Atomic Structure: Definition, Models, Quantum Numbers, and Electronic Configuration Explained Notes for UPSC, SSC, Railways.

Welcome, future government officers! Whether you're aiming for UPSC, SSC, Railways, Delhi Police, or IB ACIO, the General Science section is a scoring area. A solid grasp of Atomic Structure is crucial, as it forms the basis for many chemistry questions.

These notes are designed in simple, exam-focused language with clear examples to help you understand and remember the key concepts. Let's begin!

Table of Contents

• Atom and Their Fundamental Components
• Atomic Models
• Quantum Numbers
• Electronic Configuration
• Heisenberg’s Uncertainty Principle
• FAQs

📌 Atom and Their Fundamental Components

An atom is the smallest unit of matter that retains the chemical properties of an element. The word "atom" comes from the Greek word “atomos”, meaning “indivisible.”

But later, scientists discovered that atoms are not indivisible. They are made up of subatomic particles:

Subatomic Particle	Symbol	Charge	Relative Mass	Location in Atom	Example
Electron	e⁻	–1	~1/1836 of proton	Orbit around nucleus	Electrons in Hydrogen
Proton	p⁺	+1	1 unit	Inside nucleus	1 proton in Hydrogen
Neutron	n⁰	0 (neutral)	1 unit	Inside nucleus	Neutrons in Helium

Key Definitions & Example:

Atomic Number (Z): Number of protons in an atom. It defines the element.
Example: Every oxygen atom has 8 protons. So, its atomic number (Z) = 8.

Mass Number (A): Total number of protons and neutrons. (A = p + n).
Example: An oxygen atom with 8 protons and 8 neutrons has a mass number (A) = 16.

·       In a neutral atom: Number of Protons = Number of Electrons

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📌 Atomic Models

Over time, scientists proposed different models of the atom. Let’s see them one by one with examples.

a. Dalton’s Atomic Theory (1803)

·       John Dalton proposed that atoms are indivisible and cannot be created or destroyed.

·       Atoms of the same element are identical.

·       Compounds are formed by the combination of atoms.

👉 Limitation: Failed to explain subatomic particles (electrons, protons, neutrons).

Model	Scientist	Key Idea	Experiment/Example
Plum Pudding Model	J.J. Thomson	Atom is a positive sphere with electrons scattered in it, like plums in a pudding.	Example: Think of a watermelon. The red part is the positive charge, and the seeds are the electrons.
Nuclear Model	Ernest Rutherford	Atom has a tiny, dense, positive core called the nucleus. Electrons revolve around it.	Gold Foil Experiment: Most alpha particles passed through, but a few bounced back, proving the nucleus exists.
Planetary Model	Niels Bohr	Electrons move in fixed paths (orbits or shells) around the nucleus, like planets.	Example: Explained why hydrogen gas gives a specific colour when heated. Electrons jump orbits by absorbing energy.

Exam Tip: J.J. Thomson discovered the electron, Ernest Rutherford discovered the proton, while James Chadwick discovered the neutron.

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📌 Quantum Numbers: The Electron's Aadhaar Card

In modern theory, electrons exist in "probability zones" called orbitals. To identify each electron, we use a set of four numbers called Quantum Numbers.

Quantum Number	Describes	Allowed Values	Simple Example
Principal (n)	Energy & Shell	n = 1, 2, 3…	n=1: K-shell (closest to nucleus). Like your Country.
Azimuthal (l)	Subshell Shape	l = 0 (s), 1 (p), 2 (d), 3 (f).	l=0: 's' (sphere), l=1: 'p' (dumbbell). Like your State.
Magnetic (mₗ)	Orbital Orientation	–l to +l.	For p-subshell (l=1), mₗ = -1,0,+1 (3 orbitals: pₓ, pᵧ, p₂). Like your City.
Spin (mₛ)	Electron Spin	+1/2 or -1/2	Two electrons in one orbital must have opposite spins. Like your House Number.

Example for an electron in a 2p orbital:
n=2, l=1, mₗ=0, mₛ= +1/2 → This is the complete address of that electron.

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📌 Electronic Configuration: The Seating Plan of Electrons

Electronic configuration is the arrangement of electrons in shells, subshells, and orbitals of an atom. It follows three simple rules:

1. Aufbau Principle: "Fill from the bottom up." Electrons occupy the lowest energy orbitals first.
Trick: Remember the order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p...

2. Pauli's Exclusion Principle: "Only two per seat." An orbital can hold max two electrons with opposite spins.

3. Hund's Rule: "Prefer separate seats." Electrons will fill empty orbitals in a subshell first before pairing up.

👉 Examples:

Element	Atomic Number	Electronic Configuration
Hydrogen (H)	1	1s¹
Helium (He)	2	1s²
Oxygen (O)	8	1s² 2s² 2p⁴
Sodium (Na)	11	1s² 2s² 2p⁶ 3s¹
Chlorine (Cl)	17	1s² 2s² 2p⁶ 3s² 3p⁵

👉 In sodium (Na), the last electron is in 3s orbital, hence it belongs to period 3 and group 1

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Heisenberg’s Uncertainty Principle

Proposed by Werner Heisenberg, this is a cornerstone of modern physics.

The Principle:
"It is fundamentally impossible to know both the exact position and the exact momentum (speed & direction) of an electron at the same time."

The more accurately you measure one, the less accurately you can know the other.

Example:
Imagine you're in a dark room trying to find a flying mosquito with a flashlight. The moment the flashlight beam spots the mosquito (you know its position), you disturb its flight path, so you don't know its exact speed anymore. This is the essence of Heisenberg's principle.

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FAQs on Atomic Structure

Q1. What are the fundamental particles of an atom?

Answer: The fundamental particles of an atom are electrons, protons, and neutrons.

·       Electrons (–1 charge) revolve around the nucleus.

·       Protons (+1 charge) and neutrons (no charge) are present inside the nucleus.
👉 Example: A helium atom has 2 protons, 2 neutrons, and 2 electrons.

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Q2. Who discovered the electron, proton, and neutron?

Answer:

·       Electron → Discovered by J.J. Thomson (1897).

·       Proton → Discovered by Ernest Rutherford (1919).

·       Neutron → Discovered by James Chadwick (1932).

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Q3. What is the difference between Bohr’s model and Rutherford’s model?

Answer:

·       Rutherford’s model said electrons revolve around the nucleus like planets around the sun but did not explain stability.

·       Bohr’s model improved it by suggesting electrons revolve in fixed energy levels (shells) and absorb/release energy when jumping between levels.
👉 Example: Hydrogen electron jumps from n=2 to n=1, releasing energy as light.

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Q4. What are quantum numbers?

Answer: Quantum numbers describe the position and energy of electrons in an atom. They are:

1.   Principal quantum number (n) → energy level.

2.   Azimuthal quantum number (l) → shape of orbital.

3.   Magnetic quantum number (m) → orientation of orbital.

4.   Spin quantum number (s) → spin of electron.

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Q5. What is Heisenberg’s Uncertainty Principle?

Answer: It states that it is impossible to know both the exact position and momentum of an electron at the same time.
👉 This means electrons are found in orbitals (regions of probability), not fixed paths.

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Q6. How do you write electronic configuration of an element?

Answer:
Follow three rules:

1.   Aufbau Principle → Fill lower energy orbitals first (1s → 2s → 2p → 3s …).

2.   Pauli Exclusion Principle → No two electrons can have the same 4 quantum numbers.

3.   Hund’s Rule → Fill orbitals singly before pairing.

👉 Example: Oxygen (Z = 8) → 1s² 2s² 2p⁴.

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