Concept:Alpha emission removes a helium nucleus, so both the nucleon number and atomic number decrease in a fixed way.
Explanation:The original nucleus has nucleon number
P and atomic number
Q.
An alpha particle is represented as
24He, containing 2 protons and 2 neutrons.
When this particle leaves the nucleus, the mass decreases by 4 units, so the nucleon number becomes
P−4.
The atomic number decreases by 2 because two protons are lost, giving
Q−2.
The new element formed therefore has these two resultant numbers.
Answer:The new nucleon number is
P−4 and the new atomic number is
Q−2, i.e. option B.