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Structure of Atom: questions with answers

Class 11 Chemistry, Chapter 2 (NCERT). 15 questions by concept, 1 with pictures. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Atomic spectra, hydrogen series, and the Rydberg relation

Question 1

A hydrogen emission transition ends at n=2. To which series does it belong?

  1. (a)Balmer series
  2. (b)Brackett series
  3. (c)Paschen series
  4. (d)Lyman series
Show answer

(a) Balmer series

The fixed final level names the series; n=2 is Balmer.

Question 2

Which Balmer transition has the longest wavelength?

  1. (a)n=∞ to n=2
  2. (b)n=5 to n=2
  3. (c)n=4 to n=2
  4. (d)n=3 to n=2
Show answer

(d) n=3 to n=2

Within a series the nearest allowed upper level gives the smallest energy gap and longest wavelength.

Subatomic particles, atomic identity, isotopes, and ions

Question 3

How many protons, neutrons, and electrons are in ³⁷₁₇Cl⁻?

  1. (a)17 protons, 20 neutrons, and 18 electrons
  2. (b)17 protons, 37 neutrons, and 16 electrons
  3. (c)20 protons, 17 neutrons, and 18 electrons
  4. (d)17 protons, 20 neutrons, and 16 electrons
Show answer

(a) 17 protons, 20 neutrons, and 18 electrons

Z gives 17 protons, A−Z gives 20 neutrons, and the −1 charge means one extra electron.

Question 4

An ion has 13 protons, 14 neutrons, and 10 electrons. Which symbol represents it?

  1. (a)²³₁₃Al³⁺
  2. (b)²⁷₁₄Si³⁺
  3. (c)²⁷₁₃Al³⁺
  4. (d)¹⁴₁₃Al³⁻
Show answer

(c) ²⁷₁₃Al³⁺

Z=13 identifies Al, A=13+14=27, and 13−10=+3.

Thomson and Rutherford models: evidence and limitations

Question 5

Which description belongs to Thomson's atomic model?

  1. (a)Electrons in quantized stationary orbits
  2. (b)A tiny nucleus containing all electrons
  3. (c)Electrons embedded in a diffuse positively charged sphere
  4. (d)A probability cloud with no positive charge
Show answer

(c) Electrons embedded in a diffuse positively charged sphere

Thomson distributed positive charge throughout the atom and embedded electrons within it.

Question 6

Why is a classical electron orbit in Rutherford's model unstable?

  1. (a)Empty space prevents electrostatic attraction
  2. (b)An accelerating charge should radiate energy and spiral toward the nucleus
  3. (c)The electron has no charge
  4. (d)The nucleus should lose all its protons
Show answer

(b) An accelerating charge should radiate energy and spiral toward the nucleus

Circular motion is accelerated motion, so classical electrodynamics predicts radiation and loss of orbital energy.

Electromagnetic radiation, quantization, and photoelectric effect

Question 7

A metal has work function 2.0 eV and is illuminated by 1.5 eV photons. What happens even if the light is very intense?

  1. (a)Electrons are emitted with 0.5 eV kinetic energy
  2. (b)Electrons are emitted because intensity supplies the missing energy
  3. (c)Electrons are emitted with 3.5 eV kinetic energy
  4. (d)No photoelectrons are emitted
Show answer

(d) No photoelectrons are emitted

Each photon is below the work function, so increasing photon count cannot make one photon energetic enough.

Question 8

Photons of energy 4.5 eV eject electrons from a surface with maximum kinetic energy 1.2 eV. What is the work function?

  1. (a)5.7 eV
  2. (b)3.75 eV
  3. (c)3.3 eV
  4. (d)1.2 eV
Show answer

(c) 3.3 eV

Einstein's relation gives φ=E_photon−K_max=4.5−1.2=3.3 eV.

Bohr model of hydrogen-like species

Question 9

Which observation is explained successfully by the basic Bohr model?

