Explain the following :
(a) Electrovalent compounds conduct electricity in molten or aqueous state.
(b) Electrovalent compounds have a high melting point and boiling point while covalent compounds have low melting and boiling points.
(c) Mass of an atom is concentrated inside the nucleus of an atom.
(d) Atoms combine by transfer and sharing of electrons (s).
(e) An element has atoms with different mass number.