For an atom to be stable, it should have either a completely filled valence shell or eight electrons in its valence shell. This concept is known as the octet rule, which applies to many elements.
The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration resembling the noble gases. Noble gases, such as helium, neon, and argon, have completely filled valence shells, which makes them stable and unreactive.
Most elements strive to achieve a stable electron configuration by either gaining or losing electrons to reach a filled valence shell with eight electrons (except for hydrogen and helium, which only require two electrons for stability). This process can occur through the formation of ionic bonds or covalent bonds with other atoms.
It's important to note that the octet rule is a general guideline and doesn't apply to all elements. Some elements, such as hydrogen, lithium, and beryllium, can achieve stability with fewer than eight electrons due to their smaller atomic sizes. Additionally, elements in the third period and beyond can have expanded octets, allowing them to accommodate more than eight electrons in their valence shells.