**CBSE NOTES CLASS 12 PHYSICS**

**CHAPTER 13 NUCLEUS**

**α-rays**

α-rays consists of α-particles, which are doubly ionised helium ion.

When an α-particle is emitted by a nucleus its atomic number decreases by 2 and mass number decreases by 4.

${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}\mathrm{}\to \mathrm{}{}_{\mathrm{Z}-2}{}^{\mathrm{A}-4}\mathrm{Y}\mathrm{}+{}_{2}{}^{4}\mathrm{H}\mathrm{e}$

Example

${}_{92}{}^{238}\mathrm{U}\mathrm{}\to \mathrm{}{}_{90}{}^{234}\mathrm{T}\mathrm{h}\mathrm{}+{}_{2}{}^{4}\mathrm{H}\mathrm{e}$

Total mass of RHS is less than that of LHS.

The disintegration energy or the **Q-value of a nuclear reaction** is the difference between the initial mass energy and the total mass energy of the decay products. For α-decay

$$\mathrm{Q}\mathrm{}=\mathrm{}[{\mathrm{m}}_{\mathrm{X}}\mathrm{}\u2013\mathrm{}({\mathrm{m}}_{\mathrm{Y}}\mathrm{}+\mathrm{}{\mathrm{m}}_{\mathrm{H}\mathrm{e}}\left)\right]\mathrm{}{\mathrm{c}}^{2}$$

Q is also the net kinetic energy gained in the process or, if the initial nucleus X is at rest, the kinetic energy of the products, Q > 0 for exothermic processes such as α-decay.