An element's most stable ion forms an ionic compound with bromine, having the formula XBr. If the ion of element X has a mass number of 85 and has 36 electrons, what is the identity of the element?

The element is Rubidium. The ion only loss one electron to make the electron 36. But the proton number (atomic number) is 37. The chemical compound should RbBr

X = Rubidium (Rb)

Explanation:

The ion of element X has a mass number of 85 and electron of 36. An elements is charge if it either loss or receive electron. Base on the charge the ion has loss electron. Bromine usually possess a negatively charge ion. That makes the X electron a positively charge ion.

proton number = Atomic number

Proton number = electron number when not charged

Mass number = proton number + neutron number

Proton is usually same number with the electron to make the element neutral. But when the atom is charge the atom usually loss or gain electron.

In this case, the atom X loss one electron thereby reducing the electron number to 36. Recall, the electron is equal to proton only if the atom is uncharged. This means the proton number should be 37 as one electron was loss.

Element with proton number (atomic number) of 37 is rubidium. It is an alkali metal that is very reactive.