Question

A radioactive nucleus undergoes a series of decays according to the sequence
$A\stackrel{\beta }{\to }{A}_1\stackrel{\alpha }{\to }{A}_2\stackrel{\alpha }{\to }{A}_3$
If the mass number and atomic number of $A_3$ are $172$ and $69$respectively, then the mass number and atomic number of A is

• A. $56,23$
• B. $180,72$
• C. $120,52$
• D. $84,38$

Answer 1

The correct option is B $180,72$

${}_Z^{A}A\to A_1\to A_2{\to }_{6}9A_3^{172}$

Mass balance, $A=0+4+4+172$
$=180$
Charge balance, $Z=-1+2+2+69$
$=72$