Revision: Modern Physics >> Radioactivity Physics (English Medium) ICSE Class 10 CISCE

The atomic number of an atom is equal to the number of protons in its nucleus (which is same as the number of electrons in a neutral atom).

The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons) in its nucleus.

The total number of neutrons and protons in the nucleus is called the mass number of the element and is denoted by A.

The number of protons in the nucleus is known as the atomic number of the element and is denoted by Z.

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 atoms of different elements which have the same mass number A, but different atomic number Z, are called isobars.

The atoms having different number of protons but same number of neutrons i.e., different Z and A, but same (A-Z), are called isotones. They have different number of 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.

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.

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.

Gamma (γ) radiations are the radiations which are uncharged (neutral) and pass undeviated in both magnetic and electric fields, and are electromagnetic waves similar to light waves.

Beta (β) radiations are the radiations which are negatively charged and turn to the right in a magnetic field or towards the positive plate in an electric field, and are deviated more than alpha particles.

Alpha (α) radiations are the radiations which are positively charged and turn to the left in a magnetic field or towards the negative plate in an electric field.

The energy released due to loss in mass during the processes of nuclear fission and fusion is called nuclear (or atomic) energy.

OR

The energy released when nuclei undergo a nuclear reaction (change in structure, forming new nuclei) is called nuclear energy.

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.

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 structure of an atom and its nucleus was developed from the discovery of electrons by J.J. Thomson and alpha particle scattering experiments by Rutherford.
• An atom consists of electrons, protons, and neutrons, with protons and neutrons in the nucleus and electrons revolving in stationary orbits.
• The maximum number of electrons in a shell is given by 2n², and the shells are named K, L, M, N, O, P, and Q.

• The speed of beta particles is of the order of 10⁸ m s⁻¹, but always less than 3 × 10⁸ m s⁻¹.
• The penetrating power of beta particles is more than that of alpha particles but less than that of gamma radiation.
• Beta particles are negatively charged and get deflected in electric and magnetic fields more than alpha particles.
• Beta particles produce X-rays when stopped by metals of high atomic number and high melting point.
• Beta particles cause more biological damage than alpha particles as they can easily pass through the skin.

• In alpha emission, the atomic number (Z) decreases by 2, and the mass number (A) decreases by 4.
• In beta emission, the atomic number (Z) increases by 1 and the mass number (A) remains unchanged.
• In gamma emission, there is no change in atomic number (Z) and mass number (A); only the energy of the nucleus changes.
• Beta emission is often followed by the emission of an antineutrino (ν̅) to conserve energy and momentum.
• The daughter nucleus may still be radioactive and can undergo further α or β emission until a stable nucleus is formed.

• Radioactive fallout from nuclear power plants can release harmful radiation into the atmosphere during accidents, affecting distant areas.
• Nuclear waste from rejected fuel rods remains highly radioactive and can contaminate water and soil if not properly managed.
• Cosmic radiation and X-rays are other sources of harmful radiation, as some uncharged radiation, such as γ-rays, reaches Earth's atmosphere.

• The internal source - the radioactive substances such as potassium (K-40), carbon (C-14), and radium present inside
  our body.
• External source - cosmic rays, naturally occurring radioactive elements such as radon-222, and solar radiation.