Nuclear Chemistry and Radioactivity – NDA Chemistry Notes

The sun produces energy by nuclear fusion. Nuclear power stations generate electricity through nuclear fission. The doctor uses radiation to treat cancer. The archaeologist uses carbon dating to determine the age of ancient objects. All of these involve nuclear chemistry.

Nuclear chemistry is the study of reactions involving the nucleus of an atom. These reactions are very different from ordinary chemical reactions. They involve enormous amounts of energy and can change one element into another.

1. Radioactivity

Some elements have nuclei that are unstable. An unstable nucleus breaks down on its own to reach a more stable state. During this process, it releases radiation. This spontaneous emission of radiation from an unstable nucleus is called radioactivity.

Radioactivity was discovered by Henri Becquerel in 1896. He found that uranium salts could expose photographic plates even in the dark.

Marie Curie and her husband Pierre Curie further studied radioactivity. Marie Curie discovered the radioactive elements polonium and radium. She was the first person to win two Nobel Prizes (Physics and Chemistry).

A substance that shows radioactivity is called a radioactive substance. Common radioactive elements include uranium (U), thorium (Th), radium (Ra), polonium (Po), and radon (Rn).

2. Types of Radioactive Radiation

When a radioactive nucleus decays, it emits one or more of three types of radiation.

Alpha Radiation (α)

An alpha particle consists of 2 protons and 2 neutrons. This is the same as the nucleus of a helium atom (He-4). So an alpha particle can also be written as ₂He⁴ or ⁴₂He.

When an atom emits an alpha particle, the atomic number decreases by 2. The mass number decreases by 4. The element changes into a different element.

Properties of alpha particles: Alpha particles are the heaviest of the three types. They have a positive charge of +2. They travel very slowly. They are stopped by a sheet of paper or a few centimetres of air. They have the strongest ionising power (they ionise air most strongly).

Beta Radiation (β)

A beta particle is a high-speed electron. It has a negative charge.

When an atom emits a beta particle: A neutron in the nucleus converts to a proton and an electron. The electron is ejected as the beta particle. The atomic number increases by 1 (one more proton). The mass number stays the same (total nucleons unchanged). The element changes into a different element.

Properties of beta particles: Beta particles are faster than alpha particles. They are stopped by a thin sheet of aluminium or a metre of air. They have moderate ionising power.

Gamma Radiation (γ)

Gamma rays are high-energy electromagnetic radiation. They have no mass and no charge. They are like very high energy X-rays.

When a nucleus emits gamma rays: The atomic number does NOT change. The mass number does NOT change. The element stays the same. Only the energy of the nucleus changes.

Gamma radiation is often emitted alongside alpha or beta decay. The nucleus loses energy but stays the same element.

Properties of gamma rays: Gamma rays are the most penetrating. They can only be stopped by thick lead or concrete. They have the lowest ionising power of the three types.

Property	Alpha (α)	Beta (β)	Gamma (γ)
Nature	Helium nucleus	High-speed electron	Electromagnetic radiation
Charge	+2	-1	0
Mass	4 u	Nearly 0	0
Penetrating power	Lowest (paper stops it)	Moderate (aluminium)	Highest (lead/concrete)
Ionising power	Highest	Moderate	Lowest
Speed	Slowest	Fast	Speed of light
Change in atomic number	Decreases by 2	Increases by 1	No change
Change in mass number	Decreases by 4	No change	No change

3. Isotopes and Isobars

Isotopes: Atoms of the same element (same atomic number) but with different mass numbers (different number of neutrons) are called isotopes. [NDA 2012-I] Carbon-12 (₆C¹²) and Carbon-14 (₆C¹⁴) are isotopes of carbon. Both have 6 protons. C-12 has 6 neutrons. C-14 has 8 neutrons. Hydrogen has three isotopes: Protium (₁H¹, ordinary hydrogen), Deuterium (₁H², heavy hydrogen), and Tritium (₁H³).

Tritium: Tritium has 1 proton and 2 neutrons.

