What ion does Sulfur (S) form in an ionic compound? (Indicate the charge as + or – and specify the number.)
Explain why, using electronegativity and the octet rule in your answer.
The Correct Answer and Explanation is:
Sulfur (S) typically forms an anion with a charge of -2 when it participates in ionic compounds. This means sulfur gains two electrons, becoming ( \text{S}^{2-} ).
Explanation
Sulfur, with an atomic number of 16, has an electron configuration of ( 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^4 ). Its outer shell, the third shell, contains six electrons (two in the ( 3s ) subshell and four in the ( 3p ) subshell). According to the octet rule, atoms are more stable when they have a full valence shell of eight electrons. Sulfur, with six valence electrons, needs two additional electrons to achieve this stable configuration, akin to the noble gas argon. By gaining these two electrons, sulfur fills its 3p orbital, resulting in the ( 3s^2 \, 3p^6 ) configuration.
The tendency of sulfur to gain electrons rather than lose them is largely due to its position on the periodic table and its electronegativity. Sulfur is in Group 16, where elements commonly gain electrons to fulfill the octet rule, rather than lose them, because losing six electrons would require a significant amount of energy. Electronegativity, which is the tendency of an atom to attract electrons, is also a factor: sulfur has a relatively high electronegativity (though lower than oxygen) and is inclined to attract electrons to itself rather than give them up.
When sulfur gains two electrons to achieve a full octet, it forms the ( \text{S}^{2-} ) ion. This charge results because the sulfur atom now has 18 electrons while still having 16 protons. The resulting ( \text{S}^{2-} ) ion is stable in ionic compounds, where it typically pairs with metal cations (like ( \text{Na}^+ ) or ( \text{Ca}^{2+} )) to form salts, such as sodium sulfide (( \text{Na}_2\text{S} )) or calcium sulfide (( \text{CaS} )).
In summary, sulfur forms an ( \text{S}^{2-} ) ion in ionic compounds because it needs two additional electrons to satisfy the octet rule, and its relatively high electronegativity favors electron gain over loss. This stability arises from achieving a complete set of eight valence electrons, making the ( \text{S}^{2-} ) ion energetically favorable.