Final answer:
Atoms are most stable when they have a full set of electrons in their outermost shell, satisfying the 'octet rule'. This often requires forming chemical bonds with other atoms to share, gain, or donate electrons to achieve a complete valence shell, leading to greater atomic stability.
Step-by-step explanation:
Atoms are much 'happier' (resist breaking apart and/or making new molecules) when their electron shells are complete, meaning they have a full set of electrons. This state of maximum stability is typically achieved when an atom has eight electrons in its valence shell, as per the octet rule. Atoms with incomplete outer shells tend to form chemical bonds with other atoms to reach this stable configuration by either sharing, donating, or accepting electrons.
For instance, oxygen, which has six electrons in its outer shell, will tend to form bonds to acquire two additional electrons to complete its octet. This is evident when two hydrogen atoms share their electrons with oxygen, resulting in the formation of a water molecule (H₂O). Other examples include helium, which is happy with two electrons in its only shell, and neon, which has a full valence shell of eight electrons and is thus inert.