An element 'X' has three completely filled shells and forms a stable uninegative ion and the total number of electrons in the third and fourth shells is same. Identify the elements that form unipositive and dipositive ions that are isoelectronic with X^(-) ion. Compare the reactivities of these two elements with water. Give a reason in support of your answer.

An element 'X' has three completely filled shells and forms a stable

1.	An element 'X' has three completely filled shells and forms a stable uninegative ion and the total number of electrons in the third and fourth shells is same. Identify the elements that form unipositive and dipositive ions that are isoelectronic with X^(-) ion. Compare the reactivities of these two elements with water. Give a reason in support of your answer.
Answer» Solution :The element 'X' must be a halogen since ot forms the stable `X^(-)` ion. As there are three completely filled shells in 'X', the ELECTRONIC configuration of 'X' could be 2, 8, 18, 18, 7 and `X^(-)` is 2, 8, 18, 18, 8. Since there are 54 `e^(-)s"in" X^(-)`, the CORRESPONDING elements that form unipositive and dipositive ions should be cesium (55) and barium (56).Cesium REACTS more vigorously with water than barium. This is because reactivity of alkali metals is more due to their greater atomic size when compared with alkaline earth metals. Moreover, alkaline earth metals have greater IP values than alkali metals. Hence, they react slowly with water.