Isobars: Atoms of different elements (different atomic numbers) but with the same mass number are called isobars. [NDA 2012-I] Calcium (₂₀Ca⁴⁰) and Argon (₁₈Ar⁴⁰) are isobars. Both have mass number 40 but different atomic numbers. Isobars = same mass number, different atomic number (different elements).

Isotones: Atoms with the same number of neutrons but different atomic numbers are called isotones.

4. Nucleus of a Singly Ionised Carbon Atom

A normal carbon atom (C) has 6 protons and 6 neutrons in its nucleus. A neutral carbon atom also has 6 electrons orbiting outside.

A singly ionised carbon atom (C⁺) has lost one electron from the outer shell. This loss of an electron does NOT affect the nucleus.

Therefore, the nucleus of a singly ionised carbon atom still contains 6 protons and 6 neutrons. [NDA 2012-I] The nucleus has no electrons inside it (electrons orbit outside). So the option “6 protons, 6 neutrons and 6 electrons” in that NDA question is wrong because electrons are not inside the nucleus.

5. Half-Life

A radioactive substance decays at a characteristic rate. Half-life is the time taken for half of the radioactive atoms in a sample to decay. Different radioactive substances have very different half-lives.

Radioactive Substance	Half-Life
Carbon-14	About 5730 years
Uranium-238	About 4.5 billion years
Radium-226	About 1600 years
Iodine-131	About 8 days
Strontium-90	About 29 years

After one half-life, half the original sample remains. After two half-lives, one quarter remains. After three half-lives, one eighth remains.

6. Carbon Dating

When a living organism is alive, it constantly takes in carbon from the environment. The ratio of Carbon-12 to Carbon-14 in the living body stays the same as in the atmosphere.

When the organism dies, it stops taking in carbon. The Carbon-14 in the body begins to decay. Carbon-12 does not decay.

By measuring the ratio of Carbon-14 to Carbon-12 in an ancient fossil or artefact, scientists can calculate how long ago the organism died. This is called radiocarbon dating or carbon dating. The isotope of carbon used in carbon dating is Carbon-14.

Carbon dating can be used to date objects up to about 50,000 years old. Beyond this, the amount of C-14 becomes too small to measure accurately.

7. Nuclear Fission

Nuclear fission is the splitting of a heavy nucleus (like uranium-235 or plutonium-239) into two smaller nuclei when struck by a neutron.

U-235 + neutron → Two smaller nuclei + 2 or 3 free neutrons + enormous energy

The free neutrons released can split more uranium nuclei. This leads to a chain reaction. In a chain reaction, each fission releases neutrons which cause more fissions. The number of fissions multiplies rapidly.

Uncontrolled chain reaction: Used in nuclear bombs (atom bombs). The reaction is uncontrolled and releases energy almost instantly. Controlled chain reaction: Used in nuclear reactors. The reaction is carefully controlled to release energy at a steady rate for electricity generation.

Both nuclear reactors and nuclear bombs use chain reactions. The key difference is control.

8. Nuclear Fission Reactor

A nuclear reactor uses controlled fission to generate heat. The heat produces steam. The steam drives turbines to generate electricity. The four essential components of a reactor are fuel, moderator, control rods, and coolant.

Nuclear Fuel

The fuel used is uranium (U-235 or U-233) or plutonium (Pu-239). Uranium is the most commonly used nuclear fuel.

Moderator

The moderator slows down the fast neutrons released in fission. Slow (thermal) neutrons are more effective at causing further fission.

Heavy water (D₂O, water made with deuterium instead of ordinary hydrogen) is commonly used as a moderator. [NDA 2009-I] Ordinary water (H₂O) and graphite are also used as moderators.

Heavy water is water with a molecular weight of 20 u. Normal water has molecular weight 18 u. Heavy water contains deuterium (²H or D) instead of ordinary hydrogen (¹H).

Control Rods

Control rods absorb neutrons to control the rate of the chain reaction. When pushed further into the reactor, they absorb more neutrons and slow down the reaction. When pulled out, the reaction speeds up. Boron or cadmium are used to make control rods. They absorb neutrons very effectively.

