18. Atomic & Nuclear Physics

 Atomic Physics

An atom is like the tiniest building block of stuff that we can find. It's like the Lego piece of matter that sticks together to make everything around us. Each type of atom is kind of like its own special club with its own members who are all pretty similar to each other. These members are called electrons, protons, and neutrons.

Think of the atom as a little solar system. In the middle, there's a core called the nucleus. That's where you'll find all the protons and neutrons hanging out together. Then, whizzing around the nucleus are the electrons, sort of like planets spinning around the sun.

Here's a cool thing: in an atom, the number of electrons is always the same as the number of protons. And they have opposite charges, but they cancel each other out, so the atom as a whole is neutral, like it has no charge at all.

Particle	Mass (kg)	Charge (Coulomb)	Discoverer
Proton	1.672 x 10-27	+1.6 x 10-19	Rutherford
Neutron	1.675 x 10-27	0	Chadwick
Electron	9.108 x 1031	-1.6 x 10-19	J.J. Thomson

Note : Proton was discovered by Golastin and named by Rutherford.

So far, scientists have found lots of tiny particles hiding inside atoms. Some of the important ones are:

Particle	Mass (kg)	Charge	Discoverer
Positron	9.108 x 10-31	+1.6 x 10-19	Anderson	Antiparticle of electron
Neutrino	0	0	Pauli
Pi-meson	274 times the mass of electron	Positive and negative both	Yukawa	unstable
Photon	0	0		Velocity equal to that of light

Cathode Rays: When you put a low amount of gas in a special tube and zap it with a really strong electric shock, something cool happens. From one end of the tube, called the cathode, a bunch of super-fast tiny particles shoot out. We call these little speedsters cathode rays. They're basically a bunch of high-energy electrons. Oh, and by the way, the cathode is the part of the tube that's negatively charged.

Here are the properties of cathode rays:

(i) Cathode rays are invisible and travel in straight lines. (ii) They carry a negative charge and move from the cathode to the anode. (iii) These rays shoot out perpendicular to the cathode surface and aren't affected by where the anode is placed. (iv) They move really fast, about one-tenth the speed of light. (v) Electric and magnetic fields can bend their path. (vi) They can make gases conduct electricity. (vii) They can make things hot when they hit them. (viii) They can cause chemical changes, like affecting a photographic plate. (ix) They can go through thin pieces of metal. (x) They're made using an induction coil. (xi) When they hit heavy metals like tungsten, they can create X-rays. (xii) Cathode rays act the same no matter what material the cathode is made of or what gas is in the tube.

Positive or Canal rays :

When you poke tiny holes in the cathode of a special tube and zap it with electricity, something interesting happens: rays shoot out from the anode, moving towards the cathode and passing through those holes. These rays are positively charged and go by different names like positive rays, canal rays, or anode rays. They were discovered by Goldstein.

Here's a simplified version of the properties of positive or canal rays:

(i) Positive rays are made up of positively charged particles. (ii) They move in straight lines. (iii) They can push against things, showing they have energy. (iv) Electric and magnetic fields can push them around. (v) They can cause physical and chemical changes. (vi) They can make gases conduct electricity.

Radioactivity

Radioactivity is when the tiny center of an atom breaks apart by itself, sending out harmful stuff like radiation or tiny particles.

Henry Becquerel, Madame Curie, and Pierre Curie discovered radioactivity, and they won the Nobel Prize for it.

Atoms with 83 or more protons in their center are unstable and send out alpha, beta, and gamma particles to become stable. These are called radioactive elements, and the process is called radioactivity.

Gamma rays are released after alpha and beta rays.

Pierre and Madame Curie found a new radioactive element called radium.

Rutherford was the first to recognize the rays from radioactivity.

All natural radioactive elements eventually turn into lead after releasing their radioactive rays.

	Stable nucleus	Unstable nucleus
Atomic number	Low	High
Mass number	Low	High
Nucleus size	Small	Bigger
n/p ratio	1	Greater than 1

Properties of Alpha (α), Beta (β), Gamma (γ) Particles

 

Properties	Alpha (α) Particle	Beta (β) Particle	Gamma (γ) Particle
Origin	Nucleus	Nucleus	Nucleus
Nature	Positively charged	Negatively charged	Neutral
Composition	Helium nucleus (He )	Electron	Photon
Mass (kg)	6.4 x 10^-27	9.1 x 10^-31	0
Charge	+2e	-e	0
Chemical effect	Affects photographic plate	Affects photographic plate	Affects photographic plate
Effect of electric and magnetic field	Deflected	Deflected	No effect
Penetrating power	Minimum	In between the other two	Maximum
Ionising power	Maximum	In between the other two	Minimum
Velocity (m/s)	Between 1.4 x 10^7 and 2.2 x 10^7	1% to 99% of velocity of light	3 x 10^8 m/s (velocity of light)

