Final answer:
Atoms become chemically stable by gaining, losing, or sharing electrons to achieve a full valence shell, adhering to the octet rule. This ensures energetic stability and the formation of molecules.
Step-by-step explanation:
Atoms achieve a full outer orbital shell and become chemically stable by gaining, losing, or sharing electrons to mimic the electron configuration of the nearest noble gas. This often involves either accepting electrons from another atom, donating electrons to another atom, or sharing electrons to form covalent bonds, as exemplified by the octet rule.
According to the octet rule, with the exception of the smallest shell which holds two electrons, atoms are energetically more stable when they have eight electrons in their valence, or outermost shell. Elements such as helium and neon naturally have a full outer shell, whereas other elements like chlorine and sodium do not. Therefore, chlorine and sodium achieve stability by either gaining or losing electrons to fill their valence shell completely or by sharing electrons with other atoms to create stable molecules, like the water molecule H₂O, which consists of two hydrogen atoms and one oxygen atom chemically bonded together.
The energetic stability of an atom and its tendency to form chemical bonds are determined by the configuration of electrons in the valence shell. The filling of electron orbitals follows a systematic order, filling from the lowest energy levels up to the higher ones, and achieving a full shell often results in the formation of stable molecules.