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Name:  Darlene
Status: educator
Grade: 9-12
Location:NY
Country: N/A
Date: 12/16/2005


Question:
Hi, My question is about the periodic table and noble gases. Is a noble gas a nonmetal? Until a meeting last week at the New York State Education Department my answer would have been yes. At the meeting with the state education department I was told that the answer is no. The state said that a noble gas is only a noble gas and not a nonmetal, metal, or semimetal. The reasoning is that noble gases do not except electron(s). This is in the state core for the physical setting chemistry and all chemistry students are to learn that noble gases are only noble gases. Could you help me understand this better and why do many other references have a noble gas being a nonmetal?


Replies:
Since Xe can actually form compounds, it is certainly not correct to say that it cannot accept electrons. Compounds such as XeF2, XeF4, [XeF][Sb2F11] and others have been synthesized. In fact even Kr can form compounds under certain conditions; KrF2 can be obtained by photolysis of a Kr/F2 mixture, forming a white solid (admittedly this solid is unstable and slowly decomposes).

An authoritative, yet readable source for solid information on inorganic chemistry is Advanced Inorganic Chemistry by Cotton, Wilkinson, Murillo and Bochmann (or earlier editions by Cotton and Wilkinson). It devotes chapter 14 in its 6th edition (1999) to the Group 18 "rare gas" elements. That's my source for the information above.

All in all though, most of the VIIIA elements do not react much and it probably helps students who are learning chemistry for the first time to simply say "they do not react at all" and use that idea to "sell" the idea of a stable rare gas core which separates the valence electrons from the core electrons for elements like C, N, and so on. That idea also gets built upon to explain why certain ions are commonly observed (like O2-, F-, S2- and Na+) but not others (like Na2+).

Hope this helps,

Dr. Topper


----...Is a noble gas a nonmetal?

Yes.

Scientifically speaking, its properties are definitely "non-metallic". It does not conduct electrically, the solid (frozen) and liquid forms are dielectrics (insulators), it does not chemically donate electrons.

So in common use the term "non-metal" will include the noble gasses, regardless of New York's curriculum definitions. I would assume it will be a bit confusing for the students sometime in their future.

The term "non-metal" refers to any substance, not only to elements on the periodic table. Sounds to me like they are going to have to say the whole phrase "one of the non-metal elements" every time they use their newly defined term.

When specifically classifying the elements, I can see there is a chemical distinction between non-metals and noble gasses. "Does not accept electrons" distinguishes Argon from Chlorine quite well.

The purpose of designating areas on the periodic table is to quickly and intuitively communicate some of the character of the inhabitants. It is not hard science, and cannot be, because the behaviors are too scattered and there are always too many cross-over examples. It is more a human-sense thing than a matter of sharply defined science. I do not know whether accurately representing the characters of every element is more or less important than having easy-to-remember boundaries. The old traditional stair step boundary between metal and semi-metal always bothered me slightly because of a couple of seemingly misrepresented elements:

- Ge is brittle and has no metallically conductive forms, definitely a semi-metal in my mind.

- Sb has metallic conductivity as good as Ti or stainless steel

- Bi seems more metal than semimetal because of conductivity

As long as they are re-drawing lines across the periodic table, here is what I would like:

To the right, the "non-metal elements" (n) H; N,O,F; P,S,Cl; Br; I.

I would hope the "semi-metal elements" (s) are: B,C; Si; Ge,As,Se; Sb,Te; Bi,Po,At

Then the nearest "metal elements" (m) would be: Be; Al; Ga; Sn; Pb

Of course the "noble gas elements" are column 0 and the rest of the table are "metal elements".

This set is chosen for accuracy in my intuitive opinion; however it does give a tidy boundary between metal and semi-metal with a regular "over-1, down-2" sort of slope. The "non-metals" end up with a body and a couple of protrusions, though. And the semi-metals have a really narrow neck at Si. Oh, well. Whatever.
============================================
|H                                    | He |
----- ------------------|             |    |
Li Be         |  B    C |  N   O    F | Ne |
               |----|    |             |    |
Na Mg           Al | Si |  P   S   Cl | Ar |
                    |    |--------|    |   |
K  Ca Sc ...    Ga | Ge   As  Se | Br | Kr |
                    |----|        |    |   |
Rb Sr  Y ...    In   Sn | Sb  Te |  I | Xe |
                         |        ??   |    |
Cs Ba La ###... Tl   Pb | Bi  Po ? At | Rn |
                         |---?         ?    ?
Fr Ra Ac ###...

borderline examples:

Phosphorus:
  • (m) has positive oxidation states.
  • (n) has negative oxidation states.
  • (n) most forms are very non-conductive
  • (s) the darkest, densest special forms may be conductive or semi-conductive.


Tin:
  • (m) highly conductive & silvery in it's usual state
  • (m) highly ductile & malleable
  • (m) has only positive oxidation states
  • (s) one special form , "gray tin" is a semiconducto


Bismuth:
  • (m) metallic: more conductive than graphite, and shiny gray
  • (s) poorest conductivity of all metal elements
  • (s) large brittle crystals, not malleable
  • (n) has some negative oxidation states
  • (m) has some positive oxidation states
  • (s) many semiconducting binary compounds


Se and Te:
  • (s) semiconducting pure state, like Si and Ge
  • (s) shiny gray color due to high dielectric constant, like Si & Ge
  • (n) have negative oxidation numbers
  • (m) have postive oxidation numbers
  • (s) brittle crystals


Iodine crystals are so shiny that one wonders if astatine (At) would be a semiconductor...

Jim Swenson



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