Atomic Nucleus: Atomic Number, Mass Number, Isotopes, Isobars, Gaseous Laws, Radioactivity & Nuclear Energy: Complete Guide for UPSC, SSC, Railway Exams 2026

Atomic Nucleus, Gaseous Laws, Radioactivity & Nuclear Energy 

Introduction

Understanding atomic structure, gaseous behaviour, and nuclear processes is crucial for scoring well in the General Science section of competitive exams like UPSC, SSC, Railway, and Delhi Police. These topics appear regularly in exam papers and form the foundation of chemistry and physics concepts. This comprehensive guide breaks down complex scientific concepts into simple, easy-to-understand language with practical examples that will help you remember key points during your exam preparation.

Whether you're appearing for UPSC Prelims, SSC CGL, Railway NTPC, or Delhi Police Constable exams, this article covers everything you need to know about atomic nucleus, gaseous laws, radioactivity, and nuclear energy. Let's dive into each topic systematically with examples and tables that make learning easier.

 

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Atomic Nucleus

 

What is an Atomic Nucleus?

The atomic nucleus is the central part of an atom where most of its mass is concentrated. Think of it as the sun in our solar system—small in size but containing most of the system's mass. The nucleus contains two types of particles called nucleons: protons (positively charged) and neutrons (neutral, no charge). Electrons revolve around this nucleus in different orbits or shells.

The nucleus is extremely dense and tiny. If an atom were the size of a cricket stadium, the nucleus would be like a marble placed at the center. Despite its small size, it holds 99.9% of the atom's mass.

Table of Contents

• Introduction
• Atomic Nucleus
• Gaseous Laws
• Radioactivity
• Nuclear Energy
• Memory Tricks
• FAQs

 

Atomic Number and Mass Number

 

Atomic Number: Denoted by Z.
This represents the number of protons present in the nucleus of an atom. It determines the identity of an element. For example, all carbon atoms have 6 protons, so carbon's atomic number is 6. The atomic number also equals the number of electrons in a neutral atom.

 

Mass Number: Denoted by A.
This is the total number of protons and neutrons in the nucleus. It represents the mass of the atom. The formula is simple: Mass Number = Number of Protons + Number of Neutrons, or A = Z + N.

Example: Oxygen has 8 protons and 8 neutrons. Therefore, its atomic number is 8, and its mass number is 16.

 

Element	Symbol	Atomic Number (Z)	Neutrons (N)	Mass Number (A)
Hydrogen	H	1	0	1
Carbon	C	6	6	12
Oxygen	O	8	8	16
Sodium	Na	11	12	23
Chlorine	Cl	17	18	35

 

Atomic Number vs Mass Number

Aspect	Atomic Number (Z)	Mass Number (A)
Represents	No of Protons	Protons + Neutrons
Determines	Chemical Element	Isotope of Element
Changes when	Element changes	Isotope changes

 

Isotopes: Same Element, Different Mass

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. Imagine three brothers from the same family—they share the same surname (element) but have different weights (mass numbers). (Properties: Chemically similar, physically different.)

  Examples:

·       Hydrogen isotopes: Protium (¹H), Deuterium (²H), Tritium (³H)

·       Carbon isotopes: ¹²C (98.9%), ¹³C (1.1%), ¹⁴C (trace, radioactive)

Isobars: Different Elements, Same Mass

Isobars are atoms of different elements that have different atomic numbers but the same mass number. Think of it as two people from different families who happen to weigh the same—different identities but same weight.

Examples of Isobars:

·       Argon-40 (18 protons + 22 neutrons) and Calcium-40 (20 protons + 20 neutrons)

·       Carbon-14 (6 protons + 8 neutrons) and Nitrogen-14 (7 protons + 7 neutrons)

Isobar Pair	Element 1	Atomic Number	Element 2	Atomic Number	Mass Number
Pair 1	Argon	18	Calcium	20	40
Pair 2	Sulfur	16	Chlorine	17	37
Pair 3	Iron	26	Manganese	25	56

 

Key Difference: Isobars have different chemical properties because they are different elements with different numbers of protons and electrons.

