A chemist is testing various compounds to see which ones make an effective catalyst. She finds that the compound containing magnesium sulfate is highly effective. Based on that information, what other compounds should they test next due to an expectation of similar properties?
The periodic table can be used to answer this question. The periods, or rows, of the periodic contain elements that have similar energy levels and, therefore, the same number of atomic orbitals. The periods / families (columns) of the periodic table contain elements that have the same number of valence electrons (the number of electrons in the very outermost orbital of the atom). They are the electrons that are involved in chemical bonds with other atoms and determine the reactivity of the element. Thus, elements that are a part of the same periodic family or group can share similar characteristics or chemical properties.
Therefore, in order to answer the question, we want to find the option of the cation (first element in the chemical formula) that is in the same family as magnesium (from the magnesium sulfate in the question). The only option would be (d), or calcium sulfate. Since calcium has the same number of valence electrons as magnesium, we can anticipate similar bonding characteristics and, hopefully, similar efficacy as a catalyst.