Class-12 Ch-13 Nuclei

Nuclei class 12 notes

Ch-13 Nuclei

Topic Covered

–Latest CBSE syllabus 2023-2024

-Introduction, Atomic Number, Atomic Mass Number

-Isotopes, Isobars, Isotones

-Nuclear Radius

-Nuclear Density, Mass Defect

-Binding Energy, Binding Energy per nucleons

-Binding Energy Curve.

-Nuclear Fusion, Nuclear Fission, Nuclear Force

-All important CBSE Numerical

CBSE Nuclei Class 12 Notes Chapter 13

1. Every atom has a nucleus, which is the highly concentrated core of positive charges and masses. The nucleus of an atom contains more than 99.9% of its mass.
2. Atomic Mass Unit (amu) – Atomic mass unit is the name of the unit used to express atomic masses. One-twelfth of the mass of a carbon atom (C12) is the definition of an atomic mass unit.

3. Composition of Nucleus The composition of a nucleus can be describing by using the following term and symbols.
(i) Atomic Number (Z) Atomic number of an element is the number of protons or electrons present inside the nucleus of an isolates atom.
Atomic number = No. of protons = No. of electrons
(ii) Mass Number (A) Mass number of an element is the total number of protons and neutrons present inside the atomic nucleus of the element.
Mass number = Number of protons + Number of neutrons = Number of electrons + Number of neutrons

i.e. A=Z + N

• Isotopes:

1. The atoms of an element which have the same atomic number but different mass number are called isotopes.
2. Isotopes may have same chemical properties but different physical properties.

• Isobars: The atoms which have the same Atomic mass number (A) but different atomic number (Z) are called isobars.
• Isotones: The atoms which have the same number of neutrons are called isotones.
• Isomers: These are nuclei with same atomic numbers and same Atomic mass numbers but in different energy state.
• Electron Volt:It is represented by the symbol eV and is defined as the energy that an electron gains when it accelerates through a 1 volt potential difference.
  

• Discovery of Neutrons:

1. Neutrons were discovered by Chadwick in 1932.
2. When beryllium nuclei are bombarded by alpha-particles, highly penetrating radiations are emitted, which consists of neutral particles, each having mass nearly that of a proton. These particles were called neutrons.
3. With a half-life of roughly 900 seconds, a free neutron decays spontaneously to produce a proton, an electron, and an antineutrino. 

Nuclei class 12 notes

• Size of the Nucleus:

1. It is found that a nucleus of mass number A has a radius
   1. 

   Where, 

2. This implies that the volume of  nucleus, which is proportional to radius R³ is proportional to Atomic mass number A.

• Density of the Nucleus: Density of nucleus is constant; it is independent of  atomic mass number A, for all nuclei and density of nucleus is approximately equal to

which is very large as compared to the ordinary matter density like water which is 10³ kg m^-3.

• Mass-Energy equivalence: Albert Einstein proved that it is necessary to treat masses as another form of energy. He give the mass-energy equivalence relation as, E = mc² Where m is the mass and c is the velocity of light in vacuum it is important for nuclei class 12 notes.
• Mass Defect: The given mass of the nucleus is found to the always less the sum of the mass of Protons and Neutrons and this difference of mass is known as Mass Defect. It is given by-

• Binding Energy:

1. It may be defined as the energy required to break a nucleus into its constituent parts (protons and neutrons) and separate them to such a large distance that they may not interact with each other.
2. It can also be described as the excess energy that the nucleus releases as a result of their attraction to one another, which causes them to unite to form a nucleus.
3. The binding energy of a nucleus  is-

• Binding Energy per Nucleon: The Binding Energy per Nucleons of a Nucleus is define as the average B.E divide by the total number of nucleons.

It is obtained by dividing the total binding energy of a nucleus by its Atomic mass number.

• Nuclear Forces:

1. These are the strong in attractive forces which hold protons and neutrons together in a tiny nucleus.
2. These are very short range forces which operate over very short distance of about 0.2 – 0.3 fm of separation between any two nucleons.
3. The nuclear forces is independent on the charge of polarity and have equal attraction of pairs between Proton-Proton, Proton-Neutron, Neutron-Neutron.

• Nuclear Density: The ratio of the Nuclear mass to the Volume of the Nucleus is called Nuclear Density and it is independent of the size of the nucleus and is given by-

• Nuclei class 12 notes

  Radioactivity:

1. It is the phenomenon of spontaneous disintegration of the heavy nucleus of an atom with the emission of one or more radiations like -particle, -particle or -rays.
2. Radioactive substances are defined as those that release piercing radiation on their own.

• Radioactivity Displacement Law: It states that-

1. When a radioactive nucleus emits an -particles, atomic number decreases by 2 and Atomic mass number decreases by 4.
2. When a radioactive nucleus emits -particles, its atomic number increases by 1 but Atomic mass number remains same.
3. The emission of a -particle does not change the mass number or the atomic number of the radioactive nucleus. The -particles emission by a radioactive nucleus lower its energy state. It is important for Nuclei class 12 notes.

• Alpha Decay: It is the process of emission of the -particles from a radioactive nucleus. It may be represented as,

• Beta Decay: It is the process of emission of the electrons from a radioactive nucleus. It may be represented as,

• Gamma Decay: It is the process of emission of a -ray photon during the radioactive disintegration of a nucleus. It can be represented as,