 

Isoelectronic Species: Same Electron Count

Isoelectronic species are atoms or ions that have the same number of electrons. These can be atoms of different elements or ions. Imagine different vehicles carrying the same number of passengers—different types but same passenger count.

Examples:

·       Na⁺, Mg²⁺, Al³⁺, and Ne all have 10 electrons

·       O²⁻, F⁻, Na⁺, and Mg²⁺ all have 10 electrons

·       S²⁻, Cl⁻, Ar, K⁺, and Ca²⁺ all have 18 electrons

Comparison of Nuclear Species

Type	Same	Different
Isotope	Atomic number	Mass number, Neutrons
Isobar	Mass number	Atomic number, Elements
Isoelectronic	Number of electrons	Number of protons, species

 

Also Read: Physics- Unit, Measurement & Measuring Instruments.  

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Gaseous Laws

Introduction to Gas Laws

Gases are unique because their particles move freely and can be compressed. Understanding how gases behave under different conditions of pressure, volume, and temperature is essential for competitive exams. These relationships are described by gas laws discovered by various scientists.

Boyle's Law: Pressure-Volume Relationship

Statement: At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.

Formula: P₁V₁ = P₂V₂ or PV = constant

Simple Explanation: When you press a balloon, its volume decreases while pressure increases. When you release it, volume increases and pressure decreases.

Real-life Example: When you pump air into a bicycle tire, you're increasing pressure which decreases the volume of air molecules, making the tire hard.

Graph: Hyperbolic Curve

Charles's Law: Volume-Temperature Relationship

Statement: At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature (in Kelvin).

Formula: V₁/T₁ = V₂/T₂ or V/T = constant

Simple Explanation: When you heat a gas, it expands. When you cool it, it contracts. This is why hot air balloons rise—heated air expands and becomes less dense.

Real-life Example: A balloon left in the sun expands because the air inside heats up and increases in volume.

Numerical Example:
A gas occupies 3 liters at 300 K. What will be its volume at 450 K?

·       Using V₁/T₁ = V₂/T₂

·       3/300 = V₂/450

·       V₂ = 4.5 liters

Gay-Lussac's Law: Pressure-Temperature Relationship

Statement: At constant volume, the pressure of a fixed amount of gas is directly proportional to its absolute temperature.

Formula: P₁/T₁ = P₂/T₂ or P/T = constant

Real-life Example: Pressure cookers work on this principle. When you heat the cooker, pressure inside increases, cooking food faster.

Avogadro's Law: Volume-Moles Relationship

Statement: Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.

Formula: V₁/n₁ = V₂/n₂

Key Point: One mole of any gas at STP (Standard Temperature and Pressure: 273 K and 1 atm) occupies 22.4 liters.

Ideal Gas Equation: Combined Gas Law

All the gas laws combine into one universal equation:

Formula: PV = nRT

Where:

·       P = Pressure (atm)

·       V = Volume (liters)

·       n = Number of moles

·       R = Universal Gas Constant (0.0821 L·atm/mol·K)

·       T = Temperature (Kelvin)

Remember: To convert Celsius to Kelvin, add 273. (K = °C + 273)

Gas Law	Variables Constant	Formula	Relationship
Boyle's Law	Temperature, Moles	PV = constant	Inverse
Charles's Law	Pressure, Moles	V/T = constant	Direct
Gay-Lussac's Law	Volume, Moles	P/T = constant	Direct
Avogadro's Law	Pressure, Temperature	V/n = constant	Direct
Ideal Gas Law	None	PV = nRT	Combined

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Radioactivity

 

What is Radioactivity?

Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei. Some elements have unstable nuclei that break down naturally, releasing energy and particles. This process is called radioactive decay. Think of it as an unstable building that gradually crumbles over time, releasing pieces.

Henri Becquerel discovered radioactivity in 1896, and Marie Curie and Pierre Curie further studied this phenomenon. Marie Curie discovered two radioactive elements—Polonium and Radium.

Types of Radioactive Emissions

Alpha (α) Particles:

·       Composition: 2 protons + 2 neutrons (Helium nucleus)

·       Charge: +2

·       Mass: 4 amu

·       Penetration Power: Lowest (stopped by paper)

·       Speed: Slowest

·       Ionization Power: Highest

Example: When Uranium-238 decays, it emits an alpha particle and becomes Thorium-234.

