Which of the following statements are correct ? (a) Atoms of the same atomic number but different nucleon number are the isotopes of the element. (b) Compounds that do not contain a metallic element typically may form covalent compounds consisting of discrete molecules. (c) The oxidation number of magnesium is −2 in Mg2+ ion (Cation). (d) Covalent bonds must be polar. Choose the most appropriate answer from the options given below :