Most atoms seek to have 8 electrons on their valence shell. This is known as the octet rule, which states that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration with 8 electrons in their outermost shell (except for hydrogen and helium, which can achieve stability with 2 electrons).
Elements that already have a full valence shell, like the noble gases (Group 18 elements), are generally unreactive. Noble gases have stable configurations, and their valence shells are completely filled with 8 electrons (except for helium, which has 2 electrons). Since they already have a stable electron configuration, they do not readily gain, lose, or share electrons, making them chemically inert.
On the other hand, elements that do not have a full valence shell are typically reactive. These elements, known as the main group elements, have valence shells that are not completely filled. They tend to undergo chemical reactions in order to achieve a stable electron configuration with 8 electrons in their outermost shell. They can gain or lose electrons to either fill or empty their valence shells, or they can share electrons with other atoms to form covalent bonds. By doing so, these elements aim to achieve a more stable electron arrangement, following the octet rule.
In summary, elements that have a full valence shell are unreactive because they already possess stable electron configurations. Elements with incomplete valence shells are reactive as they strive to gain, lose, or share electrons to attain a full octet and achieve greater stability.