Question

${}_{\to }{}_2^{4}He+{}_{82}^{208}B$

• A. ${}_{84}^{216}A$
• B. ${}_{80}^{204}A$
• C. ${}_{84}^{212}A$

Answer 1

The correct option is C ${}_{84}^{212}A$

In alpha decay, the atomic number is decreased by 2 and the mass number is decreased by 4. Thus, the resulting nucleus has 2 protons and 2 neutrons less than parent (A) nucleus.

Therefore, parent nucleus A has composed of 84 protons and 128 neutrons. The parent nucleus A emits an alpha particle and is transformed into daughter nucleus B. This can be written as:
${}_{84}^{212}A\to {}_2^{4}He+{}_{82}^{208}B$