Beta (β) Particles:

·       Composition: High-speed electrons or positrons

·       Charge: -1 (electron) or +1 (positron)

·       Mass: Very small (1/1840 amu)

·       Penetration Power: Medium (stopped by aluminum sheet)

·       Speed: Fast

·       Ionization Power: Medium

Example: When Carbon-14 decays, it emits a beta particle and becomes Nitrogen-14.

Gamma (γ) Rays:

·       Composition: Electromagnetic radiation

·       Charge: 0 (neutral)

·       Mass: 0 (no mass)

·       Penetration Power: Highest (requires thick lead)

·       Speed: Speed of light

·       Ionization Power: Lowest

Comparison Table:

Property	Alpha (α)	Beta (β)	Gamma (γ)
Nature	Helium nucleus	Electrons	EM radiation
Charge	+2	-1	0
Mass	4 amu	1/1840 amu	0
Penetration	Paper stops	Aluminum stops	Lead required
Speed	Slowest	Fast	Fastest (light speed)
Ionization	Strongest	Medium	Weakest

 

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Half-Life: Measuring Radioactive Decay

Half-life is the time required for half of the radioactive atoms in a sample to decay. Each radioactive isotope has a characteristic half-life.

Examples:

·       Carbon-14: 5,730 years (used in archaeological dating)

·       Uranium-238: 4.5 billion years

·       Iodine-131: 8 days (used in medical treatments)

·       Polonium-214: 0.00016 seconds

Simple Calculation:
If you start with 100 grams of a radioactive substance with a half-life of 10 years:

·       After 10 years: 50 grams remain

·       After 20 years: 25 grams remain

·       After 30 years: 12.5 grams remain

Applications of Radioactivity

Medical Field:

·       Cancer treatment using Cobalt-60

·       Medical imaging and diagnosis

·       Sterilization of medical equipment

Agriculture:

·       Insect control

·       Food preservation

·       Studying plant uptake mechanisms

Industry:

·       Detecting cracks in metals

·       Measuring thickness of materials

·       Smoke detectors using Americium-241

Archaeology:

·       Carbon-14 dating for ancient artifacts

·       Determining age of rocks and fossils

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Nuclear Energy

 

Nuclear Fission: Splitting the Atom

Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei, releasing enormous energy. This process powers nuclear reactors and atomic bombs.

How it Works:
When a Uranium-235 nucleus absorbs a neutron, it becomes unstable and splits into two smaller nuclei (like Barium and Krypton), releasing 2-3 neutrons and tremendous energy. These neutrons can then split more uranium atoms, creating a chain reaction.

Example Reaction:
U-235 + neutron → Ba-141 + Kr-92 + 3 neutrons + Energy

Advantages:

·       Produces massive amounts of energy from small fuel amounts

·       No greenhouse gas emissions during operation

·       Reliable baseload power

Disadvantages:

·       Radioactive waste disposal problems

·       Risk of accidents

·       High initial setup costs

·       Potential for weapon proliferation

Nuclear Fusion: Combining Atoms

Nuclear fusion is the combining of two light nuclei to form a heavier nucleus, releasing energy. This is the process that powers the sun and stars.

How it Works:
Two hydrogen nuclei combine under extreme temperature and pressure to form helium, releasing enormous energy.

Example Reaction:
2 Hydrogen nuclei → 1 Helium nucleus + Energy

Advantages:

·       Fuel (hydrogen) is abundant in seawater

·       Produces much more energy than fission

·       Minimal radioactive waste

·       No risk of runaway reactions

Challenges:

·       Requires extremely high temperatures (millions of degrees)

·       Difficult to control and sustain

·       Technology still under development

·       Expensive research and development

Nuclear Power Plants in India

India has been developing nuclear power capabilities for energy security. Major nuclear power plants include:

·       Tarapur Atomic Power Station (Maharashtra)

·       Kudankulam Nuclear Power Plant (Tamil Nadu)

·       Kakrapar Atomic Power Station (Gujarat)

·       Rajasthan Atomic Power Station (Rajasthan)

·       Kaiga Nuclear Power Station (Karnataka)

India's Nuclear Program: India is not a signatory to the Nuclear Non-Proliferation Treaty (NPT) but maintains a "No First Use" policy for nuclear weapons.

