Answer
(a)molecule is group of atoms, ion has negative or positive charge
(b) covalent bonding: electrons are shared ionic bonding: gain or loss of elcetrons
(c) Molecule consists of same elements and compound consists of different elements
(d) Cations have positive charge and anions have negative charge
Work Step by Step
(a) A molecule is made up of group of atoms that are held together by a chemical bond. A molecule is neutral i.e it has neither negative nor positive charge.
On the other hand, when an atom loses or gains electrons then it is said to be an ion.
(b) The type of bonding in which electrons are shared between two elements is known as covalent bonding.
Ionic bonding occurs when one element loses electrons and the other element gains electrons.
(c) A molecule is made up of two or more elements while a compound is made up of two or more different elements. For example, $O_2$ is a molecule but it is not a compound as it does not have different elements.
(d) An ion having a positive charge is called cation while an ion having a negative charge is called anion.
