Final answer:
Atoms gain or lose electrons to emulate the electron configuration of noble gases because such a filled valence shell state is the most stable. Alkali metals and halogens are most reactive towards achieving this noble gas configuration through ion formation.
Step-by-step explanation:
Through the loss and gain of electrons, atoms strive to achieve the same electron configuration as noble gases. Atoms form ions by losing or gaining electrons because this gives them the stability of a full valence shell, similar to the noble gases. For instance, alkali metals like lithium, in group 1, lose their single valence electron easily, transforming into positively charged ions. On the other hand, halogens in group 17, with seven electrons in their valence shell, tend to gain one electron to complete their octet. Transition metals have variable oxidation states and can lose electrons, but they don't follow this trend as consistently as groups 1 and 17.
The octet rule is a guiding principle that helps predict ion formation. Metals, especially from groups 1, 2, and 13, lose electrons to achieve the electron configuration of the noble gas before them, whereas nonmetals in groups 15 to 17 gain electrons to complete their valence shell, mimicking the noble gas configuration following them on the periodic table. This desire for stability is the driving force behind atoms losing or gaining electrons.