The maximum number of valence electrons in an atom is not limited to 8 for all elements. The octet rule, which suggests that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration with eight valence electrons, applies primarily to elements in the second period (row) of the periodic table. This includes elements such as carbon, nitrogen, oxygen, and fluorine.
The octet rule is based on the observation that atoms in these elements' valence shells tend to have eight electrons, which corresponds to a stable electron configuration similar to the noble gases (such as helium, neon, and argon) that naturally have a full outer shell with eight electrons. This arrangement provides a high degree of stability and is energetically favorable.
However, it's important to note that the octet rule is a guideline rather than an absolute rule. It is applicable to elements in the second period because they have valence shells that can accommodate a maximum of eight electrons. Elements in higher periods, such as the third period and beyond, can have more than eight valence electrons due to the availability of additional orbitals.
Elements in the third period and beyond have access to the d orbitals, which can accommodate additional electrons. These elements can expand their valence shells beyond eight electrons. For example, elements in the third period, like phosphorus and sulfur, can have more than eight valence electrons.
In summary, the octet rule is a useful guideline for understanding the chemical behavior of many elements, especially those in the second period of the periodic table. While the number 8 is not universally applicable to all elements, it arises from the stability associated with having a full outer electron shell similar to the noble gases. Elements in higher periods can exceed the octet rule due to the presence of additional orbitals.