Final answer:
Sulfur and oxygen are in Group 6 because they have six valence electrons, which allows them to form 2- ions and achieve an octet by accepting or sharing two additional electrons. Sulfur can also expand its octet in some compounds.
Step-by-step explanation:
Sulfur and oxygen are both in Group 6 of the periodic table because they have six valence electrons in their outermost shell. This electronic configuration allows them to easily accept two electrons to form 2- ions, such as the sulfide ion (S²-) for sulfur and oxide ion for oxygen.
For example, in the formation of sulfur dioxide (SO₂), the sulfur atom is located lower in the periodic table (Row 3 vs. Row 2 for oxygen), and thus comes first in the compound's name. Additionally, the prefix 'di' indicates that there are two oxygen atoms present. The electron configuration of sulfur (16 electrons) shows that it has six electrons in its outer shell, analogous to oxygen and its fellow Group 6 elements, except with a principal quantum number of n = 3.
Both sulfur and oxygen achieve an octet by sharing electrons. Sulfur can expand its octet in cases like sulfur dioxide, where according to LibreTexts, it can have a formal charge of zero, signifying that it still obeys the octet rule even when it shares more than eight electrons.