When one $\alpha$ -particle is absorbed and onc neutron is emitted then the balance equation shows that resulting nucleus will be carbon. The balance equation is
$_{4} Be^{9}+ \underset{\text{($\alpha$ -particle)}}{{_2He^4}}\to \underset{\text{(neutron)}}{{_{0}n^{1}}}+_6C^{12}$
$(\alpha$ -particle) (ncutron) From this equation, we find that atomic wcight is 12 and atomic mass number is 6 . Since both these values are meant for carbon (C), thercfore resulting nucleus will be, ${ }_{6} C ^{12}$.