7) E

A – Wrong
The definition of nuclear fission is splitting of a nucleus into two smaller nuclei. The definition given in this statement is wrong.

B – Wrong
The definition of half life is wrong. Half life is the time taken for the half of the substance to decay.

C – Wrong
Number of neutrons = Mass number – Atomic number. (Not Atomic number – Mass number)

D – Wrong
Fission is used in nuclear power stations.

E – Correct
When a nucleus emits a ß-particle, the number of particles in the nucleus itself remains the same (since mass number does no change).

F – Wrong
When a nucleus emits an α-particle, it loses two neutrons and two protons.