Krypton (Kr) is an element that belongs to Group 18, also known as the noble gases or Group 8A. The noble gases are characterized by their stable electronic configurations and generally have low reactivity.
In the case of krypton, it has 8 valence electrons rather than 18 because the valence electrons are determined by the outermost electron shell, specifically the highest energy level or "noble gas configuration." Krypton's electron configuration is [Kr] 5s^2 4d^10 5p^6, which means it has a total of 36 electrons.
The noble gases have complete outer electron shells, which make them highly stable and chemically unreactive. Their outermost energy level is already filled with electrons, so they do not readily gain, lose, or share electrons to form chemical bonds. As a result, the valence electrons for noble gases are considered to be the electrons in the outermost "s" and "p" subshells, which is a maximum of 8 electrons.
Although krypton has a total of 36 electrons, only the 8 electrons in the 5s and 5p orbitals are considered as valence electrons because they are in the outermost shell. The 28 electrons in the inner shells are not involved in chemical bonding and do not contribute to krypton's valence.