Answer
Missing atom: $^{239}_{92}U$
Complete reaction:
$^1_0n + ^{238}_{92}U -- \gt ^{239}_{92}U $
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.
$1 + 238 = ?$
$? = 239$
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.
$0 + 92 = ?$
$? = 92$
3. Identify the symbol of the element with atomic number equal to 92:
It is "U".
4. Now, write the complete symbol, and substitute it on the reaction.
$^{239}_{92}U$
$^1_0n + ^{238}_{92}U -- \gt ^{239}_{92}U $
