Question

ranium-235 undergoes nuclear fission as shown in the diagram below.

A small gray ball labeled n impacts a large mass of red and gray balls labeled superscript 235 subscript 92 upper U. The impact gives off a smaller mass of balls labeled superscript 139 subscript 56 upper B a, a starburst labeled 200 megavolts, which in turn gives off 2 balls labeled n; and another smaller mass of balls labeled X.

Which element does X represent?
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Answer 1

Use conservation of mass number and atomic number.

Mass: 235 + 1 = 236 = 139 + A_X + 2 ⇒ A_X = 236 − 141 = 95.
Charge: 92 = 56 + Z_X ⇒ Z_X = 36.

Element with Z = 36 is krypton. So X is 95_36Kr (krypton-95).