When an alpha particle is emitted from nucleus the nucleus loses two proton and two neutrons. This means the atomic mass number decreases by 4 and the atomic number decreases by 2.

In beta decays, the mass number of the atoms remains the same, but the atomic number changes: the atomic number increases in the negative beta decay and decreases in the positive beta decay by $1$, respectively.

In gamma decay, mass number and atomic number remains unchanged.

Step by step variation is given below:

${}_{84}{}^{218} \mathrm{A}\stackrel{\alpha }{⟶}{}_{82}{}^{214} {\mathrm{A}}_1\stackrel{{\beta }^{-}}{⟶}{}_{83}{}^{214} {\mathrm{A}}_2\stackrel{\gamma }{⟶}{}_{83}{}^{214} {\mathrm{A}}_3\stackrel{\mathrm{\alpha }}{⟶}{}_{81}{}^{210} {\mathrm{A}}_4\stackrel{{\mathrm{\beta }}^{+}}{⟶}{}_{80}{}^{210} {\mathrm{A}}_5\stackrel{\mathrm{\gamma }}{⟶}{}_{80}{}^{210} {\mathrm{A}}_6$