Hi Diane, for your first question, sulfur has six valence electrons, and by gaining two more (forming the sulfide ion S2�?/SUP>) it will have eight valence electrons, just like its nearest noble gas, argon. All of the noble gases except helium have eight valence electrons (helium has two).
The word "adopt" in the question is a little confusing, but I think it means "gain" as in, "How many electrons would sulfur have to gain to reach a noble gas configuration."
You can also compare the electron configurations:
S 1s2 2s2 2p6 3s2 3p4
and for argon,
Ar 1s2 2s2 2p6 3s2 3p6
When a sulfur atom gains two electrons, it will have the same electron configuration as argon.
The "ground state" configuration of an atom is the most stable configuration. Any atom can be in an "excited state," noble gas or otherwise. For example, the ground state configuration of argon is,
Ar 1s2 2s2 2p6 3s2 3p6 ground state
If we promote an electron, from, say, the 3p orbital to a 3d orbital, that would be an excited state:
Ar 1s2 2s2 2p6 3s2 3p5 3d1 excited state
The electron in the 3d orbital will "fall" back to the 3p orbital so that the atom will be back in its ground state.
2. That is right, aluminum has three valence electrons, the same as its group number 3A. Some periodic tables now have the groups numbered from 1 to 18, with aluminum being in Group 13. I prefer the old labeling, as it makes the number of valence electrons clear. Note that the transition metal group numbers do not necessarily correspond to the number of valence electrons of those elements.
3. ICl, iodine monochloride, can obey the octet rule (the dot structure will be just like that of I2 and Cl2 and the other halogens), as do the other compounds, but chlorine trifluoride, ClF3, cannot obey the octet rule. This is found out when you try to draw the dot structure. With 28 valence electrons total, you have two extra electrons which have to go on the central chlorine atom, giving it 10 electrons, an "expanded octet."
Steve
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