Coolant

The coolant carries the heat generated in the reactor to the steam generator. Water, heavy water, or liquid sodium are used as coolants.

What is NOT needed in a nuclear fission reactor: An accelerator is NOT needed in a nuclear fission reactor. Accelerators are used in particle physics experiments to speed up particles. In a reactor, slow neutrons (moderated) are needed, not fast ones.

9. Nuclear Fusion

Nuclear fusion is the combining of two light nuclei to form a heavier nucleus. This process releases enormous energy.

The Sun produces its energy by nuclear fusion. In the Sun, hydrogen nuclei (protons) fuse to form helium nuclei. This releases vast amounts of energy as light and heat.

⁴ × ₁H¹ → ₂He⁴ + 2 positrons + 2 neutrinos + energy

Feature	Nuclear Fission	Nuclear Fusion
Process	Heavy nucleus splits	Light nuclei combine
Fuel	Uranium-235, Plutonium-239	Hydrogen (deuterium, tritium)
Energy released	Very large	Even larger per kg of fuel
Used in	Nuclear reactors, atom bomb	Sun, hydrogen bomb
Requirement	Only needs a neutron trigger	Requires extremely high temperature (millions of °C)
Controlled use	Yes (nuclear reactors)	Not yet achieved in sustained controlled form
Waste	Radioactive waste produced	Minimal radioactive waste

Hydrogen bomb uses nuclear fusion (triggered by a fission bomb to achieve the required temperature). It is far more powerful than an atomic bomb.

10. Mass-Energy Equivalence

In nuclear reactions, a tiny amount of mass disappears. This mass is converted into a large amount of energy.

Albert Einstein proposed the famous equation: E = mc²

E = energy released. m = mass lost (mass defect). c = speed of light (3 × 10⁸ m/s).

Because c² is an enormous number, even a tiny loss of mass produces a tremendous amount of energy.

In an atomic explosion, the release of energy is due to conversion of mass into energy. This mass-energy conversion is what makes nuclear reactions so much more powerful than ordinary chemical reactions.

11. Radioactive Isotopes and Their Uses

Radioactive isotopes (radioisotopes) have many important uses in medicine and industry.

Radioisotope	Use
Cobalt-60 (Co-60)	Treatment of cancer (radiotherapy)
Iodine-131 (I-131)	Treatment of thyroid cancer and thyroid disorders
Strontium-90 (Sr-90)	Bone cancer treatment; also used in nuclear batteries
Uranium-235 (U-235)	Fuel for nuclear reactors and atom bombs
Carbon-14 (C-14)	Carbon dating of ancient specimens
Technetium-99m (Tc-99m)	Medical imaging (diagnostic scans)
Radium-226	Formerly used in cancer treatment and luminous paints

Radioactive substances that enter the human body through the food chain and cause physiological disorders include: Strontium-90, Iodine-131, and Cesium-137. All three of these can accumulate in different organs and cause harm.

12. Radioactivity and the Food Chain

Nuclear accidents (like Chernobyl in 1986 and Fukushima in 2011) release radioactive substances into the environment.

Strontium-90 behaves like calcium in the body. It accumulates in bones. It causes bone cancer and leukaemia.

Iodine-131 accumulates in the thyroid gland. It causes thyroid cancer. Potassium iodide tablets are given after nuclear accidents to protect the thyroid.

Cesium-137 behaves like potassium in the body. It distributes throughout the body. It causes various cancers.

All three (Strontium-90, Iodine-131, Cesium-137) enter the body through food (milk, vegetables, meat) grown in contaminated areas.

13. Rutherford’s Alpha Particle Scattering Experiment

Rutherford shot alpha particles at a thin gold foil and observed where they went.

Most alpha particles passed straight through. This showed that most of the atom is empty space.

A few alpha particles were deflected at large angles. A very few bounced back. This proved that the positive charge and most of the mass of the atom is concentrated in a very small, dense region at the centre. Rutherford called this the nucleus.

This experiment led to Rutherford’s nuclear model of the atom. [Covered in Chapter 2]