Shapes of Atomic Orbitals-s, p, d, and f Orbital-Class 11 Chemistry Notes

What are Atomic Orbitals?

Atomic orbitals are mathematical functions that describe the wave nature of electrons (or electron pairs) in an atom.

They offer a way to calculate the probability of finding an electron in a specified region around the nucleus of the atom.

Orbitals Chemistry

There are four different kinds of orbitals, denoted s, p, d and f each with a different shape. Of the four, s and p orbitals are considered because these orbitals are the most common in organic and biological chemistry. An s-orbital is spherical with the nucleus at its centre, a p-orbitals is dumbbell-shaped and four of the five d orbitals are cloverleaf shaped. The fifth d orbital is shaped like an elongated dumbbell with a doughnut around its middle. The orbitals in an atom are organized into different layers or electron shells.

According to the quantum atomic model, an atom can have many possible numbers of orbitals. These orbitals can be categorized on the basis of their size, shape or orientation. A smaller sized orbital means there is a greater chance of getting an electron near the nucleus. The orbital wave function or ϕ is a mathematical function used for representing the coordinates of an electron. The square of the orbital wave function represents the probability of finding an electron.

This wave function also helps us in drawing boundary surface diagrams. Boundary surface diagrams of the constant probability density for different orbitals help us understand the shape of orbitals.

Let us represent the shapes of orbitals with the help of boundary surface diagrams:

The Shape of s Orbitals

  • The boundary surface diagram for the s orbital looks like a sphere having the nucleus as its centre which in two dimensions can be seen as a circle.
  • Hence, we can say that s-orbitals are spherically symmetric having the probability of finding the electron at a given distance equal in all the directions.
  • The size of the s orbital is also found to increase with the increase in the value of the principal quantum number (n), thus, 4s > 3s> 2s > 1s.
Atomic orbitals are mathematical functions that describe the wave nature of electrons (or electron pairs) in an atom.

The Shape of p Orbitals

  • Each p orbital consists of two sections better known as lobes which lie on either side of the plane passing through the nucleus.
  • The three p orbitals differ in the way the lobes are oriented whereas they are identical in terms of size, shape, and energy.
  • As the lobes lie along one of the x, y or z-axis, these three orbitals are given the designations 2px, 2py, and 2pz. Thus, we can say that there are three p orbitals whose axes are mutually perpendicular.
  • Similar to s orbitals the size, and energy of p orbitals increase with an increase in the principal quantum number (4p > 3p > 2p).
Atomic orbitals are mathematical functions that describe the wave nature of electrons (or electron pairs) in an atom.

The Shape of d Orbitals

  • The magnetic orbital quantum number for d orbitals is given as (-2,-1,0, 1,2). Hence, we can say that there are five d-orbitals.
  • These orbitals are designated as dxy, dyz, dxz, dx2–y 2 and dz2.
  • Out of these five d orbitals, the shapes of the first four d-orbitals are similar to each other, which is different from the dz2 orbital whereas the energy of all five d orbitals is the same.
Atomic orbitals are mathematical functions that describe the wave nature of electrons (or electron pairs) in an atom.

Solved Example

Assuming 2s-2p mixing is NOT operative, the paramagnetic species among the following is :
(A) Be2
(B) B2
(C) C2
(D) N2

Solution:

The answer is C2
C2 = valence electrons =8
(σ2s)2(σ∗2s)2(σ2pz)2(π2px)1(π2py)1
Because if the question says 2s-2p orbital intermixing is not operative it indicates you to fill the molecular orbitals according to the energy order of the system having electrons >14
The electrons are unpaired in a bonding molecular orbital. Rest in all other molecules the electrons are paired all others are diamagnetic.

Frequently Asked Questions – FAQs

Q1

How many orbitals are there in chemistry?

The four different orbital forms (s, p, d, and f) have different sizes and one orbital will accommodate up to two electrons at most. The orbitals p, d, and f have separate sub-levels and will thus accommodate more electrons. As shown, each element’s electron configuration is unique to its position on the periodic table.

