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The number of isotopes doesn't directly determine the group of an element. To find the group, you need to know the element's electron configuration. However, since it's a transition metal, it will be located in groups 3-12 of the periodic table.
To determine the chemical formula of the sulfide, we need the most stable oxidation state of the transition metal (X). Without knowing the element's identity, we cannot determine its most stable oxidation state and thus cannot write the chemical formula of its sulfide.