Masses of two isobars ${}_{29}{}^{64}\mathrm{Cu}$ and ${}_{30}{}^{64}\mathrm{Zn}$ are 63.9298 u and 63.9292 u respectively. It can be concluded from these data that:

The expected atomic mass of stable Cu should be smaller than that of Zn. Since the given mass of Cu is greater than that of Zn, ⁶⁴Cu will be unstable. Cu will decay to Zn through $\beta -decay.$

In $\beta$-decay, the atomic number is increased by one while the mass number remains unchanged. Thus:

${}_{29}{}^{64}\mathrm{Cu}\stackrel{\mathrm{\beta }-\mathrm{decay}}{\to }{}_{30}{}^{64}\mathrm{Zn}+{}_{-1}{}^0\mathrm{e}$