Q2

How do orbitals work?

An atomic orbital is a mathematical term in atomic theory and quantum mechanics that describes the wave-like behaviour of either one electron or a pair of electrons in an atom. Every such orbital will occupy a maximum of two electrons, each having its own quantity of spin.

Q3

How many orbitals are there?

The s sublevel has only one orbital, so max. 2 electrons can be present. The p sublevel has 3 orbitals, so max. 6 electrons can be present. The d sublevel has 5 orbitals, so max. 10 electrons can be present. And the 4 sub-levels have seven orbitals, and they can hold max of 14 electrons.

Q4

Why is the s orbital spherical?

All s orbitals are shaped spherically and have spherical symmetry. That means the function of the wave will depend only on the distance from the nucleus and not on the direction. For any particle, as the central quantum number of the orbital decreases, the size of the orbital decreases, but the geometry stays spherical.

Q5

What is sigma and pi bond?

Sigma and pi bonds are formed by atomic orbital overlap. Sigma bonds are formed by overlapping end-to-end and Pi bonds occur where one atomic orbital lobe overlaps another. As seen along the axis of the bond, both derived their names from the Greek letters and the bond.

Q6

What does P orbital stand for?

The s, p, d, and f, respectively stand for sharp, primary, diffuse and fundamental. The letters and words refer to the visual impression left by the spectral lines’ fine structure that occurs because of the first relativistic corrections, particularly the spin-orbital interaction.

Q7

Which orbitals have the highest energy?

The orbital 1s hold the highest energy. You will appreciate it by talking of different things: But first let’s be super clear: an electron’s energy is the energy it will take to pull it out of the electrical bubble of the atom.

Q8

What is the difference between a shell and an orbital?

A shell in an atom is a set of subshells of the same quantum number theory, n. Orbitals contain two electrons each, and electrons are part of the same orbital in an orbital of the same definition of size, angular momentum size, and magnetic quantum number.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.

JEE Syllabus for Chapter - ATOMIC STRUCTURE

According to the JEE syllabus, the "Atomic Structure" chapter covers key concepts like: Nature of electromagnetic radiation, photoelectric effect, spectrum of the hydrogen atom, Bohr model of ahydrogen atom - its postulates, derivation of the relations for the energy of the electron and radii of the different orbits, limitations of Bohr's model, dual nature of matter, de Broglie's relationship, Heisenberguncertainty principle, elementary ideas of quantum mechanics, the quantum mechanical model of the atomand its important features, concept of atomic orbitals as one-electron wave functions, variation of Ψ and Ψ2 with r for 1s and 2s orbitals, various quantum numbers (principal, angular momentumand magneticquantum numbers) and their significance, shapes of s, p and d - orbitals, electron spin and spin quantumnumber, rules for filling electrons in orbitals – Aufbau principle, Pauli's exclusion principle and Hund'srule, electronic configuration of elements and extra stability of half-filled and completely filled orbitals.

NEET Syllabus for Chapter - ATOMIC STRUCTURE

According to the NEET syllabus, the "Atomic Structure" chapter covers key concepts like: subatomic particles (protons, electrons, neutrons), atomic number and mass number, various atomic models (Dalton's, Thomson's, Rutherford's, Bohr's), quantum mechanical model (Schrödinger's equation, quantum numbers - principal, azimuthal, magnetic, and spin), shapes of orbitals (s, p, d), electronic configuration of elements based on Aufbau principle, Pauli exclusion principle, and Hund's rule; including the limitations of Bohr's model and the concept of dual nature of matter with de Broglie's relationship and Heisenberg's uncertainty principle. 

