit must contain an integral number of wavelengths, equal to n.

30. After np⁶ electronic configuration, the next will be:

• (a) (n+1)d
• (b) (n+1)s
• (c) (n+1)f
• (d) None of these

View Answer & Explanation

Correct Answer: (b) (n+1)s

After filling np⁶, the next orbital according to the Aufbau principle is (n+1)s.

31. From the discharge tube experiment it was concluded that:

• (a) Mass of proton is in fraction
• (b) Matter contains electrons
• (c) Nucleus contains positive charge
• (d) Positive rays are heavier than protons

View Answer & Explanation

Correct Answer: (b) Matter contains electrons

The discharge tube experiment demonstrated the existence of electrons as fundamental constituents of matter.

32. The characteristic not associated with Planck's theory is:

• (a) Radiations are associated with energy
• (b) The magnitude of energy associated with a quantum is proportional to frequency
• (c) Radiation energy is neither emitted nor absorbed continuously
• (d) Radiation energy is neither emitted nor absorbed discontinuously

View Answer & Explanation

Correct Answer: (d)

Planck’s theory states radiation energy is emitted or absorbed in discrete quanta, not continuously.

33. Einstein's theory of photoelectric effect is based on:

• (a) Maxwell's electromagnetic theory of light
• (b) Planck's quantum theory of light
• (c) Both of the above
• (d) None of the above

View Answer & Explanation

Correct Answer: (b) Planck's quantum theory of light

Einstein applied Planck’s quantum hypothesis to explain the photoelectric effect, introducing the concept of photons.

34. Choose the incorrect statement:

• (a) Every object emits radiation whose predominant frequency depends on its temperature
• (b) The quantum energy of a wave is proportional to its frequency
• (c) Photons are quanta of light
• (d) The value of Planck's constant is energy dependent

View Answer & Explanation

Correct Answer: (d)

Planck’s constant is a universal constant and does not depend on energy.

35. Which of the following transitions are not allowed in the normal electronic emission spectrum of an atom?

• (a) 2s → 1s
• (b) 2p → 1s
• (c) 3d → 4p
• (d) 5p → 3s

View Answer & Explanation

Correct Answer: (a) 2s → 1s

Transitions between orbitals of the same l value (like s → s) are forbidden by selection rules.

36. The mass of an electron is m, its charge e and it is accelerated from rest through a potential difference V. The velocity of electron will be calculated by formula:

• (a) √V/m
• (b) √eV/m
• (c) √2eV/m
• (d) None of these

View Answer & Explanation

Correct Answer: (c) √2eV/m

Kinetic energy gained = eV = ½mv², hence v = √(2eV/m).

37. In photoelectric effect the number of photo-electrons emitted is proportional to:

• (a) Intensity of incident beam
• (b) Frequency of incident beam
• (c) Wavelength of incident beam
• (d) All of the above

View Answer & Explanation

Correct Answer: (a) Intensity of incident beam

The number of emitted electrons depends on the intensity (number of photons), while frequency determines their kinetic energy.

38. Einstein's photoelectric equation states that Ek = hν − W. Here, Ek refers to:

• (a) Kinetic energy of all ejected electrons
• (b) Mean kinetic energy of emitted electrons
• (c) Minimum kinetic energy of emitted electrons
• (d) Maximum kinetic energy of emitted electrons

View Answer & Explanation

Correct Answer: (d)

Einstein’s equation gives the maximum kinetic energy of photoelectrons, corresponding to electrons emitted without energy loss.

39. If Ee, Eα and Ep represent the kinetic energies of an electron, α-particle and a proton respectively, each moving with same de Broglie wavelength then:

• (a) Ee = Eα = Ep
• (b) Ee > Eα > Ep
• (c) Eα > Ep > Ee
• (d) Ee > Ep > Eα

View Answer & Explanation

Correct Answer: (c)

For same wavelength, momentum is same. Kinetic energy = p²/2m, so heavier particles (α) have lower velocity but higher kinetic energy.

40. Choose the correct statement among following:

• (a) There are 8 principal electron energy levels
• (b) The second principal energy level can have 4 sub energy levels and contain a maximum of 8 electrons
• (c) The M energy level can have a maximum of 32 electrons
• (d) The 4s level has lower energy than 3d level

View Answer & Explanation

Correct Answer: (d)

According to the Aufbau principle, 4s orbital is filled before 3d, showing it has lower energy.

41. The line spectra of two elements are not identical because:

• (a) The elements do not have the same number of neutrons
• (b) They have different mass numbers
• (c) Their outermost electrons are at different energy levels
• (d) All of the above

View Answer & Explanation

Correct Answer: (c)

Line spectra depend on electronic transitions. Different elements have different electron configurations, leading to unique spectra.

42. Choose the correct statement among the following:

• (a) ψ² represents the atomic orbital
• (b) The number of peaks in radial distribution is n−l
• (c) A node is a point in space around nucleus where the wave function ψ has zero value
• (d) All of the above

View Answer & Explanation

Correct Answer: (d)

All statements are correct: ψ² gives probability density, radial nodes depend on n−l, and nodes are regions of zero probability.

