Answer
The missing atom is: $^{27}_{13}Al$
And the complete reaction is:
$^1_0n + ^{27}_{13}Al -- \gt ^{24}_{11}Na + ^4_2He$
Work Step by Step
1. Find the mass number of the missing element.
The total mass number of the left side must be equal to that of the right side.
$1 + ? = 24 + 4$
Solve for: "?"
$? = 28 - 1 = 27$
2. Find the atomic number of the missing element.
The total atomic number of the left side must be equal to that of the right side.
$0 + ? = 11 + 2$
$? = 13$
3. Identify the symbol of the element with atomic number equal to 13:
It is "Al" (Aluminium).
4. Now, write the complete symbol, and substitute it on the reaction.
$^{27}_{13}Al$
$^1_0n + ^{27}_{13}Al -- \gt ^{24}_{11}Na + ^4_2He$
