Final answer:
The noble gases, excluding helium, have eight valence electrons, following the octet rule. This full valence shell renders them inert. Exceptions to the octet rule occur, such as with elements that can have fewer or more than eight valence electrons.
Step-by-step explanation:
Noble Gases Valence Electrons
The noble gases, with the exception of helium, contain eight valence electrons. This full valence shell configuration contributes to their lack of reactivity, making them the least reactive or most inert group of elements on the periodic table. Helium, however, only has two valence electrons, following the duet rule due to it only having the 1s orbital available for electron occupancy.
The eight valence electrons make noble gases exceptionally stable. This concept of having filled valence shells is also known as the octet rule, which states that atoms gain, lose, or share electrons to achieve a noble gas configuration of eight valence electrons, resulting in greater stability.
Groups within the periodic table contain elements with the same number of valence electrons. As an example, the noble gases are found in Group 18, and all members (apart from helium) possess eight valence electrons. This periodic trend explains the predictable reactivity of elements within the periodic table.
Exceptions to the Octet Rule
While the octet rule is a guiding principle in chemistry, there are exceptions. For example, certain elements such as boron and aluminum form compounds with less than eight valence electrons. Furthermore, heavier elements especially those located in period three and beyond, such as phosphorus and sulfur, can form compounds with more than eight valence electrons owing to the presence of d or f orbitals in addition to their valence s and p orbitals.