Answer
$^{10}_{5}B$
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:
$252+A=257+(5\times1)\implies A=257+5-252=10$
As the sum of charges must be equal:
$98+Z=103+(5\times0)$ or $Z=103-98=5$
The symbol of the element with atomic number 5 is $B$.
Therefore, the missing reactant is $\,^{10}_{5}B$.
