Structure of Atom

31 NEET-UG practice questions on Structure of Atom, part of Chemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
How many unpaired electrons does a gaseous Fe³⁺ ion (Z = 26) have in its ground state?
1. A5
2. B4
3. C6
4. D3
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Answer: A · 5
Iron is [Ar] 3d⁶ 4s², and the 4s electrons leave first, so Fe³⁺ is [Ar] 3d⁵. By Hund's rule, the five 3d electrons occupy five orbitals singly.
- B: This is the count for Fe²⁺, which is 3d⁶.
- C: This counts the six 3d electrons of the neutral atom as unpaired.
- D: This removes all three electrons from 3d, leaving 3d³ 4s².
Remember this
Transition metal ions lose 4s electrons before 3d, and Hund's rule then fixes the number of unpaired electrons.
Question 2 · Hard
The first Balmer line of hydrogen comes from the transition n = 3 → n = 2. Which transition in Li²⁺ (Z = 3) emits light of the same wavelength? (Use 1/λ ∝ Z²(1/n₁² − 1/n₂²).)
1. An = 6 → n = 4
2. Bn = 27 → n = 18
3. Cn = 9 → n = 6
4. Dn = 3 → n = 2
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Answer: C · n = 9 → n = 6
For the same wavelength, Z²(1/n₁² − 1/n₂²) must match, and Z² = 9 is cancelled when each n is multiplied by Z = 3. So 9(1/36 − 1/81) = 1/4 − 1/9, and the transition is n = 9 → n = 6.
- A: This scales n by 2, which is the He⁺ answer with Z = 2.
- B: This scales n by Z² = 9 instead of by Z = 3.
- D: This leaves n unchanged and ignores the Z² factor.
Remember this
A hydrogen line reappears in a one-electron ion when both n values are multiplied by Z.
Question 3 · Easy
A 3p orbital has how many radial nodes?
1. A3
2. B1
3. C2
4. D0
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Answer: B · 1
Radial nodes = n − l − 1 = 3 − 1 − 1 = 1. A p orbital also has one angular node, which gives two nodes in all.
- A: This takes the principal quantum number as the node count.
- C: This is the total number of nodes, radial plus angular.
- D: This assumes a p orbital has only an angular node.
Remember this
Radial nodes = n − l − 1, angular nodes = l, total nodes = n − 1.
Question 4 · Medium
Particles X and Y have masses in the ratio 1 : 4. The uncertainties in their velocities are in the ratio 2 : 1. In what ratio are the minimum uncertainties in their positions, Δx(X) : Δx(Y)?
1. A1 : 2
2. B8 : 1
3. C2 : 1
4. D1 : 8
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Answer: C · 2 : 1
From Heisenberg, Δx × m × Δv ≥ h/4π, so the minimum Δx is proportional to 1/(mΔv). The values are 1/(1 × 2) = 1/2 for X and 1/(4 × 1) = 1/4 for Y, a ratio of 2 : 1.
- A: This compares the products mΔv directly, without taking the inverse.
- B: This inverts the mass ratio but not the velocity uncertainty ratio.
- D: This inverts the velocity uncertainty ratio but not the mass ratio.
Remember this
Minimum Δx varies inversely with the product of mass and Δv.
Question 5 · Medium
Arrange the following orbitals of a multi-electron atom in increasing order of energy. A. 4p B. 3d C. 4s D. 3p
1. AD, C, B, A
2. BD, C, A, B
3. CD, B, C, A
4. DC, D, B, A
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Answer: A · D, C, B, A
The n + l values are 4p = 5, 3d = 5, 4s = 4 and 3p = 4. For equal n + l, the orbital with lower n has lower energy, so 3p < 4s < 3d < 4p.
- B: This places 4p below 3d, ignoring the lower n rule at equal n + l.
- C: This places 3d below 4s, using n alone.
- D: This places 4s below 3p, ignoring the lower n rule at equal n + l.
Remember this
The (n + l) rule: lower sum fills first; for the same sum, lower n fills first.
Question 6 · Medium
What is the de Broglie wavelength of a ball of mass 0.1 kg moving at 33 m/s? (Take h = 6.6 × 10⁻³⁴ J s)
1. A2 × 10⁻³⁷ m
2. B2 × 10⁻³⁴ m
3. C6.6 × 10⁻³³ m
4. D2 × 10⁻³³ m
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Question 7 · Medium
Which sets of quantum numbers are allowed for an electron? A. n = 3, l = 2, m = −2, s = +1/2 B. n = 2, l = 2, m = 0, s = −1/2 C. n = 4, l = 0, m = 0, s = −1/2 D. n = 3, l = 1, m = +2, s = +1/2
1. AA and C only
2. BA and D only
3. CB and C only
4. DA, C and D only
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Question 8 · Hard
The energy levels of a one-electron species are drawn in the figure; the lowest level is its ground state. Using the Bohr model, with Bohr radius 52.9 pm and hydrogen ground-state energy −13.6 eV, find the radius of the orbit for level X.
Figure: MedExams
1. A238 pm
2. B159 pm
3. C79 pm
4. D476 pm
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Question 9 · Easy
Four emission transitions of the hydrogen atom are marked in the figure. Match List-I with List-II. List-I (transition) A. Arrow A B. Arrow B C. Arrow C D. Arrow D List-II (spectral series) I. Lyman II. Balmer III. Paschen IV. Brackett
Figure: MedExams
1. AA-III, B-II, C-IV, D-I
2. BA-II, B-I, C-III, D-IV
3. CA-III, B-I, C-IV, D-II
4. DA-IV, B-I, C-III, D-II
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Question 10 · Medium
A hydrogen orbital with azimuthal quantum number l = 1 has the radial probability curve shown. Which orbital is it?
