Answer
$^{16}_{8}O$
Work Step by Step
Let the missing reactant be $\,^{A}_{Z}X$ where $X$ is the symbol, $Z$ is the atomic number and $A$ is the mass number.
As the sum of masses on both sides must be equal:
$238+A=249+(5\times1)\implies A=249+5-238=16$
As the sum of charges must be equal:
$92+Z=100+(5\times0)$ or $Z=100-92=8$
The symbol of the element with atomic number 8 is $O$.
Therefore, the missing reactant is $\,^{16}_{8}O$.
