It's important to know that the reaction must have the total mass involved before and after due to the law of conservation of mass. This means that the total mass number of the left side of the reaction must be equal to the right side. So, we have the following
[tex]235+1=X+141+3[/tex]Notice that 235 is the mass number of U, 1 is the mass number of the neutron, X represents the mass number of the unknown element, 141 represents the mass number of Ba, and 3 represents the mass number of three neutrons. This means X should have a mass number of 92, which is the element Kr.
Therefore, the mass number is 92 and the chemical symbol is Kr.
