An atomic nucleus A is composed of 84 protons and 128 neutrons. The nucleus A emits and alpha particle and is transformed into a nucleus B.
(i) What is the composition of B ?
(ii) The nucleus B emits a beta particle and is transformed into a nucleus C. What is the composition of C?
(iii) What is mass number of the nucleus A ?
(iv) Does the composition of C change if it emits gamma radiations ?

Explanation

Atomic number of A = Number of protons
= 84
Number of neutrons in A = Mass number of A - Atomic number of A
Thus, Mass number of A = Number of neutrons in A + Atomic number of A
= 128 + 84
= 212
When an alpha particle is emitted,
Atomic number of B = Atomic number of A - 2
= 84 - 2 = 82
Thus, Number of protons in B = Number of electrons in B
= Atomic number
= 82
Mass number of B = 212 - 4
= 208
Number of neutrons in B = Mass number of B - Atomic number of B
= 208 - 82
= 126
`""_(84)^(121)Ato""_(2)^(4)He+""_(82)^(208)B`