MCQs on Structure of Atom for Class 11 CBSE Board Exam

Here are some multiple-choice questions (MCQs) on Structure of Atom for Class 11 CBSE along with detailed explanations:


1. Which of the following statements about the nucleus of an atom is correct?

A) It contains protons and neutrons
B) It has a negative charge
C) It occupies most of the volume of the atom
D) It is responsible for chemical properties of an atom

Answer: A) It contains protons and neutrons

Explanation:
The nucleus of an atom contains protons (positively charged) and neutrons (neutral). The electrons revolve around the nucleus in different energy levels. The nucleus has a positive charge due to the presence of protons. It occupies a very small volume but contributes to almost the entire mass of the atom.


2. The total number of electrons that can be accommodated in the second shell (L-shell) is:

A) 2
B) 8
C) 18
D) 32

Answer: B) 8

Explanation:
The maximum number of electrons in a shell is given by 2n², where n is the shell number.
For the second shell (n = 2):
Max electrons = 2(2²) = 8.


3. Which of the following is NOT a postulate of Bohr’s atomic model?

A) Electrons revolve around the nucleus in fixed orbits
B) Electrons emit or absorb energy when they jump between orbits
C) Energy levels are quantized
D) Electrons can have any random energy value

Answer: D) Electrons can have any random energy value

Explanation:
Bohr's model states that electrons revolve in fixed energy levels, and they cannot have arbitrary energy. Electrons only gain or lose energy when they transition between these discrete orbits.


4. The isotope of hydrogen that contains one proton and two neutrons is:

A) Protium
B) Deuterium
C) Tritium
D) None of these

Answer: C) Tritium

Explanation:

  • Protium (¹H) → 1 proton, 0 neutrons
  • Deuterium (²H) → 1 proton, 1 neutron
  • Tritium (³H) → 1 proton, 2 neutrons

5. The wave nature of electrons was proposed by:

A) Bohr
B) Heisenberg
C) de Broglie
D) Rutherford

Answer: C) de Broglie

Explanation:
Louis de Broglie proposed that electrons exhibit both particle and wave nature (wave-particle duality). His equation λ = h/mv relates the wavelength (λ) of a moving particle to its momentum.


6. The uncertainty principle was proposed by:

A) Bohr
B) Heisenberg
C) Rutherford
D) Schrodinger

Answer: B) Heisenberg

Explanation:
Heisenberg's Uncertainty Principle states that it is impossible to simultaneously determine the exact position and momentum of an electron.


7. The quantum number that describes the shape of an orbital is:

A) Principal quantum number (n)
B) Azimuthal quantum number (l)
C) Magnetic quantum number (m)
D) Spin quantum number (s)

Answer: B) Azimuthal quantum number (l)

Explanation:
The Azimuthal quantum number (l) determines the shape of orbitals:

  • s-orbital (l = 0) → Spherical
  • p-orbital (l = 1) → Dumbbell
  • d-orbital (l = 2) → Complex
  • f-orbital (l = 3) → More complex

8. Which of the following orbitals cannot exist?

A) 1s
B) 2p
C) 3f
D) 4d

Answer: C) 3f

Explanation:
For an orbital to exist, the Azimuthal quantum number (l) must satisfy:
l = 0 to (n-1), where n is the principal quantum number.
For n = 3, possible l values: 0 (s), 1 (p), 2 (d) → No f-orbital.


9. Which of the following elements has the electronic configuration: 1s² 2s² 2p⁶ 3s¹?

A) Sodium (Na)
B) Magnesium (Mg)
C) Aluminium (Al)
D) Potassium (K)

Answer: A) Sodium (Na)

Explanation:

  • 1s² 2s² 2p⁶ 3s¹ is the electronic configuration of sodium (Na) (Atomic number 11).
  • Magnesium (Mg) = 1s² 2s² 2p⁶ 3s²
  • Aluminium (Al) = 1s² 2s² 2p⁶ 3s² 3p¹

10. The shape of an s-orbital is:

A) Spherical
B) Dumbbell
C) Double dumbbell
D) Complex

Answer: A) Spherical

Explanation:
The s-orbital is spherically symmetric around the nucleus. p-orbitals are dumbbell-shaped.


