What occurs when atoms and molecules lose or gain electrons?

When atoms and molecules lose or gain electrons, they become ions. This process involves the alteration of an atom's or molecule's charge—losing electrons leads to a positive charge (cation), whereas gaining electrons results in a negative charge (anion). This transformation is critical in many chemical reactions and is fundamental to understanding chemical bonding, particularly in ionic compounds.

The formation of ions is essential because it allows for various types of chemical interactions, such as the attraction between oppositely charged ions, which is the basis for ionic bonding. This concept is foundational in chemistry, indicating how substances interact on a molecular level.

The other options do not accurately describe what happens when atoms or molecules gain or lose electrons. Isotopes involve variations in neutron numbers rather than electron transfers. Water molecules and compounds result from the chemical combination of different elements rather than the direct gain or loss of electrons by individual atoms or molecules.