Which one of the following statements concerning elements in the Periodic Table is correct? A Elements of the same group all have the same number of electrons in the outermost occupied electron shell. B Elements of Group 16 occur as cations in ionic compounds. C Oxides of elements in Groups 16 and 17 are basic. D The halogens (Group 17) are all gases at room temperature. E The Group 13 elements are all metals.