Format numeric characters as subscript

I'm trying to help out my daughter (who is a Chemical Engineering student) with her various Excel files. She has lists of chemical names in their molecular formulas and wants to subscript the numeric characters that follow non-numeric characters. I figured out how to do it with a single cell but for some reason I can't get it to work with a range of cells.

Examples
H2O The 2 is subscripted
H2SO4 The 2 and the 4 are subscripted
2CCl4 The 2 is not subscripted but the 4 is


The following code will work only on the first cell of the selection and then stops.

Could someone please point out where I'm going wrong?

Sub MyRangeChemFormat()
Dim c As Integer
Dim s As String
Dim rng As Range
For Each rng In Selection
s = rng.Value
    For c = 2 To Len(s)
    If IsNumeric(Mid(s, c, 1)) And IsNumeric(Mid(s, c - 1, 1)) = False Then
        ActiveCell.Characters(c, 1).Font.Subscript = True
    Else
    End If
    Next c
Next rng
End Sub

Thanks for any help you can give