Figure: MedExams
1. A2p
2. B4p
3. C3p
4. D5p
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Question 11 · Medium
Given below are two statements: Statement I: In a hydrogen atom, the 3s, 3p and 3d orbitals have the same energy. Statement II: In a multi-electron atom, the 2s and 2p orbitals have the same energy. Choose the correct answer.
1. ABoth Statement I and Statement II are correct
2. BStatement I is incorrect but Statement II is correct
3. CStatement I is correct but Statement II is incorrect
4. DBoth Statement I and Statement II are incorrect
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Question 12 · Medium
Light of frequency 1.0 × 10¹⁵ s⁻¹ falls on a metal whose threshold frequency is 5.0 × 10¹⁴ s⁻¹. The maximum kinetic energy of an ejected electron is (h = 6.6 × 10⁻³⁴ J s)
1. A9.9 × 10⁻¹⁹ J
2. B3.3 × 10⁻¹⁹ J
3. C6.6 × 10⁻¹⁹ J
4. D1.65 × 10⁻¹⁹ J
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Question 13 · Medium
Take the energy of the electron in the ground state of a hydrogen atom as −2.18 × 10⁻¹⁸ J. In the Bohr model, the energy of the electron in the n = 2 orbit of He⁺ is
1. A−8.72 × 10⁻¹⁸ J
2. B−2.18 × 10⁻¹⁸ J
3. C−5.45 × 10⁻¹⁹ J
4. D−1.09 × 10⁻¹⁸ J
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Question 14 · Easy
The maximum number of electrons that a subshell with azimuthal quantum number l = 3 can hold is
1. A10
2. B18
3. C7
4. D14
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Question 15 · Hard
In a ground-state chromium atom (Z = 24), how many electrons occupy orbitals for which n + l = 4?
1. A7
2. B8
3. C11
4. D6
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Question 16 · Medium
Assertion A: The ground-state electronic configuration of copper (Z = 29) is [Ar] 3d¹⁰ 4s¹ and not [Ar] 3d⁹ 4s². Reason R: Completely filled and half-filled subshells have extra stability because of their symmetrical electron distribution and larger exchange energy. Choose the correct answer.
1. AA is not correct but R is correct
2. BBoth A and R are correct but R is not the correct explanation of A
3. CBoth A and R are correct and R is the correct explanation of A
4. DA is correct but R is not correct
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Question 17 · Medium
Arrange these four orbitals of a many-electron atom from lowest to highest energy, as given by the (n + l) rule. A. 4f B. 5p C. 6s D. 4d Choose the correct answer from the options given below.
1. AD, C, B, A
2. BD, B, C, A
3. CB, D, C, A
4. DA, D, B, C
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Question 18 · Easy
For the shell with principal quantum number n = 4, what is the total number of orbitals?
1. A32
2. B9
3. C16
4. D4
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Question 19 · Hard
In a hydrogen-like ion, the radius of the third Bohr orbit is three times the radius of the first Bohr orbit of the hydrogen atom. What is the energy of the electron of this ion in its ground state? (Ground-state energy of the hydrogen atom = −13.6 eV)
1. A−40.8 eV
2. B−13.6 eV
3. C−122.4 eV
4. D−54.4 eV
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Question 20 · Medium
When the electrons in a large sample of hydrogen atoms fall from the n = 5 level back to the ground state, by every possible route, how many different spectral lines can appear in the emission spectrum?
1. A10
2. B15
3. C5
4. D4
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Question 21 · Medium
Assertion A: When light of a frequency above the threshold frequency falls on a metal, making the light brighter increases the number of electrons ejected per second but not their maximum kinetic energy. Reason R: Each electron is ejected by a single photon, and the energy of a photon depends only on the frequency of the light. Choose the correct answer.
1. AA is not correct but R is correct
2. BBoth A and R are correct and R is the correct explanation of A
3. CA is correct but R is not correct
4. DBoth A and R are correct but R is not the correct explanation of A
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Question 22 · Easy
Carbon-14 (Z = 6) and nitrogen-14 (Z = 7) are best described as
1. Aisotopes
2. Bisobars
3. Callotropes
4. Disoelectronic species
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Question 23 · Hard
A He⁺ ion is in its first excited state. What minimum energy will ionise it? (Ground-state energy of the hydrogen atom = −13.6 eV)
1. A40.8 eV
2. B54.4 eV
3. C13.6 eV
4. D3.4 eV
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Question 24 · Medium
In the Bohr model, what is the ratio of the radius of the second orbit of He⁺ to the radius of the third orbit of Li²⁺?
1. A3 : 2
2. B1 : 1
3. C2 : 3
4. D4 : 9
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Question 25 · Medium
In a ground-state phosphorus atom (Z = 15), how many electrons have magnetic quantum number m = 0?
1. A9
2. B8
3. C6
4. D10
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6 more Structure of Atom questions

