# Nuclei Class 12 Notes Physics Chapter 13 - CBSE

## What are Nuclei ?

 Topic/Term Formula Symbol Representation Important Points Radius Nucleus R = R0A1/3Most of the space in atom is empty.N(θ) ∝(1/sin4(θ/2))where N(θ) = number of α particlesθ = scattered angle R0 = an empirical constant = 1.1 × 10–15 mA = mass number of atom Nuclear radius is measured in fermi.1 fermi = 10–15 m Nuclear Density p= (M/V) M = mass of nucleusV = volume of nucleus Density of nuclear matter is of the order of 1017 kg/m3Density of nuclear matter is independent of the mass number. Mass Defect ∆m = [Zmp + (A – Z)mn – M] Z = atomic numbermp = mass of protonA = atomic massmn = mass of neutronM = mass of nucleus Binding Energy ∆Eb = ∆mc2or∆Eb = ∆m × 931.5 MeV ∆m = mass defect Binding energy is the minimum energy required to break the nucleus into its constituent particles.

## Some Important Concepts

 Term Definition / Explanation Composition of Nucleus The atomic nucleus consists of two types of elementary particles i.e., protons and neutrons, protons and neutrons are collectively known as nucleons. Atomic number = Number of protons (Z) Mass number (A) = Number of protons (Z) + Number of neutrons (N) Nuclear mass is the total mass of the protons and neutrons measured in amu, 1 atomic mass unit (u) = 931.5 MeV/c2 Nuclear Fission Fusing two or more lighter nuclei to a single heavy nucleus is known as nuclear fusion. Fusion reactions take place in stars and Hydrogen bomb.