11. The number of orbitals present in the third energy level (n = 3) is:

A) 3
B) 9
C) 18
D) 5

Answer: B) 9

Explanation:
Total orbitals in an energy level =
For n = 3, orbitals = 3² = 9
(1 s-orbital, 3 p-orbitals, 5 d-orbitals)


12. If an electron has quantum numbers n = 3, l = 2, what type of orbital is it in?

A) 3s
B) 3p
C) 3d
D) 3f

Answer: C) 3d

Explanation:

  • n = 3 (Third shell)
  • l = 2 corresponds to d-orbital3d orbital

13. The maximum number of electrons that can be accommodated in an orbital is:

A) 1
B) 2
C) 4
D) 6

Answer: B) 2

Explanation:
Each orbital can hold a maximum of 2 electrons with opposite spins, as per Pauli's exclusion principle.


14. Which quantum number determines the energy of an electron in a hydrogen atom?

A) Principal quantum number (n)
B) Azimuthal quantum number (l)
C) Magnetic quantum number (m)
D) Spin quantum number (s)

Answer: A) Principal quantum number (n)

Explanation:
For hydrogen-like atoms, the energy of an electron depends only on n.


15. The concept of orbitals was introduced by:

A) Bohr
B) Rutherford
C) Schrodinger
D) Heisenberg

Answer: C) Schrodinger

Explanation:
Schrodinger’s wave equation introduced orbitals (regions of high probability of finding an electron), replacing Bohr’s fixed orbits.

Assertion and Reason (A-R) type questions on Structure of Atom for Class 11 CBSE Board Exam

Here are some Assertion and Reason (A-R) type questions on Structure of Atom for Class 11 CBSE, along with detailed explanations.


How to Answer Assertion-Reason Questions?

Each question consists of two statements:

  • Assertion (A): A statement of fact.
  • Reason (R): An explanation for the assertion.

You must choose the correct option:

  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false, but R is true.

1. Assertion (A): The nucleus of an atom contains protons and neutrons.

Reason (R): The electrons in an atom revolve around the nucleus in fixed orbits.

Answer: Option (2) (Both A and R are true, but R is not the correct explanation of A.)

🔹 Explanation:

  • The nucleus is composed of protons and neutrons, which was confirmed by Rutherford’s experiment.
  • Electrons revolve around the nucleus in fixed orbits, as described in Bohr’s atomic model.
  • However, the presence of electrons in orbits does not explain why the nucleus contains protons and neutrons.

2. Assertion (A): The maximum number of electrons in a shell is given by 2n².

Reason (R): Electrons in an atom are arranged in different shells around the nucleus.

Answer: Option (1) (Both A and R are true, and R is the correct explanation of A.)

🔹 Explanation:

  • The Bohr-Bury rule states that the maximum number of electrons in a shell = 2n², where n is the shell number.
  • Electrons are distributed in different shells, and this follows the energy level distribution principle.

Example:

  • K shell (n = 1): 2(1²) = 2 electrons
  • L shell (n = 2): 2(2²) = 8 electrons
  • M shell (n = 3): 2(3²) = 18 electrons

Thus, R correctly explains A.


3. Assertion (A): The mass of an atom is concentrated in the nucleus.

Reason (R): The electrons have negligible mass compared to protons and neutrons.

Answer: Option (1) (Both A and R are true, and R is the correct explanation of A.)

🔹 Explanation:

  • The nucleus contains protons and neutrons, which are heavy particles.
  • Electrons are much lighter (mass = 1/1836 of a proton), so their contribution to atomic mass is negligible.
  • Thus, the nucleus contains almost all the mass of an atom, and R explains A correctly.

4. Assertion (A): The energy of electrons in an atom is quantized.

Reason (R): Electrons can exist at any random energy level.

Answer: Option (3) (A is true, but R is false.)

🔹 Explanation:

  • According to Bohr’s model, electrons can only occupy specific, discrete energy levels (quantized energy states).
  • Electrons cannot have arbitrary energy values.
  • Hence, A is true, but R is false because electrons follow quantized energy levels.

