Chemistry: The Molecular Science (5th Edition)

$^{45}Ti\rightarrow \,^{45}Sc+\text{positron}$
a Positron is a positively charged electron. Positron emission leads to a unit decrease in the atomic number. Therefore, the missing product has an atomic number of one less than that of $Ti$. That is, Z= (Z of $Ti$)-1=22-1=21. The element with atomic number 21 is scandium ($Sc$). There is no change in mass number as the mass number of a positron is 0. Therefore, the missing product is $\,^{45}Sc$.