Comparison	Nuclear Fission	Nuclear Fusion
Process	Splitting heavy atoms	Combining light atoms
Fuel	Uranium, Plutonium	Hydrogen isotopes
Temperature	Lower	Extremely high
Waste	Significant radioactive	Minimal radioactive
Current Use	Power plants, weapons	Research stage
Energy Release	High	Very high

 

Nuclear Safety and Radiation Protection

Safety Measures:

·       Thick concrete containment structures

·       Multiple backup cooling systems

·       Trained personnel and strict protocols

·       Regular monitoring and maintenance

Radiation Protection Principles:

·       Time: Minimize exposure duration

·       Distance: Stay away from radiation sources

·       Shielding: Use protective barriers (lead, concrete)

Famous Nuclear Accidents:

·       Chernobyl (1986) - Ukraine (then USSR)

·       Fukushima (2011) - Japan

·       Three Mile Island (1979) - USA

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Memory Tricks

 

·       Isotopes: "Same House (element), Different Weight (mass)"

·       Isobars: "Different Houses (elements), Same Weight (mass)"

·       Gas Laws: "PV=nRT" - "Please Visit New Royal Temple"

·       Radioactivity: "Alpha Big Slow, Beta Medium Fast, Gamma Small Fastest"

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Frequently Asked Questions (FAQs)

 

Q1: What is the difference between isotopes and isobars?

Isotopes are atoms of the same element with different mass numbers (same protons, different neutrons). Isobars are atoms of different elements with the same mass number but different atomic numbers.

 

Q2: Which gas law is used in pressure cookers?

Gay-Lussac's Law is used in pressure cookers. When heated at constant volume, the pressure of gas increases, allowing food to cook at higher temperatures.

 

Q3: Why is Carbon-14 used for dating ancient objects?

Carbon-14 has a half-life of 5,730 years, making it ideal for dating objects up to 50,000 years old. Living organisms constantly exchange carbon with the environment, but after death, C-14 decays at a known rate.

 

Q4: Which type of radioactive emission is most dangerous?

Gamma rays are most dangerous for external exposure because of their high penetration power. However, alpha particles are most dangerous if ingested or inhaled because of their high ionization power.

 

Q5: What is the difference between nuclear fission and fusion?

Fission splits heavy atoms (like Uranium) into lighter ones, releasing energy. Fusion combines light atoms (like Hydrogen) into heavier ones, releasing more energy. Fission is used in current power plants; fusion is still being researched.

 

Q6: What is STP in gas laws?

STP stands for Standard Temperature and Pressure: 273 K (0°C) temperature and 1 atmosphere (101.325 kPa) pressure. At STP, one mole of any gas occupies 22.4 liters.

 

Q7: Can radioactivity be destroyed or stopped?

No, radioactivity is a nuclear property and cannot be stopped by chemical means or physical conditions. It will continue until the nucleus becomes stable.

 

Q8: Which Indian scientist is famous for nuclear physics?

Dr. Homi J. Bhabha is considered the father of India's nuclear program. Dr. APJ Abdul Kalam also contributed significantly to India's missile and nuclear programs.

 

Q9: What is the most common isotope of Hydrogen?

Protium (¹H) is the most common isotope of Hydrogen, with one proton and no neutrons. It makes up 99.98% of all hydrogen atoms on Earth.

 

Q10: How do nuclear power plants generate electricity?

Nuclear fission heats water to produce steam. This steam drives turbines connected to generators, producing electricity. The nuclear reaction provides heat instead of burning coal or gas.

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Understanding atomic nucleus, gaseous laws, radioactivity, and nuclear energy is crucial for scoring well in competitive exams. These topics frequently appear in the General Science section of UPSC, SSC, Railway, and Delhi Police examinations. Regular revision of formulas, concepts, and examples will help you tackle questions confidently.

Focus on understanding the fundamental concepts rather than rote memorization. Practice numerical problems related to gas laws, memorize key differences between isotopes and isobars, and stay updated with India's nuclear program developments for current affairs sections.

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Best of luck with your preparation! Stay consistent, stay focused, and success will follow.