Modern Physics

Model of an atom

Radioactivity

• When size of nucleus enlarges → Electrostatic force > Nuclear force, which leads to radioactivity
• Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability.
• The decomposition process is called radioactivity. The energy and particles which are released during the decomposition process are called radiation.

• When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.
• When unstable nuclei decompose in nature, the process is referred to as natural radioactivity.

Three major types of natural radioactivity

Alpha radiation

• Consists of a stream of positively charged particles, called alpha particles
• Alpha particles have an atomic mass of 4 and a charge of +2 (a helium nucleus).
• When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. For example:

 ²³⁸₉₂U → ⁴₂He + ²³⁴₉₀Th        (The helium nucleus is the alpha particle)

Beta Radiation

• Consists of a stream of electrons, called beta particles.
• When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit. For example:

 ²³⁴₉₀Th    → ⁰_-1e + ²³⁴₉₁Pa (The electron is the beta particle)

 Gamma Radiation

• Gamma rays are high-energy photons with a very short wavelength (0.0005 to 0.1 nm).
• The emission of gamma radiation results from an energy change within the atomic nucleus.
• Gamma emission changes neither the atomic number nor the atomic mass.
• Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

Applications of Radioactivity

• Used as a tracer for chemical reactions. You can put an isotope in a living organism and it will do the same reactions as the regular element but you will be able to trace what it reacts with and where it goes
• Detecting how old something is by seeing how much of the isotope of the element is left → Carbon Dating →  C 14 (Used for living organisms) & Uranium dating →  For non-living organism ex. rocks
• Used for finding out the faults in metal structures esp. in airplanes → radioactive material will penetrate more through the cracked areas
• Act as a fuel for nuclear reactors to produce electricity
• Some isotopes are used in the treatment of cancer→ to kill the cancer mutated cells
• Some isotopes are used to study the proper functioning of internal organs
• Gamma radiations are used to sterilize the surgical instruments
• Radio phosphorous is used for studying the rate of phosphorous assimilation by the plant
• Preservation of food grains and seeds
• Used for preparing synthetic elements (artificial transmutation)
• Detecting leaks in natural gas pipes

Nuclear fission

• In nuclear fission the nucleus of an atom breaks up into two lighter nuclei.
• The process is accompanied by the release of a large amount of energy.
• The process may take place spontaneously in some cases or may be induced by the excitation of the nucleus with a variety of particles (e.g., neutrons, protons, deuterons, or alpha particles) or with electromagnetic radiation in the form of gamma rays.

Atomic bomb → Only by fissile U 235 i.e. Enriched Uranium (90%)

For Nuclear reactors → 6 % of U-235

Nuclear fusion

• Process by which nuclear reactions between light elements form heavier elements (up to iron).
• During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy) → substantial amounts of energy are released.

Hydrogen bomb → Requires an atomic bomb to detonate

Nuclear Reactor

• A nuclear reactor, formerly known as atomic pile, is a device used to initiate and control a sustained nuclear chain reaction.
• Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships.

• Heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines.
• These either drive a ship’s propellers or turn electrical generators.

Sequential Process