Question

${}_{90}T{h}^{228}$ emits four alpha and one beta particles. Number of neutrons in daughter element is :

• A. 129
• B. 190
• C. 232
• D. 138

Answer 1

The correct option is A 129

When an alpha particle is lost, atomic number decreases by 2 and mass number decreases by 4. When a beta particle is lost, atomic number increases by 1 and the mass number remains unchanged.
${}_{90}T{h}^{228}$ emits four alpha and one beta particles. Number of neutrons in daughter element is 129.
${}_{90}T{h}^{228}\underset{-4_2^{4}He}{\overset{{-}_{-1}^{0}e}{\to }}{=}_{83}^{212}Bi$
Number of neutrons $=212-83=129$