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Which Lewis Dot Diagram could represent a noble gas?

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3 Answers

Here are some things to think about with this question:
1. Two dots at top (or on either side, or on the bottom) could represent any element with two valence electrons.  In a ground-state atom, the first two valence electrons go into the "s" orbital, so they'll be paired and it's appropriate to put a pair of dots into a Lewis dot diagram.  The elements with two valence electrons include helium and all of the alkaline earth metals (Be, Mg, Ca...).  Helium is a noble gas, so #1 is a correct answer here.
2. Three pairs of dots and one single dot indicates an atom with 7 valence electrons.  These are the halogens (F, Cl, Br...), each of which has one less electron than the noble gases that are next to them in the periodic table.  Noble gases have a complete set of valence electrons (2 for helium and 8 for the others), so this one can't be a noble gas.
3. One single dot indicates an atom with 1 valence electron.  These include hydrogen and the alkali metals (Li, Na, K...), each of which has one more electron than the noble gases that they follow in the periodic table.  
4. This is the same as #2.  It doesn't matter what side the dots are on.  There are a maximum of eight valence electrons in any atom, and the Lewis diagram has spaces for a maximum of eight dots (two on top, two on bottom, two on right, and two on left).  
So, the only correct answer is 1.
Note: If we were talking about ions, not neutral atoms, then any of the answers (1, 2, 3, or 4) could be correct.  #2 and #4 could be noble gas ions with a +1 charge and #3 could be a noble gas ion with a -1 charge.

Most noble gases have 8 dots, two on each side but Helium has only two dots to complete its energy level, so 1 would be the correct response