## Chemistry: The Molecular Science (5th Edition)

$^{10}_{5}B$
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 $\,^{10}_{5}B$.