Given : Composition of atomic nucleus
$\mathrm{A}=84$ protons and 128 neutrons.
$\mathit{A}=84+128$
$=212$
Atomic no. of $\mathrm{A}=84$
The nucleus A emits an alpha particle, the mass number decreases by 4 and the atomic no. decreases by 2 .
${}_{84}^{212}\mathrm{A}\stackrel{-\mathit{\alpha }}{\mathit{─}\mathit{─}\mathit{─}▸}{}_{82}^{208}\mathrm{B}$
No. of protons in $\mathit{B}=82$ .

$=126.$