(a) Deduce the expression, $N=N_0 e^{-\lambda t}$, for the law of radioactive decay. (b) (i) Write symbolically the process expressing the $\beta^+$ decay of ${}_{11}^{22}\mathrm{Na}$. Also write the basic nuclear process underlying this decay. (ii) If the nucleus formed in the decay of the nucleus ${}_{11}^{22}\mathrm{Na}$, and isotope or isobar?