The extra two electrons go on the sulfur atom. It will have 10 electrons around it, an "expanded octet", rather than the usual octet. This is possible for any element in row 3 or below in the periodic table, because these elements have as of yet empty d-orbitals in their valence level which can be used to make additional bonds. Also, as we go downward in the periodic table, the atoms get larger and are able to accomodate more bonds and/or lone pairs without undue crowding.
Here is the dot structure of sulfur tetrafluoride, SF4:
While we are violating the octet rule on sulfur, note that there is a favorable aspect also, namely, the formal charge on the sulfur atom is zero. This is often the case when we draw a dot structure with an expanded octet on an atom.
The structure of selenium tetrafluoride will be the same, since selenium and sulfur are in the same family. The overall "electron" or "electron domain" geometry is trigonal bipyramidal and the molecular geometry is "see-saw". Remember, lone electron pairs always go in equatorial positions when the overall geometry is trigonal bipyramidal.
Steve
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