When aluminium is bombarded with fast neutrons, it changes into sodium with emission of particle x according to the equation:

${}_{13}{}^{27}\mathrm{Al} + {}_0{}^1\mathrm{n} \to {}_{11}{}^{24}\mathrm{Na} + \mathrm{x}$

What is x?

The mass number of x = 27 + 1 –  24 = 4 and its atomic number = 13 + 0 – 11 = 2. 
Hence particle x is helium nucleus having atomic number as 2 and mass number as 4, which is alpha particle.