SO₄²⁻ · sulfate ion
The strict-octet structure uses four S–O single bonds with charges on every atom; the expanded-octet forms (two S=O bonds, allowed because sulfur is period 3) lower formal charges and generate six equivalent resonance structures.
The small red + and blue − numbers beside atoms are , not the ion's overall charge. The small numbered circles just tell identical atoms apart, so “O #2” in the table is the same oxygen as “2” in the drawing.
A must account for exactly the , so total them first: sulfur is a Group 16 element, so it has 16 - 10 = 6 valence electrons; each oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, and with 4 of them that is 6 × 4 = 24; the overall -2 charge means 2 additional electrons were gained, so add 2, giving 32 valence electrons that the finished structure has to use in full, with none added and none left over.
Sulfur is the because it is the least electronegative of the present. The least electronegative atom holds its electrons most loosely and therefore shares them with several neighbors most readily, which is exactly what a central atom must do. (Hydrogen and fluorine are never central.)
Connect the to every with a . Because each single bond represents one shared electron pair (2 electrons), the 4 bonds use 8 electrons, leaving 24 of the original 32 to place.
The terminal O are filled to an next: 12 among them, using 24 electrons and reducing the number still available to 0.
With the terminal octets complete, no electrons remain, so S receives no at this stage.
S already satisfies the with single bonds, but doing so leaves it with a of +2. Because S is a period-3 (or lower) element, it can hold more than eight electrons, so 2 from the terminal are shared as 2 additional bonds. This lowers the formal charges toward zero, and the structure with the smallest formal charges is the lower-energy, preferred one.
Verify each in turn, counting shared and together. S: 12 shared + 0 nonbonding = 12 (12 electrons); each O: 4 shared + 4 nonbonding = 8 (a full ); each O: 2 shared + 6 nonbonding = 8 (a full octet).
Assign , where each 's formal charge is its minus its minus half its (bonding electrons are halved because a shared pair is split evenly between the two atoms): O = -1, O = -1. These sum to -2, which equals the overall charge of the species, and the negative formal charge is placed on the most electronegative atom, which is where it is most stable, so this arrangement is the preferred structure.
The multiple bond shown here could be drawn in 5 other, equally valid positions, giving 6 in all. The true species is not any one of these drawings but a single resonance hybrid, the weighted average of them, in which every equivalent bond has the same order of 1.50. The electrons are delocalized over all positions at once; they do not shift back and forth between structures.
Note that S is surrounded by 12 electrons, more than an . This is a legitimate : a period-3 (or lower) is physically large enough to hold more than eight electrons in its shell, and doing so lowers the . A small second-period atom such as nitrogen or oxygen cannot expand its octet and is strictly limited to eight, a distinction students often overlook. (The older explanation invoking d is now regarded as incorrect; the effect is due to atomic size and charge distribution.)
Watch out
The classic slip with SO4^2-: forgetting to add 2 electrons for the -2 charge. Get the count wrong by even one and every later step quietly breaks.
Apply by counting the around S: 4 bonding domains plus 0 gives 4 electron domains. Domains positioned to minimize their mutual repulsion adopt , which corresponds to sp³ .
No are hiding in that arrangement, so the you see is the shape you get: tetrahedral, with 109.5° .
: each S-O bond is genuinely polar on its own, but the arrangement points those pulls symmetrically against each other. They cancel to nothing. Nonpolar overall.
Worth knowing
Sulfate is everywhere you look: gypsum in your walls is calcium sulfate, epsom bath is magnesium sulfate. All four S-O bonds are identical in the real , which is doing its averaging trick.