Sulfur

[AhYeon_Kwon_2H]

Why does sulfur in SO₂ have a normal octet, but sulfate (SO₄^2-) have an expanded octet? Shouldn't it be consistent?

[timschaeffer Dis 1J]

it doesn't have to be consistent. When doing lewis structures for molecules like these, all that matters is that each molecule has at least an octet and the formal charges are relatively low.

For S02, all that matters is that both oxygens and the sulfur have full octets (it doesn't matter that sulfur has more valence electrons in other molecules)

[Tiffany_Cacy_3D]

If you draw the lewis structure for (SO4)2- with only single bonds it would not have an expanded octet but in order to draw the lewis structure with the least amount of formal charges you have to draw the lewis structure with two single bonds and two double bonds so that only two Oxygens have negative charges and this would give the central atom a formal charge of 0 as well. Having two double bonds and two single bonds would give the sulfur and extended octet but this also gives it the most stable form.

[AhYeon_Kwon_2H]

Cool, so when in doubt, just check the formal charge. Got it, thank you so much!