One example of an element that has more than its expected number of outer shell (valence) electrons is sulfur. Sulfur normally has six valence electrons, but it can expand its octet and accommodate more than eight electrons in its outer shell.
This phenomenon is known as the "expanded octet" rule and occurs when elements from the third period onwards (n = 3 or higher) can utilize their empty d orbitals to accommodate additional electrons. By incorporating these d orbitals, elements like sulfur can exceed the usual octet rule of having eight electrons in the outer shell.
In the case of sulfur, it has six valence electrons in its 3s and 3p orbitals. However, it can accommodate additional electrons in its 3d orbitals, allowing it to expand its valence shell. This ability to expand its octet arises due to the availability of empty 3d orbitals that can accommodate extra electrons.
It's important to note that not all elements can expand their octets. This phenomenon is primarily observed in elements from the third period onwards, and it is particularly prevalent in elements from the third to fifth periods of the periodic table.