5. Assertion (A): The Heisenberg Uncertainty Principle states that the exact position and momentum of an electron cannot be simultaneously determined.

Reason (R): Electrons move in fixed circular orbits around the nucleus.

Answer: Option (3) (A is true, but R is false.)

🔹 Explanation:

  • Heisenberg’s Uncertainty Principle states:
    Δx × Δp ≥ h/4π
    (where Δx = uncertainty in position, Δp = uncertainty in momentum)
  • This means we cannot know both the exact position and momentum of an electron at the same time.
  • Bohr’s model (fixed orbits) was later replaced by Schrodinger’s model (probability orbitals).
  • R is false because electrons do not move in fixed orbits but in probabilistic regions (orbitals).

6. Assertion (A): The azimuthal quantum number (l) determines the shape of an orbital.

Reason (R): The principal quantum number (n) determines the size of the orbital.

Answer: Option (1) (Both A and R are true, and R is the correct explanation of A.)

🔹 Explanation:

  • The Azimuthal quantum number (l) determines the shape of an orbital (s, p, d, f).
  • The Principal quantum number (n) determines the size and energy of an orbital.
  • Since both statements are correct and R correctly explains A, the answer is Option (1).

7. Assertion (A): The 3f orbital exists.

Reason (R): The azimuthal quantum number (l) for an f-orbital is 3.

Answer: Option (4) (A is false, but R is true.)

🔹 Explanation:

  • l = 3 represents an f-orbital, but for an orbital to exist, n must be greater than l.
  • For n = 3, the possible values of l are 0 (s), 1 (p), and 2 (d).
  • f-orbital (l = 3) is possible only when n ≥ 4, so 3f does not exist.
  • Hence, A is false, but R is true.

8. Assertion (A): The Pauli Exclusion Principle states that no two electrons in an atom can have the same set of four quantum numbers.

Reason (R): An orbital can accommodate a maximum of two electrons with opposite spins.

Answer: Option (1) (Both A and R are true, and R is the correct explanation of A.)

🔹 Explanation:

  • Pauli's Exclusion Principle states that no two electrons in an atom can have identical quantum numbers.
  • This is because each orbital can hold only two electrons with opposite spins.
  • Since R explains A correctly, the answer is Option (1).

9. Assertion (A): The probability of finding an electron is maximum near the nucleus.

Reason (R): The probability density of an electron is given by the wave function (ψ²).

Answer: Option (1) (Both A and R are true, and R is the correct explanation of A.)

🔹 Explanation:

  • The Schrodinger wave equation gives ψ², which represents the probability density of finding an electron in a region.
  • For s-orbitals, the highest probability is near the nucleus.
  • Since R explains A correctly, the answer is Option (1).

FAQs (Important Questions & Answers) on Structure of Atom – Class 11

1. What is the structure of an atom?

The structure of an atom consists of a nucleus containing protons and neutrons, surrounded by electrons moving in discrete energy levels or shells around the nucleus.

2. Who discovered the atom?

The concept of the atom was first proposed by John Dalton in his Atomic Theory (1808). Later, significant contributions were made by J.J. Thomson, Rutherford, and Bohr to explain its structure.

3. What are the fundamental particles of an atom?

The three fundamental particles of an atom are:

  • Proton (p⁺) – Positively charged, found in the nucleus.
  • Neutron (n⁰) – Neutral, found in the nucleus.
  • Electron (e⁻) – Negatively charged, revolves around the nucleus.

4. What is Dalton’s Atomic Theory?

Dalton’s Atomic Theory (1808) states that:

  • Matter is made of indivisible atoms.
  • Atoms of an element are identical in mass and properties.
  • Atoms combine in whole-number ratios to form compounds.
  • Atoms cannot be created or destroyed in a chemical reaction.

