Final answer:
The unknown element in the nuclear reaction is Barium (Ba).
Step-by-step explanation:
The nuclear reaction given is: 138/55CS → ? + ⁰−1e⁰ + ¹⁰ 5 Ba. In this reaction, a cesium-138 nucleus (138/55CS) decays into an unknown nucleus (represented by ?), an electron (⁰−1e⁰), and a barium-138 nucleus (¹⁰ 5 Ba).
To determine the element represented by the unknown nucleus, we need to look up the atomic number of ¹⁰ 5 Ba, which is 56. Therefore, the element represented by the unknown nucleus is element number 56, which is Barium (Ba).