Revision: Atomic and Nuclear Physics Physics HSC Science Class 12 Tamil Nadu Board of Secondary Education

The impact parameter is defined as the perpendicular distance between the centre of the gold nucleus and the direction of the velocity vector of the alpha particle when it is at a large distance.

One atomic mass unit (u) is defined as the 1/12^th of the mass of the isotope of carbon \[\ce{^12_6C}\].

The minimum energy required to remove an electron from an atom in the ground state is known as binding energy or ionization energy.

Ionization potential is defined as ionization energy per unit charge.

The atoms of the same element, having same atomic number Z, but different mass number A, are called isotopes.

OR

Atoms having the same atomic number (Z) but different mass numbers (A).

As nucleus is positively charged it strongly attracts the negative charged electrons. The electron orbit close to the nucleus are tightly bound by strong attractive force of nucleus. These electrons are known as bound electrons.

Electrons in outer orbits are weakly bound with the nucleus. In solids these weakly bound electrons leave their individual atom and become a part of it. These electrons are known as free electrons.

The phenomenon of spontaneous disintegration of an unstable nucleus of a naturally occurring isotope accompanied by emission of active radiations, α particles, β particles and γ radiations is called radioactivity.

Radioactivity is a nuclear phenomenon. It is the process of spontaneous emission of α or β and γ radiations from the nucleus of atoms during their decay.

Nuclear fission is the process in which a heavy nucleus splits into two lighter nuclei of nearly the same size, when bombarded with slow neutrons. In each fission reaction, a tremendous amount of energy (≈ 190 MeV) is released.

OR

The process of splitting of a heavy nucleus (₉₂U²³⁵ or ₉₂U²³⁹) into two lighter nuclei of comparable masses along with the release of a large amount of energy after being bombarded by slow neutrons is called Nuclear Fission.

• Nuclear fusion is the process in which two light nuclei combine to form a heavy nucleus. In this process also, huge amount of energy is released.
• The phenomenon in which two light nuclei fuse to form a larger nucleus and energy is released is called Nuclear Fusion.

\[\text{Fission Reaction of Uranium-235:}\_{92}\mathrm{U}^{235}+_0n^1\longrightarrow\left[_{92}\mathrm{U}^{236}\right]\longrightarrow_{56}\mathrm{Ba}^{144}+_{36}\mathrm{Kr}^{89}+3_0n^1+200\mathrm{~MeV}\]

\[_1\mathrm{H}^2+_1\mathrm{H}^2\longrightarrow_2\mathrm{He}^3+_0n^1+3.27\mathrm{~MeV}\]

\[_1\mathrm{H}^2+_1\mathrm{H}^2\longrightarrow_1\mathrm{H}\mathrm{e}^3+_1\mathrm{H}^1+4.03\mathrm{~MeV}\]

\[_1\mathrm{H}^2+_1\mathrm{H}^3\longrightarrow_2\mathrm{H}\mathrm{e}^4+_0n^1+17.59\mathrm{~MeV}\]

\[_1\mathrm{H}^2+_2\mathrm{He}^3\longrightarrow_2\mathrm{He}^4+_1\mathrm{H}^1+18.3\mathrm{~MeV}\]

Isotopes are atoms of the same element that have the same atomic number but different mass numbers (different number of neutrons).

Same in isotopes:

• Atomic number (Z)
• Number of protons and electrons
• Electronic configuration
• Position in periodic table
• Chemical properties (nearly identical)

Different in isotopes:

• Mass number (A)
• Number of neutrons
• Physical properties

Examples: \[_1H^1and_1H^2\]

• In a fission reaction, a heavy atomic nucleus is split into smaller nuclei, other particles and radiation.
• Uranium-235 absorbs a neutron and splits into barium and krypton, emitting neutrons and radiation.
• Each fission of U²³⁵ releases approximately 200 MeV of energy.
• 3 neutrons are released per fission, which can trigger further fissions — leading to a chain reaction.
• Nuclear power plants exploit the process of fission to create energy.
• If an incoming neutron strikes a uranium nucleus, fragments produced are chemical elements like barium or krypton, while some are free neutrons.

• In a fusion reaction, two or more light atomic nuclei fuse to form a single heavier nucleus.
• The mass change in the process is the source of nuclear energy.
• Fusion within the cores of the sun and other stars generates their radiating energy by fusing two hydrogen atoms to produce a helium atom.
• The product nucleus has less mass than the total mass of the combining nuclei — the difference is released as energy.
• Fusion of deuterium (²H) and tritium (³H) produces helium-4 and releases 17.59 MeV — the most energy-rich reaction listed.
• Fusion releases far more energy per unit mass than fission.