Final answer:
An atom is unreactive or stable when its valence shell is full, preventing electrons from interacting with other atoms. Atoms with incomplete valence shells are reactive and form chemical bonds to achieve stability. The stability of an atom is also influenced by the balanced forces within its nucleus.
Step-by-step explanation:
An atom's reactivity is primarily determined by the number of electrons in its valence shell, which is the outermost shell around the nucleus. Atoms with a full valence shell are generally unreactive or stable because their electrons are less likely to interact with other atoms.
On the other hand, atoms with unfilled valence shells tend to seek stability by reacting with other atoms, either by gaining, losing, or sharing electrons to achieve a full valence shell configuration similar to that of noble gases. This is why elements like hydrogen, with just one electron, are highly reactive.
Elements with low atomic numbers seek to fill their outermost shell by forming chemical bonds, whereas the nucleus also plays a role in an atom's overall stability. The number of protons and neutrons, and the balance of forces within the nucleus, such as the strong nuclear force and electrostatic repulsion, influences nuclear stability.