Some atoms are not stable. They break down on their own and give out rays. This is called radioactivity. Exams ask a fixed set of questions on it. They ask who discovered it, the three types of rays, half-life, fission and fusion, and uses in medicine. This post explains radioactivity and nuclear energy in simple words. It ends with 15 previous year questions.
Key Points (Quick Revision)
- Radioactivity was discovered by Henri Becquerel in 1896.
- Alpha rays are the least penetrating. Gamma rays are the most penetrating.
- Half-life is the time in which half of a radioactive sample decays.
- Fission splits a heavy nucleus. Fusion joins light nuclei. The Sun runs on fusion.
- Carbon-14 is used to find the age of old fossils and wood.
1. What radioactivity is
Every atom has a small centre called the nucleus. It holds protons and neutrons. In most atoms, the nucleus is stable. In some heavy atoms, it is not.
An unstable nucleus breaks down on its own. While doing so, it gives out radiation. This process is called radioactivity, or radioactive decay. It does not depend on heat, pressure or any outside condition.
Elements that show this are called radioactive elements. Uranium, thorium, radium and polonium are common examples.
The history is asked often:
| Scientist | Contribution |
|---|---|
| Henri Becquerel | Discovered radioactivity in uranium salts in 1896 |
| Marie Curie and Pierre Curie | Discovered polonium and radium in 1898 |
| Ernest Rutherford | Named and studied alpha and beta rays |
| Otto Hahn and Fritz Strassmann | Discovered nuclear fission in 1938 |
Becquerel and the Curies shared the Nobel Prize in Physics in 1903. Marie Curie later won the Nobel Prize in Chemistry in 1911. She was the first person to win Nobel Prizes in two different sciences.
2. The three types of radiation
| Point | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| What it is | Helium nucleus (2 protons, 2 neutrons) | Fast-moving electron | Electromagnetic wave |
| Charge | Positive (+2) | Negative (-1) | No charge |
| Penetrating power | Least | Medium | Most |
| Stopped by | A sheet of paper | A thin sheet of aluminium | Thick lead or concrete |
| Ionising power | Most | Medium | Least |
Note the pattern. Alpha rays go the shortest distance. But they cause the most ionisation along the way. Gamma rays go the farthest, but cause the least ionisation.
Gamma rays have no charge. So they are not bent by electric or magnetic fields. Alpha and beta rays are bent in opposite directions, because their charges are opposite.
Decay also changes the element:
- Alpha decay - atomic number falls by 2, mass number falls by 4.
- Beta decay - atomic number rises by 1, mass number stays the same.
- Gamma decay - no change in atomic number or mass number.
3. Half-life and measuring radioactivity
The half-life of a radioactive element is the time in which half of its atoms decay. After one half-life, half the sample is left. After two half-lives, one-fourth is left. After three, one-eighth is left.
Each element has its own half-life. Some last a fraction of a second. Others last billions of years. Half-life cannot be changed by heat or pressure.
Carbon-14 has a half-life of about 5,730 years. Living things take in carbon all their life. After death, their carbon-14 slowly decays. By measuring what is left, scientists find the age of old wood, bones and fossils. This is called carbon dating, or radiocarbon dating. It was developed by Willard Libby.
For very old rocks, scientists use elements with much longer half-lives, such as uranium.
| What is measured | Unit or tool |
|---|---|
| Activity (SI unit) | Becquerel (Bq) - one decay per second |
| Activity (older unit) | Curie (Ci) |
| Radiation dose on the body | Sievert (Sv) |
| Instrument to detect radiation | Geiger-Muller counter |
4. Nuclear fission and fusion
Both fission and fusion release huge energy from a small mass. The link is Einstein's equation, E = mc². A tiny loss of mass turns into a very large amount of energy.
| Point | Nuclear fission | Nuclear fusion |
|---|---|---|
| What happens | A heavy nucleus splits into two smaller nuclei | Two light nuclei join to form a heavier one |
| Example | Uranium-235 hit by a neutron | Hydrogen nuclei join to form helium |
| Where used or seen | Nuclear power reactors, atom bomb | The Sun and stars, hydrogen bomb |
| Condition | Needs a neutron to start | Needs extremely high temperature |
Fission also gives out new neutrons. These neutrons split more nuclei. This goes on and on. It is called a chain reaction. In a bomb, it is uncontrolled. In a reactor, it is kept under control.