5. What was J.J. Thomson’s Model of the Atom?

J.J. Thomson proposed the Plum Pudding Model (1897), where an atom was visualized as a positively charged sphere with negatively charged electrons embedded in it, like plums in a pudding. However, this model was later disproven by Rutherford’s experiment.

6. What is Rutherford’s Atomic Model?

Rutherford’s Gold Foil Experiment (1911) led to the discovery that:

  • An atom has a dense positively charged nucleus.
  • Electrons revolve around the nucleus.
  • Most of the atom is empty space.
    This model could not explain atomic stability, which was later addressed by Bohr’s model.

7. What is Bohr’s Atomic Model?

Niels Bohr (1913) proposed that:

  • Electrons revolve in fixed energy levels (shells).
  • They do not lose energy while in stable orbits.
  • Energy is absorbed or emitted when an electron jumps between energy levels.

8. What are Quantum Numbers?

Quantum numbers define the position and energy of an electron in an atom:

  1. Principal Quantum Number (n) – Represents the main energy level.
  2. Azimuthal Quantum Number (l) – Defines the shape of the orbital.
  3. Magnetic Quantum Number (mₗ) – Indicates the orientation of the orbital.
  4. Spin Quantum Number (mₛ) – Describes the spin of an electron (+½ or -½).

9. What is the Heisenberg Uncertainty Principle?

Proposed by Werner Heisenberg, it states that it is impossible to simultaneously determine the exact position and momentum of an electron in an atom.

10. What is the difference between an orbit and an orbital?

  • Orbit – A fixed path in which electrons revolve (Bohr’s Model).
  • Orbital – A 3D region in space where the probability of finding an electron is highest (Quantum Mechanical Model).

11. What is the Aufbau Principle?

The Aufbau Principle states that electrons fill atomic orbitals in order of increasing energy levels, i.e., lower-energy orbitals are filled first before higher ones.

12. What is Hund’s Rule?

Hund’s Rule states that in degenerate (equal energy) orbitals, electrons fill each orbital singly before pairing up.

13. What is Pauli’s Exclusion Principle?

Pauli’s Exclusion Principle states that no two electrons in an atom can have the same set of four quantum numbers. This means an atomic orbital can hold a maximum of two electrons with opposite spins.

14. What is the Quantum Mechanical Model of the Atom?

Developed by Schrödinger, this model describes electrons as wave-like particles with a probability distribution around the nucleus instead of fixed orbits. It introduces atomic orbitals (s, p, d, f) as probable electron locations.

15. What are the types of orbitals in an atom?

There are four types of orbitals:

  • s-orbital – Spherical shape (holds max 2 electrons).
  • p-orbital – Dumbbell shape (holds max 6 electrons).
  • d-orbital – Complex shape (holds max 10 electrons).
  • f-orbital – Complex shape (holds max 14 electrons).

16. What is the significance of the atomic number and mass number?

  • Atomic Number (Z) – Number of protons in an atom.
  • Mass Number (A) – Sum of protons and neutrons in an atom’s nucleus.

17. What are Isotopes, Isobars, and Isotones?

  • Isotopes – Same atomic number, different mass number (e.g., Hydrogen: ¹H, ²H, ³H).
  • Isobars – Same mass number, different atomic number (e.g., ¹⁴C and ¹⁴N).
  • Isotones – Same number of neutrons, different atomic and mass numbers (e.g., ¹⁴C and ¹⁵N).

18. What is the Dual Nature of Electrons?

Proposed by de Broglie, it states that electrons exhibit both particle and wave-like properties, known as wave-particle duality.

19. What is the Electronic Configuration of an Atom?

The electronic configuration of an atom describes how electrons are distributed in different orbitals. It follows the Aufbau principle, Hund’s rule, and Pauli’s exclusion principle (e.g., Oxygen (O) = 1s² 2s² 2p⁴).

20. Why is the Bohr model still used despite its limitations?

Although the Bohr model fails for multi-electron atoms and does not explain fine spectral lines, it is still useful because it provides a simple and understandable representation of electron energy levels.

