Answer
Missing atom: $^{4}_{2}He$
Complete reaction:
$^4_2He + ^{40}_{18}Ar -- \gt ^{43}_{19}K + ^1_1H$
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.
$? + 40 = 43 + 1$
$? + 40= 44$
$? = 40 - 44$
$? = 4$
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.
$? + 18 = 19 + 1$
$? + 18 = 20$
$? = 20 - 18$
$? = 2$
3. Identify the symbol of the element with atomic number equal to 8:
It is "He".
4. Now, write the complete symbol, and substitute it on the reaction.
$^{4}_{2}He$
$^4_2He + ^{40}_{18}Ar -- \gt ^{43}_{19}K + ^1_1H$