  1. (a)All fine-structure and magnetic-field splittings
  2. (b)The line spectrum of a hydrogen-like species
  3. (c)Simultaneous exact position and momentum of an electron
  4. (d)The detailed spectrum of every many-electron atom
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(b) The line spectrum of a hydrogen-like species

The model's central success is the spectrum of one-electron atoms and ions.

Matter waves and Heisenberg uncertainty

Question 10

If a particle's momentum doubles, what happens to its de Broglie wavelength?

  1. (a)It becomes half as large
  2. (b)It becomes four times as large
  3. (c)It doubles
  4. (d)It is unchanged
Show answer

(a) It becomes half as large

Because λ=h/p, wavelength is inversely proportional to momentum.

Orbital probability, shapes, and nodes

Question 11

Which statement about all s orbitals is correct?

  1. (a)Their probability density is constant at every radius
  2. (b)They have exactly one electron
  3. (c)They have two lobes on an axis
  4. (d)They are spherically symmetric
Show answer

(d) They are spherically symmetric

Spherical symmetry means equal probability density in every direction at a fixed radius.

Quantum-mechanical orbitals and quantum numbers

Question 12

What physical meaning is assigned to |ψ|² at a point?

  1. (a)Electron probability density at that point
  2. (b)A fixed electron trajectory
  3. (c)The electron's exact speed
  4. (d)A negative probability
Show answer

(a) Electron probability density at that point

Born's interpretation assigns probability density to the squared magnitude of the wave function.

Orbital energies, shielding, and effective nuclear charge

Question 13

Which orbitals are degenerate in the basic hydrogen atom model?

  1. (a)2s and 2p
  2. (b)1s and 2p
  3. (c)3p and 4s
  4. (d)2s and 3s
Show answer

(a) 2s and 2p

For a one-electron hydrogen atom, energy depends only on n.

Electronic configurations, filling rules, and subshell stability

Question 14

A student answers ‘Both electrons paired in one p orbital’ to this question: Which 2p orbital-box arrangement is the ground-state pattern for carbon's 2p² electrons? Which evaluation is correct?

The student’s proposed arrangementThree 2p orbital boxes. The student places two opposite-spin electrons in the first box, leaving the others empty. QUESTION DIAGRAM The student’s proposed arrangement Schematic · not to scale Student’s response: both electrons paired 2p² electrons · three 2p orbitals
  1. (a)The response is correct as written.
  2. (b)The response is incorrect only because the question has no unique answer.
  3. (c)The response is incorrect; the correct answer is Two singly occupied p orbitals with parallel spins
Show answer

(c) The response is incorrect; the correct answer is Two singly occupied p orbitals with parallel spins

A p subshell has three equal-energy orbitals. Carbon has two electrons to place in its 2p subshell.

By Hund's rule, put one electron in each of two different orbitals with parallel spins before pairing: [↑]  [↑]  [  ][\uparrow]\;[\uparrow]\;[\;].

Therefore, placing both electrons in one p orbital is not the ground-state arrangement. The correct evaluation is that two p orbitals are singly occupied with parallel spins.

Question 15

Which arrangement violates Pauli's exclusion principle?

  1. (a)One electron in an s orbital
  2. (b)Two electrons in one orbital with opposite spins
  3. (c)Two electrons in one orbital with the same spin
  4. (d)Two electrons in different p orbitals with parallel spins
Show answer

(c) Two electrons in one orbital with the same spin

Two electrons sharing one orbital already share n, l, and mₗ, so they must have opposite mₛ.

Common misconception in Structure of Atom

Open full misconception guide →

Is an orbital a path drawn around the nucleus?

Wrong

An electron travels on the line shown for its orbital.

Why it feels right. The diagram looks like a planet’s orbit.

Right

An orbital describes the electron’s quantum state and spatial probability distribution.

Worked example

The nested regions suggest higher probability density nearer the centre of a 1s orbital. They are not electron paths.
  1. A 1s picture uses a cloud or boundary to represent where the electron is likely to be found.
  2. It supplies no track for a moving point.