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.

Ionization potential is defined as ionization energy per unit charge.

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.

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).

The nuclear radius is the effective distance from the centre of the nucleus to its outer boundary. Since the nucleus does not have a perfectly sharp surface, this radius is treated as an approximate effective value in physics.

The nuclear force is the strong attractive force between nucleons (protons and neutrons) that binds them together inside the nucleus.

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.

Radioactivity is the spontaneous emission of high-energy radiation from the nucleus of an unstable atom.

A radioactive substance is a substance whose nuclei are unstable and therefore emit radiation on their own.

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.

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.

Nuclear fission is a neutron-induced nuclear reaction in which a heavy nucleus, such as uranium-235, breaks into two intermediate-mass nuclear fragments.

Nuclear fusion is the process in which light nuclei combine to form a more tightly bound heavier nucleus, and energy is released during the process.

Because both nuclei carry a positive charge, they repel each other through electrostatic force. This repulsion is called the Coulomb barrier in the context of fusion.

Thermonuclear fusion is fusion initiated by extremely high temperature, which gives nuclei enough kinetic energy to approach one another closely.

The experimentally observed relation is: R = R₀A^1/3

where:

• R = radius of the nucleus
• R₀​ = radius constant
• A = mass number of the nucleus.

For nuclei,

• R₀ ≈ 1.2 × 10⁻¹⁵ m.

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: \[_1H^1and_1H^2\]