43. In the ground state of Cu⁺, the number of shells occupied, sub-shells occupied, filled orbitals and unpaired electrons respectively are:

• (a) 4, 8, 15, 0
• (b) 3, 6, 15, 1
• (c) 3, 6, 14, 0
• (d) 4, 7, 14, 2

View Answer & Explanation

Correct Answer: (a)

Cu⁺ has configuration [Ar]3d¹⁰. Thus 4 shells, 8 subshells, 15 filled orbitals, and 0 unpaired electrons.

44. Choose the incorrect statement:

• (a) The shape of an atomic orbital depends upon the azimuthal quantum number
• (b) The orientation of an atomic orbital depends upon the magnetic quantum number
• (c) The energy of an electron in an atomic orbital of multi-electron atom depends on principal quantum number
• (d) The number of degenerate atomic orbitals of one type depends on the value of azimuthal and magnetic quantum numbers

View Answer & Explanation

Correct Answer: (c)

In multi-electron atoms, energy depends on both principal (n) and azimuthal (l) quantum numbers, not only n.

45. The magnitude of the spin angular momentum of an electron is given by:

• (a) S = √s(s+1)h/2π
• (b) S = sh/2π
• (c) S = 1.5 h/2π
• (d) None of these

View Answer & Explanation

Correct Answer: (a)

Spin angular momentum is given by √s(s+1)ħ, where s = ½ for electron.

46. The number of spherical nodes in 3p orbitals is:

• (a) 0
• (b) 1
• (c) 2
• (d) 3

View Answer & Explanation

Correct Answer: (b) 1

Number of spherical nodes = n − l − 1. For 3p (n=3, l=1), nodes = 1.

47. Which of the following statement(s) are incorrect?

• (a) Lyman series of hydrogen atom lies in the UV region
• (b) Balmer series lies in the visible region
• (c) Paschen series lies in the visible region
• (d) All of the above

View Answer & Explanation

Correct Answer: (c)

Paschen series lies in the infrared region, not visible. Hence statement (c) is incorrect.

48. The principal quantum number of H atom orbital, if the electron energy is −3.4 eV, will be:

• (a) 1
• (b) 2
• (c) 3
• (d) Zero

View Answer & Explanation

Correct Answer: (2)

Energy of hydrogen atom = −13.6/n² eV. For −3.4 eV, n² = 4, so n = 2.

49. Degenerate orbitals are those which contain:

• (a) Same wave function
• (b) Same orientation
• (c) Same energy
• (d) Both (a) and (b)

View Answer & Explanation

Correct Answer: (c)

Degenerate orbitals are orbitals with the same energy, though they may differ in orientation.

50. The velocity an electron must possess to acquire a momentum equal to the photon of wavelength 5200 Å will be:

• (a) 1398 m/s
• (b) 1298 m/s
• (c) 1400 m/s
• (d) 1300 m/s

View Answer & Explanation

Correct Answer: (c) 1400 m/s

Momentum of photon = h/λ. Equating to mv gives v ≈ 1400 m/s for λ = 5200 Å.

51. Consider the following statements:
1. Electron density in XY plane in 3d_x²−y² orbital is zero
2. Electron density in XY plane in 3d_z² orbital is zero
3. 2s orbital has only one spherical node
4. For 2p_z orbital, YZ is the nodal plane

• (a) 2 and 3
• (b) 1, 2, 3, 4
• (c) Only 2
• (d) 1 and 3

View Answer & Explanation

Correct Answer: (d) 1 and 3

3d_x²−y² orbital lies along x and y axes, so electron density in XY plane is zero. The 2s orbital has one spherical node. Statements 2 and 4 are incorrect.

52. Which orbital is represented by wave function ψ₃₁₀?

• (a) 3d
• (b) 3p
• (c) 3s
• (d) Data is insufficient

View Answer & Explanation

Correct Answer: (b) 3p

ψ_nlm notation indicates n=3, l=1, m=0, which corresponds to the 3p orbital.

53. A particle A moving with a certain velocity has the de Broglie wavelength of 1 Å. For particle B with mass 25% of A and velocity 75% of A, calculate the de Broglie wavelength.

• (a) 3 Å
• (b) 5.33 Å
• (c) 6.88 Å
• (d) 0.48 Å

View Answer & Explanation

Correct Answer: (b) 5.33 Å

λ = h/mv. For particle B, λ_B = λ_A × (m_Av_A)/(m_Bv_B) = 1 × (1)/(0.25 × 0.75) ≈ 5.33 Å.

54. The radiation is emitted when a hydrogen atom goes from a higher energy state to a lower energy state. The wavelength of one line in visible region of atomic spectrum of hydrogen is 6.63 × 10⁻⁷ m. The energy difference between two states is:

• (a) 3.0 × 10⁻¹⁹ J
• (b) 1.0 × 10⁻¹⁸ J
• (c) 5.0 × 10⁻¹⁰ J
• (d) None of these

View Answer & Explanation

Correct Answer: (a) 3.0 × 10⁻¹⁹ J

Energy difference ΔE = hc/λ. Substituting h = 6.626 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s, λ = 6.63 × 10⁻⁷ m gives ≈ 3 × 10⁻¹⁹ J.

55. Atomic emission spectra of an element cannot be used to:

• (a) Identify the atom
• (b) Determine the mass number of the nucleus of atom
• (c) Measure the difference in energy between pairs of stationary states of atom
• (d) All of the above

View Answer & Explanation

Correct Answer: (b)

Emission spectra provide information about electronic transitions, not nuclear properties such as mass number.

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