1. When an alpha (α) particle is emitted, the atomic number decreases by 2 and the mass number decreases by 4.
2. When a beta (β) particle is emitted, the atomic number increases by one, but the mass number stays the same.
3. The effects of gamma (γ), beta (β), and alpha (α) radiation on atomic and mass numbers are determined by Group-displacement Law or Soddy-Fajan Law.
4. Radioactivity is detected using a Geiger-Muller (G.M.) Counter.
5. The time it takes for half of the nuclei in a radioactive substance to decay is called its half-life.

Cloud chamber: A cloud chamber helps us see radioactive particles and measure their energy. It was invented by C.R.T. Wilson. Carbon-14 is a radioactive form of carbon used to find the age of fossils and plants through carbon dating. We determine age by comparing the ratio of 6C12 to 6C14.

Nuclear Fission and Fusion: Nuclear fission is when a big nucleus splits into two smaller ones. This releases a lot of energy, which we call nuclear energy. Strassmann and O. Hahn were the first to show nuclear fission. They found that when uranium-235 absorbs a neutron, it breaks into two smaller nuclei: barium-142 and krypton-92.

Chain Reaction: When uranium atoms are hit by slow neutrons, they split, releasing more neutrons that split more uranium atoms, creating a chain reaction.

There are two types: uncontrolled and controlled.

Uncontrolled Chain Reaction : In an uncontrolled chain reaction, each fission reaction produces three neutrons. These neutrons then cause the fission of more uranium nuclei, creating even more neutrons. This cycle continues, with the number of neutrons increasing rapidly until all the fissionable material is used up. This reaction happens very quickly and releases a massive amount of energy in a short time, which is why it's called an uncontrolled or explosive chain reaction.

Atom bomb : An atom bomb relies on nuclear fission, using uranium-235 and plutonium as the fissionable materials. The United States first used this bomb during World War II, dropping it on Hiroshima on August 6, 1945, and Nagasaki on August 9, 1945.

Controlled Chain Reaction : In a controlled chain reaction, the fission process happens slowly and without any explosion. This allows us to control the release of energy. Only one of the neutrons produced in each fission event is able to cause further fission, keeping the reaction rate constant.

Nuclear Reactor or Atomic Pile : A nuclear reactor, also known as an atomic pile, is a setup where controlled nuclear fission reactions occur. The first nuclear reactor was established at the University of Chicago under Prof. Fermi's supervision.

Here are its components:

1. Fissionable Fuel: Uranium-235 or Plutonium-239 is used.
2. Moderator: This slows down neutrons so they can trigger more fission reactions. Heavy water or graphite is commonly used.
3. Control Rod: These rods, made of cadmium or boron, absorb excess neutrons to regulate the chain reaction.
4. Coolant: Heat is generated during fission. Coolant absorbs this heat to prevent overheating. Water, heavy water, or gases like helium or carbon dioxide can be used as coolants.

Uses of nuclear reactor

1. Nuclear reactors are used to generate electricity by harnessing the energy released during nuclear fission.
2. They're also used to create various isotopes for medical, scientific, and agricultural purposes.

Nuclear Fusion : Nuclear fusion happens when two or more light nuclei join to form a heavier nucleus, releasing huge amounts of energy. A typical example of nuclear fission is ₁H²+ ₁H³ →₂He⁴+_{0n¹+17.6 Mev.}

The sun and other stars produce energy through nuclear fusion. It requires incredibly high temperatures, around 10 million Kelvin.

Hydrogen bomb : The hydrogen bomb, created by American scientists in 1952, relies on nuclear fusion. It's a thousand times more powerful than an atom bomb.

Mass Energy Relation : In 1905, Einstein showed that mass and energy are related through his famous equation E=mc2, where E is energy, m is mass, and c is the speed of light. This means mass can be converted into energy and vice versa.

Albert Einstein, a German-born American scientist, received the Nobel Prize in Physics in 1921.

The Sun continuously emits energy, with Earth receiving about 4×1017 joules per second. As a result, the Sun's mass decreases by approximately 4×109 kilograms per second. Despite this, the Sun's enormous mass means it will continue to provide energy for another billion years.