Question

For VSEPR theory, why do ions follow the exact same process? Why don't they gain/lose electrons?

Answer 1

They do follow the same process. We apply VSEPR theory to the ions after they have gained or lost electrons.

Example:

Determine the VSEPR shape for SO₃²⁻.

We first draw the Lewis structures.

Step 1: Draw a skeleton structure with only a single bond between the two atoms. This gives S three bonds to O atoms.

Step 2: Draw a trial structure, in which you put enough electron pairs around every atom to give it an octet of valence electrons.

Step 3: Count all the valence electrons in the trial structure.

We have 20 lone pair electrons plus 6 electrons in the S-O single bonds. This makes 26 valence electrons.

Step 4: Count the valence electrons we can have in our structure.

S = 6; O = 6. SO₃²⁻ has 6 + 3×6 + 2 = 26 valence electrons. Our trial structure is correct.

Step 4: Apply VSEPR theory to determine the shape.

SO₃²⁻ has an AX₃E structure and a trigonal pyramidal geometry.

In every case, the steps are the same for the ion as for the molecule.