📚 CBSE Class 11 Chemistry: Structure of Atom – Complete Syllabus Overview

Are you a Class 11 CBSE student aiming to master "Structure of Atom"? Here’s a comprehensive breakdown of the chapter to help you focus your studies!

Case Study-Based MCQs on Structure of Atom – Class 11 CBSE

Case study-based questions are designed to test your analytical skills and conceptual understanding. Below are five case studies with multiple-choice questions (MCQs) and detailed explanations.


Case Study 1: Discovery of Atomic Structure

The structure of an atom has been explored through various experiments. J.J. Thomson discovered electrons using a cathode ray tube experiment, while Goldstein discovered protons using canal rays. Rutherford’s alpha particle scattering experiment led to the discovery of a dense nucleus at the center of the atom. Later, Bohr’s model refined our understanding by proposing that electrons move in discrete energy levels.

1.1 What was the key observation in Rutherford’s experiment?

A) Most alpha particles were deflected back
B) Alpha particles passed through the gold foil without deflection
C) All alpha particles got absorbed by the gold foil
D) Electrons were ejected from the gold foil

Answer: B) Alpha particles passed through the gold foil without deflection

🔹 Explanation: Rutherford’s experiment showed that most of the space in an atom is empty, as most alpha particles passed straight through the foil. However, a few were deflected, indicating the presence of a dense, positively charged nucleus.


1.2 What did J.J. Thomson’s experiment conclude?

A) Atoms have a nucleus
B) Atoms are indivisible
C) Atoms contain negatively charged particles
D) Atoms are mostly empty space

Answer: C) Atoms contain negatively charged particles

🔹 Explanation: J.J. Thomson’s cathode ray experiment showed the presence of negatively charged electrons, leading to the "plum pudding model" of the atom.


Case Study 2: Bohr’s Atomic Model

Niels Bohr proposed that electrons move in fixed orbits (energy levels) around the nucleus without losing energy. He introduced the quantization of energy levels and explained the emission spectra of hydrogen.

2.1 According to Bohr’s model, what happens when an electron jumps from a higher to a lower energy level?

A) The atom becomes unstable
B) The electron absorbs energy
C) The electron loses energy in the form of radiation
D) The electron disappears

Answer: C) The electron loses energy in the form of radiation

🔹 Explanation: When an electron moves from a higher energy level to a lower one, it releases energy in the form of light (photon), which creates the atomic emission spectrum.


2.2 Which of the following is NOT a postulate of Bohr’s model?

A) Electrons revolve around the nucleus in circular orbits
B) Electrons can have any energy within an orbit
C) Energy levels are quantized
D) Electrons do not lose energy while revolving

Answer: B) Electrons can have any energy within an orbit

🔹 Explanation: According to Bohr, electrons can only exist in specific, quantized energy levels, meaning they cannot have any arbitrary energy.


Case Study 3: Quantum Mechanical Model

The quantum mechanical model, developed by Schrodinger, replaced Bohr’s model. It introduced the concept of orbitals, where electrons are found as a probability distribution rather than fixed orbits. The model is based on wave-particle duality and Heisenberg’s Uncertainty Principle.

3.1 What does Heisenberg’s Uncertainty Principle state?

A) Electrons are present in fixed orbits
B) We cannot determine both position and momentum of an electron simultaneously
C) Electrons revolve around the nucleus like planets
D) Electrons follow a predictable path

Answer: B) We cannot determine both position and momentum of an electron simultaneously

🔹 Explanation: Heisenberg’s Uncertainty Principle states that it is impossible to simultaneously know the exact position and momentum of an electron, leading to the concept of probability orbitals.


3.2 Which scientist introduced the wave equation to describe electron behavior?

A) Bohr
B) Heisenberg
C) Schrodinger
D) Rutherford

Answer: C) Schrodinger

🔹 Explanation: Erwin Schrodinger developed the wave equation, which describes the probability of finding an electron in a given region of space, known as an orbital.


