Answer
(a) "X" represents Strontium (Sr)
(b) "X" represents Bromine (Br)
Work Step by Step
The charge of an ion can be calculated by:
$(charge) = Z - n^{\circ}e^-$
Where: $Z$ is the atomic number (Because it represents the number of protons, and each one has a positive charge).
$ n^{\circ}e^-$ is the number of electrons, each one gives a negative charge to the ion/atom.
Adding ($n^{\circ}e^-$) to both sides of the equation:
$(charge) + n^{\circ}e^- = Z$
(a)
$(2+) + 36 = 38$
Therefore, "X" has an atomic number of 38, which looking in the periodic table gives us the Strontium element.
(b)
$(-1) + 36 = 35$
Therefore, "X" has an atomic number of 35, which looking in the periodic table gives us the Bromine element.
