Atomic and Nuclear Physics

Introduction

Atomic and Nuclear Physics deals with the study of the structure of atoms, their energy levels, and nuclear reactions such as radioactivity, nuclear fission, and fusion. This chapter is crucial for NEET aspirants as it explains the fundamental principles behind atomic models, nuclear forces, and applications of nuclear energy. Understanding atomic and nuclear physics helps in various medical and technological applications like MRI, X-rays, nuclear power plants, and radiation therapy.

Atomic Physics

Structure of the Atom

An atom consists of three fundamental particles:

• Protons (positively charged)

• Neutrons (neutral)

• Electrons (negatively charged)

The protons and neutrons are located in the nucleus, while electrons revolve around the nucleus in discrete energy levels.

Atomic Models

1. Thomson’s Model (Plum Pudding Model)

   • Proposed by J.J. Thomson.

   • Suggested that an atom is a sphere of positive charge with electrons embedded like raisins in a pudding.

   • Could not explain the stability of the atom.

2. Rutherford’s Model (Nuclear Model)

   • Proposed by Ernest Rutherford based on his gold foil experiment.

   • Stated that an atom has a small, dense nucleus with electrons orbiting around it.

   • Failed to explain why electrons do not fall into the nucleus due to attraction.

3. Bohr’s Model (Quantized Energy Levels)

   • Proposed by Niels Bohr.

   • Electrons revolve in fixed orbits (energy levels).

   • Electrons can jump between energy levels by absorbing or emitting energy.

   • Explained hydrogen spectra but failed for complex atoms.

Energy Levels and Spectra

• Atoms absorb and emit energy in discrete amounts called quanta.

• When an electron moves from a higher energy level to a lower energy level, it emits light, forming emission spectra.

• The spectrum of hydrogen consists of Lyman, Balmer, Paschen, Brackett, and Pfund series.

Nuclear Physics

Nucleus and Its Properties

The nucleus of an atom consists of protons and neutrons (nucleons). The number of protons determines the atomic number (Z), and the total number of protons and neutrons determines the mass number (A).

Nuclear Forces

• The strong nuclear force binds protons and neutrons together in the nucleus.

• It is the strongest force in nature but acts only at very short distances.

Radioactivity

Discovered by Henri Becquerel, radioactivity is the spontaneous emission of radiation from unstable atomic nuclei. Marie Curie and Pierre Curie contributed significantly to this field.

Types of Radioactive Decay

1. Alpha (α) Decay

   • The nucleus emits an alpha particle (2 protons and 2 neutrons).

   • Decreases the atomic number by 2 and the mass number by 4.

2. Beta (β) Decay

   • A neutron converts into a proton and emits a beta particle (electron or positron).

   • Changes the atomic number without affecting mass number.

3. Gamma (γ) Decay

   • Emission of high-energy electromagnetic radiation (gamma rays).

   • Does not change atomic number or mass number.

Half-Life and Radioactive Decay Law

• The half-life of a radioactive substance is the time taken for half of its nuclei to decay.

• The rate of decay follows an exponential law.

Nuclear Fission

• Nuclear fission is the splitting of a heavy nucleus (e.g., uranium-235) into smaller nuclei, releasing a huge amount of energy.

• Used in nuclear power plants and atomic bombs.

• A chain reaction occurs when neutrons released in fission trigger further fission reactions.

Nuclear Fusion

• Nuclear fusion is the combining of two light nuclei (e.g., hydrogen isotopes) to form a heavier nucleus, releasing enormous energy.

• Occurs in the Sun and stars.

• Produces much more energy than fission and does not produce harmful radioactive waste.

Applications of Nuclear Physics

1. Nuclear Power Plants: Use fission to generate electricity.

2. Medical Applications:

   • X-rays and CT scans for imaging.

   • Radiotherapy for cancer treatment.

3. Carbon Dating: Used in archaeology to determine the age of fossils.

4. Nuclear Weapons: Atomic bombs use uncontrolled fission reactions.

5. Hydrogen Bombs: Use nuclear fusion for massive explosions.

Conclusion

Atomic and nuclear physics provides a fundamental understanding of matter, energy, and radiation. The Bohr model explains atomic structure, while nuclear reactions such as fission and fusion help in energy production and medical advancements. Mastering this chapter is essential for NEET aspirants to understand real-world applications in power generation, medicine, and astrophysics.