First slide

Radio activity

Question

A radioactive nucleus undergoes a series of decays according to the scheme
$A \overset{α}{\to} A_{1} \overset{β}{\to} A_{2} \overset{α}{\to} A_{3} \overset{γ}{\to} A_{4}$
If the mass number and atomic number of A are 180 and 72 respectively, then what are these number for $A_{4} ?$

Easy

A

172 and 69
B

174 and 70
C

176 and 69
D

176 and 70.

Solution

Two $α$ -particles reduce mass number by 8.
$∴$ New mass number = 180 — 8 = 172
Emission of two $α$ -particles reduces charge number by 4. Emission of $β$ -particles increases charge number by 1.
$∴$ New charge number = 72 — 4 + 1 = 69.

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