Question 5.3: Draw a Lewis structure for SO4^2-, a polyatomic ion in which......
Draw a Lewis structure for SO_{4} ^{2-}, a polyatomic ion in which S is the central atom and all O atoms are bonded to the S atom (see Figure 5.7).
Step 1: Both S and O are Group VIA elements. Thus each of the atoms has 6 valence electrons. Two extra electrons are also present, which accounts for the -2 charge on the ion. The total electron count is 6 + 4(6) + 2 = 32.
Step 2: Drawing the molecular skeleton with single covalent bonds between bonded atoms gives
\quad\quad\quad\quad \left[\begin{matrix} O\\ O:\overset{.\ .}{\underset{.\ .}{S}}:O\\O \end{matrix}\right]^{2-}
Step 3: Adding nonbonding electron pairs to give each oxygen atom an octet of electrons yields
\quad\quad\quad\quad \left[\begin{matrix}:\overset{.\ .}{O}:\\:\overset{.\ .}{\underset{.\ .}{O}}:\overset{.\ .}{\underset{.\ .}{S}}:\overset{.\ .}{\underset{.\ .}{O}}:\\:\underset{.\ .}{O}:\end{matrix}\right]^{2-}
Step 4: The Step 3 structure has 32 electrons, the total number available. No more electrons can be added to the structure, and indeed, none need to be added because the central S atom has an octet of electrons. There is no need to proceed to Step 5.