Case Study 4: Quantum Numbers

Each electron in an atom is described by four quantum numbers:

  1. Principal quantum number (n) → Determines the energy level
  2. Azimuthal quantum number (l) → Determines the shape of orbitals
  3. Magnetic quantum number (m) → Determines the orientation of orbitals
  4. Spin quantum number (s) → Determines electron spin

4.1 What does the principal quantum number (n) determine?

A) Shape of the orbital
B) Orientation of the orbital
C) Size and energy of the orbital
D) Spin of the electron

Answer: C) Size and energy of the orbital

🔹 Explanation: The principal quantum number (n) determines the size of the electron cloud and the energy level where the electron resides.


4.2 How many orbitals are present in the third energy level (n = 3)?

A) 3
B) 9
C) 18
D) 5

Answer: B) 9

🔹 Explanation: The number of orbitals in an energy level is given by .
For n = 33² = 9 orbitals (1s, 3p, 5d).


Case Study 5: Electronic Configuration and Periodicity

The electronic configuration of an atom follows the Aufbau principle, Pauli Exclusion Principle, and Hund’s Rule. Elements in the periodic table are arranged based on their atomic number and valence electron configuration.

5.1 Which rule states that electrons fill orbitals in order of increasing energy?

A) Hund’s Rule
B) Aufbau Principle
C) Pauli’s Exclusion Principle
D) Heisenberg’s Principle

Answer: B) Aufbau Principle

🔹 Explanation: The Aufbau Principle states that electrons occupy the lowest energy orbitals first before filling higher ones.


5.2 The electronic configuration of an element is 1s² 2s² 2p⁶ 3s². What is the element?

A) Magnesium (Mg)
B) Sodium (Na)
C) Aluminium (Al)
D) Oxygen (O)

Answer: A) Magnesium (Mg)

🔹 Explanation:

  • 1s² 2s² 2p⁶ 3s² corresponds to an atomic number of 12, which is magnesium (Mg).

CBSE Board Exam Syllabus for Chapter 2: Structure of Atom

Key Topics

  1. Discovery of Subatomic Particles
    • Electron, Proton, and Neutron
    • Experiments by J.J. Thomson and Rutherford
  2. Atomic Models
    • Thomson’s Model – The "Plum Pudding" model
    • Rutherford’s Nuclear Model – Gold foil experiment
    • Bohr’s Model of the Hydrogen Atom
  3. Dual Nature of Matter and Radiation
    • Photoelectric Effect – Einstein’s explanation
    • de Broglie’s Hypothesis – Matter waves
  4. Heisenberg’s Uncertainty Principle
    • Limitation in determining position and momentum simultaneously
  5. Quantum Mechanical Model of Atom
    • Introduction to Schrödinger’s Wave Equation
    • Concept of orbitals and shapes of s, p, and d orbitals
  6. Quantum Numbers
    • Principal (n), Azimuthal (l), Magnetic (m), and Spin (s)
    • Significance and rules of electron filling
  7. Electronic Configuration of Atoms
    • Aufbau Principle – Electrons fill lower energy orbitals first
    • Pauli’s Exclusion Principle – No two electrons can have identical quantum numbers
    • Hund’s Rule of Maximum Multiplicity
  8. Hydrogen Spectrum
    • Explanation of line spectra based on Bohr’s theory
  9. Limitations of Bohr’s Model
    • Transition to modern quantum mechanics

Tips to Master The Chapter Structure of Atom

  • Understand and visualize atomic models and orbitals for better retention.
  • Focus on the photoelectric effect and Heisenberg’s principle, as they are conceptually important.
  • Practice writing electronic configurations using rules like the Aufbau Principle and Hund’s Rule.
  • Solve questions on quantum numbers to strengthen your grasp of the concept.

💡 Structure of Atom forms the foundation for further chapters like Chemical Bonding and Periodic Properties. Mastering it will make future topics easier to understand.

Happy studying, and good luck with your preparations! 😊

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