Structure of Atom: Comprehensive NEET Chemistry Notes

1. Discovery of Subatomic Particles

The discovery of subatomic particles was a significant milestone in understanding the structure of the atom.

1.1 Discovery of Electron

Michael Faraday, in 1830, demonstrated that electric current passing through a solution of electrolytes leads to chemical reactions at the electrodes, suggesting the particulate nature of electricity. Later, the cathode ray discharge experiments by J.J. Thomson in 1897 led to the discovery of electrons.

Did You Know?

The cathode ray tube used in early televisions was based on the same principle as Thomson's experiments.

Real-life Application:

The discovery of electrons paved the way for developing electronic devices, including TVs, computers, and smartphones.

2. Atomic Models

Several atomic models were proposed to explain the structure of atoms. The most notable ones include Thomson's model and Rutherford's nuclear model.

2.1 Thomson's Model

J.J. Thomson proposed that an atom is a sphere of positive charge with electrons embedded in it, resembling a "plum pudding."

Common Misconception:

Thomson's model incorrectly suggested that the positive charge is spread out over the entire atom rather than concentrated in the nucleus.

3. Rutherford’s Nuclear Model

Ernest Rutherford's gold foil experiment in 1911 led to the nuclear model of the atom, where a dense nucleus is surrounded by electrons.

3.1 Rutherford’s Gold Foil Experiment

Rutherford bombarded a thin gold foil with alpha particles and observed their scattering. He concluded that most of the atom's mass and positive charge are concentrated in a tiny nucleus.

NEET Tip:

Remember the key observations from Rutherford's experiment: most alpha particles passed through, some were deflected, and a few were reflected back.

Real-life Application:

Rutherford's model laid the foundation for understanding nuclear reactions, including those in nuclear power plants.

4. Bohr's Model of Atom

Niels Bohr improved upon Rutherford’s model by incorporating quantum theory, explaining how electrons occupy specific orbits around the nucleus.

4.1 Postulates of Bohr's Model

1. Electrons orbit the nucleus in fixed paths called orbits or shells.
2. Each orbit corresponds to a specific energy level.
3. Electrons can jump between energy levels by absorbing or emitting energy.

Visual Aid:

Mnemonic:

"King Louis Never Says No" - K, L, M, N, O, P, Q (names of electron shells).

5. Quantum Mechanical Model of Atom

The quantum mechanical model describes the behavior of electrons using principles of quantum mechanics, emphasizing probabilities rather than fixed orbits.

5.1 Schrödinger Wave Equation

Erwin Schrödinger formulated a wave equation that describes how the quantum state of a physical system changes over time.

5.2 Quantum Numbers

• Principal Quantum Number (n): Indicates the main energy level.
• Angular Momentum Quantum Number (l): Indicates the shape of the orbital.
• Magnetic Quantum Number (m): Indicates the orientation of the orbital.
• Spin Quantum Number (s): Indicates the spin of the electron.

NEET Problem-Solving Strategy:

Use quantum numbers to determine the probable location of an electron in an atom.

6. Atomic Spectra and Quantum Theory

Atomic spectra are the result of the interaction of electromagnetic radiation with matter, providing insights into the electronic structure of atoms.

6.1 Electromagnetic Radiation

Electromagnetic radiation exhibits both wave-like and particle-like properties.

6.2 Planck’s Quantum Theory

Max Planck proposed that energy is quantized and can be emitted or absorbed in discrete amounts called quanta.

6.3 Photoelectric Effect

Albert Einstein explained the photoelectric effect using quantum theory, demonstrating that light consists of particles called photons.

NEET Tip:

Understand the photoelectric effect as it frequently appears in NEET questions related to quantum mechanics.

Visual Aid:

7. Quantum Numbers and Electron Configuration

Quantum numbers describe the position and energy of electrons in an atom, crucial for understanding electron configuration.

7.1 Aufbau Principle

Electrons occupy the lowest energy orbitals first.

7.2 Pauli Exclusion Principle

No two electrons in an atom can have the same set of four quantum numbers.

7.3 Hund’s Rule

Electrons fill degenerate orbitals singly first, with parallel spins, to maximize stability.

Mnemonic:

"Aunt Polly Helps" - Aufbau Principle, Pauli Exclusion Principle, Hund’s Rule.

Quick Recap

• Subatomic particles include electrons, protons, and neutrons.
• Atomic models evolved from Thomson’s "plum pudding" model to Rutherford’s nuclear model and Bohr’s quantum theory-based model.
• Quantum mechanical model uses quantum numbers to describe electron behavior.
• Atomic spectra and quantum theory explain the interaction of electromagnetic radiation with matter.

Concept Connection

Understanding the structure of atoms is fundamental for topics in chemistry and physics, such as chemical bonding and nuclear reactions.

Practice Questions

1. Question: Define the term "isotope." Solution: Isotopes are atoms of the same element with different numbers of neutrons.
2. Question: Explain Rutherford’s gold foil experiment. Solution: Rutherford directed alpha particles at a thin gold foil and observed their scattering, concluding that atoms have a small, dense nucleus.
3. Question: State Planck's quantum theory. Solution: Energy is quantized and can be emitted or absorbed in discrete amounts called quanta.
4. Question: Describe Bohr’s model of the hydrogen atom. Solution: Bohr proposed that electrons orbit the nucleus in fixed paths with specific energy levels.
5. Question: Calculate the energy of a photon with a wavelength of 500 nm. Solution: $E=\frac{hc}{\lambda }=\frac{6.626\times 10^{-34}\times 3.0\times 10^8}{500\times 10^{-9}}=3.98\times 10^{-19}J$
6. Question: Determine the number of protons, neutrons, and electrons in ${(}_{11}^{23}Na)$. Solution: Protons = 11, Neutrons = 23 - 11 = 12, Electrons = 11 (for neutral atom).
7. Question: What is the significance of quantum numbers? Solution: Quantum numbers describe the position, energy, and spin of electrons in an atom.
8. Question: Explain the photoelectric effect. Solution: The photoelectric effect occurs when light of sufficient frequency strikes a metal surface, causing the ejection of electrons.
9. Question: Write the electronic configuration of carbon (Z=6). Solution: $1s^{2}2s^{2}2p^2$
10. Question: What is the Heisenberg Uncertainty Principle? Solution: It states that it is impossible to simultaneously determine the exact position and momentum of an electron.

Quick Reference Guide and Glossary

• Isotope: Atoms of the same element with different numbers of neutrons.
• Photon: A particle representing a quantum of light.
• Quantum Number: Numbers that describe the position and energy of an electron.
• Electromagnetic Radiation: Waves of electric and magnetic fields that propagate through space.
• Photoelectric Effect: Ejection of electrons from a metal surface when exposed to light of sufficient frequency.
• Aufbau Principle: Electrons fill the lowest energy orbitals first.
• Pauli Exclusion Principle: No two electrons can have the same set of four quantum numbers.
• Hund’s Rule: Electrons fill degenerate orbitals singly first with parallel spins.