Given : Composition of atomic nucleus $A=84$ protons and 128 neutrons. $\; \text{ Mass no. of }\;$ $A=84+128$$\;=212$ Atomic no. of $A=84$The nucleus A emits an alpha particle, the mass number decreases by 4 and the atomic no. decreases by 2 . ${}_{84}^{212}A \overset{-\alpha}{\longrightarrow} {}_{82}^{208}B$No. of protons in $B=82$ . $\;\; \text{ No. of neutrons in }\; B=208-82$ $\;=126.$