The energy of the Sun comes from fusion. In the Sun's core, hydrogen nuclei join to form helium and release energy.
5. Parts of a nuclear reactor
| Part | Job | Materials used |
|---|---|---|
| Fuel | Undergoes fission and gives energy | Uranium-235, Plutonium-239 |
| Moderator | Slows down the neutrons | Heavy water, graphite |
| Control rods | Absorb extra neutrons and control the reaction | Cadmium, boron |
| Coolant | Carries heat away from the core | Water, heavy water, liquid sodium |
| Shield | Stops harmful radiation from escaping | Thick concrete |
Heavy water is D2O. In it, normal hydrogen is replaced by deuterium, a heavier isotope of hydrogen.
India's nuclear plan was designed by Homi J. Bhabha. It has three stages. The final stage aims to use thorium as fuel. India has large thorium reserves in the monazite sands of the Kerala coast. India's first research reactor, Apsara, started in 1956 at Trombay.
6. Uses and dangers
Radioactive isotopes have many useful jobs:
- Cobalt-60 - used in radiotherapy to treat cancer.
- Iodine-131 - used to find and treat thyroid problems.
- Carbon-14 - used for carbon dating of fossils and old objects.
- Gamma rays - used to kill germs in food and on medical tools.
But radiation is also harmful. Too much exposure damages body cells. It can cause burns, radiation sickness and cancer. That is why workers in these places wear protection and carry dose meters.
Two nuclear accidents are asked often. The Chernobyl disaster happened in 1986, in present-day Ukraine. The Fukushima disaster happened in 2011, in Japan, after an earthquake and tsunami.
7. Previous year questions
- Who discovered radioactivity? – Henri Becquerel
- In which year was radioactivity discovered? – 1896
- Radium and polonium were discovered by whom? – Marie Curie and Pierre Curie
- Which radiation has the highest penetrating power? – Gamma rays
- An alpha particle is the nucleus of which atom? – Helium
- Which radiation is not deflected by a magnetic field? – Gamma rays
- What is the SI unit of radioactivity? – Becquerel
- Which isotope is used to find the age of fossils? – Carbon-14
- Which instrument is used to detect radiation? – Geiger-Muller counter
- The energy of the Sun comes from which process? – Nuclear fusion
- Which process is used in nuclear power reactors? – Nuclear fission
- What is the job of a moderator in a nuclear reactor? – To slow down neutrons
- Which element is used in control rods? – Cadmium (or boron)
- Which isotope is used to treat cancer? – Cobalt-60
- Which isotope is used for thyroid problems? – Iodine-131
8. How to revise this topic
Start with the alpha, beta and gamma table. Learn what each is, its charge, and what stops it. Many questions come from this one table.
Next, learn the names and years in order: Becquerel 1896, the Curies 1898, Hahn and Strassmann 1938.
Then keep fission and fusion side by side - split versus join, reactor versus Sun.
Finally, make a small list of reactor parts and isotopes with their uses. Practise a few half-life sums too.
9. Frequently Asked Questions
What is radioactivity?
Radioactivity is the process in which an unstable atomic nucleus breaks down on its own and gives out radiation - alpha, beta or gamma rays. It does not depend on heat, pressure or any outside condition. It was discovered by Henri Becquerel in 1896.
What is half-life?
Half-life is the time in which half of the atoms in a radioactive sample decay. After one half-life, half the sample is left; after two, one-fourth; after three, one-eighth. Each radioactive element has its own fixed half-life.
Which radiation is the most penetrating?
Gamma rays. They have no charge and can pass through thick materials, so they are stopped only by thick lead or concrete. Alpha rays are the least penetrating and are stopped by a sheet of paper.
What is the difference between nuclear fission and fusion?
In fission, a heavy nucleus like uranium-235 splits into smaller nuclei and releases energy. It is used in nuclear reactors. In fusion, light nuclei like hydrogen join to form a heavier nucleus. Fusion is the source of the Sun's energy.
How does carbon dating work?
Living things take in carbon, including a small amount of radioactive carbon-14. After death, the carbon-14 decays with a half-life of about 5,730 years. By measuring how much is left, scientists find the age of old wood, bones and fossils.
For more General Science topics in simple language, see our Atomic Structure, Periodic Table and Nuclear and Thermal Power Plants posts, or browse the General Science section. Preparing for a specific exam? Start with the RRB NTPC guide or join the 100 Hour GS Course.