Question

A radioactive element A decays by the sequence and with half-lives given below :
$A\underset{30min}{\overset{\alpha }{\to }}B\underset{2days}{\overset{2\beta }{\to }}C$
Which of the following statements about this system are correct?

• A. The mass number of B is greater than A
• B. After two hours, less than 10% of the intial A is left
• C. Maximum amount of B present at any time is less than 50% of the initial amount of AQ
• D. The atomic numbers of A and C are the same

Answer 1

Answer: (B), (D)

After two hours, less than 10% of the intial A is left
The atomic numbers of A and C are the same

2 hours = 4 half lives, therefore, amount left = 6.25% of original. Loss of one $\alpha$ particle and two $\beta$ particle does not change atomic number.