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Structure of Atom

How many unpaired electrons does a gaseous Fe³⁺ ion (Z = 26) have in its ground state?

The first Balmer line of hydrogen comes from the transition n = 3 → n = 2. Which transition in Li²⁺ (Z = 3) emits light of the same wavelength? (Use 1/λ ∝ Z²(1/n₁² − 1/n₂²).)

A 3p orbital has how many radial nodes?

Particles X and Y have masses in the ratio 1 : 4. The uncertainties in their velocities are in the ratio 2 : 1. In what ratio are the minimum uncertainties in their positions, Δx(X) : Δx(Y)?

Arrange the following orbitals of a multi-electron atom in increasing order of energy. A. 4p B. 3d C. 4s D. 3p

What is the de Broglie wavelength of a ball of mass 0.1 kg moving at 33 m/s? (Take h = 6.6 × 10⁻³⁴ J s)

Which sets of quantum numbers are allowed for an electron? A. n = 3, l = 2, m = −2, s = +1/2 B. n = 2, l = 2, m = 0, s = −1/2 C. n = 4, l = 0, m = 0, s = −1/2 D. n = 3, l = 1, m = +2, s = +1/2

The energy levels of a one-electron species are drawn in the figure; the lowest level is its ground state. Using the Bohr model, with Bohr radius 52.9 pm and hydrogen ground-state energy −13.6 eV, find the radius of the orbit for level X.

Four emission transitions of the hydrogen atom are marked in the figure. Match List-I with List-II. List-I (transition) A. Arrow A B. Arrow B C. Arrow C D. Arrow D List-II (spectral series) I. Lyman II. Balmer III. Paschen IV. Brackett

A hydrogen orbital with azimuthal quantum number l = 1 has the radial probability curve shown. Which orbital is it?

Given below are two statements: Statement I: In a hydrogen atom, the 3s, 3p and 3d orbitals have the same energy. Statement II: In a multi-electron atom, the 2s and 2p orbitals have the same energy. Choose the correct answer.

Light of frequency 1.0 × 10¹⁵ s⁻¹ falls on a metal whose threshold frequency is 5.0 × 10¹⁴ s⁻¹. The maximum kinetic energy of an ejected electron is (h = 6.6 × 10⁻³⁴ J s)

Take the energy of the electron in the ground state of a hydrogen atom as −2.18 × 10⁻¹⁸ J. In the Bohr model, the energy of the electron in the n = 2 orbit of He⁺ is

The maximum number of electrons that a subshell with azimuthal quantum number l = 3 can hold is

In a ground-state chromium atom (Z = 24), how many electrons occupy orbitals for which n + l = 4?

Arrange these four orbitals of a many-electron atom from lowest to highest energy, as given by the (n + l) rule. A. 4f B. 5p C. 6s D. 4d Choose the correct answer from the options given below.

For the shell with principal quantum number n = 4, what is the total number of orbitals?

In a hydrogen-like ion, the radius of the third Bohr orbit is three times the radius of the first Bohr orbit of the hydrogen atom. What is the energy of the electron of this ion in its ground state? (Ground-state energy of the hydrogen atom = −13.6 eV)

When the electrons in a large sample of hydrogen atoms fall from the n = 5 level back to the ground state, by every possible route, how many different spectral lines can appear in the emission spectrum?

Carbon-14 (Z = 6) and nitrogen-14 (Z = 7) are best described as

A He⁺ ion is in its first excited state. What minimum energy will ionise it? (Ground-state energy of the hydrogen atom = −13.6 eV)

In the Bohr model, what is the ratio of the radius of the second orbit of He⁺ to the radius of the third orbit of Li²⁺?

In a ground-state phosphorus atom (Z = 15), how many electrons have magnetic quantum number m = 0?