The statement that there are no elements with more than two electrons in their valence shells, except for helium, is not entirely accurate. It is true that for many elements in the periodic table, the valence shell (the outermost electron shell) can accommodate a maximum of eight electrons, following the octet rule. This is known as the "rule of eight" or the "octet rule."
The octet rule is based on the observation that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration similar to the noble gases (Group 18 elements) which have eight electrons in their valence shells, except for helium which has two electrons in its valence shell. This stability is attributed to the filled electron shells of noble gases.
However, there are exceptions to the octet rule, particularly for elements that can accommodate more than eight electrons in their valence shells. These elements are found in the third period and beyond of the periodic table. They can extend their valence shells beyond the octet by utilizing empty d orbitals in addition to s and p orbitals. This phenomenon is referred to as "expanded octet" or "hypervalency."
Elements such as phosphorus (P), sulfur (S), and chlorine (Cl) can form compounds where they exceed the octet and have more than eight electrons in their valence shells. For instance, sulfur can have 12 electrons in its valence shell in compounds such as sulfur hexafluoride (SF6). Phosphorus can also exhibit expanded octets in compounds like phosphorus pentachloride (PCl5) with 10 electrons in its valence shell.
It's worth noting that not all elements can exceed the octet rule, and the octet rule remains a useful guideline for understanding the chemical behavior of many elements. However, exceptions to the octet rule do exist, particularly for elements beyond the second period of the periodic table.