uranium 234 alpha emission Solution Ans. During alpha ( 4 a 2 ) decay/emission- I. Atomic number is decreased by 2 units. II. Atomic mass (= n + p) is decreased by 4 unit. Balanced Equation:  1 234 U 92 ---- a-decay ----> 1 230 Th 90 + 1 4 a 2 # The coefficient value of 1 is generally NOT shown in the balanced equation. If there is no coefficient written before a chemical species, it’s understood to be 1. SO, the balanced equation can also be written as- 234 U 92 ---- a-decay ----> 230 Th 90 + 4 a 2 # The element with atomic number 90 is thorium. .