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 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 number of protons in the nucleus of an atom, which is characteristic of a chemical element and determines its place in the periodic table. Atomic number is also equal to the number of electrons in an 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 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.

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.

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.

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.

OR

The energy released during the transformation of nuclei is called Nuclear Energy.

The difference between the sum of the masses of the nucleons composing a nucleus and the rest mass of the nucleus is called the mass defect.

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.

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

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

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

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

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

• 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.

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: \[_1H^1and_1H^2\]

Isobars are atoms of different elements that have the same mass number but different atomic numbers.

Same in isobars:

• Mass number (A)
• Number of nucleons

Different in isobars:

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

Examples: \[_{18}Ar^{40}\mathrm{and}_{19}K^{40}\]

Isotones are atoms of different elements that have the same number of neutrons but different atomic and mass numbers.

Same in isotones:

• Number of neutrons

Different in isotones:

• Atomic number and mass number
• Number of protons and electrons
• Electronic configuration
• Position in periodic table

Examples: \[_1\mathrm{H}^3\mathrm{~and~}_2\mathrm{H}\mathrm{e}^4\]

• 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.

• 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.