Metal Models Three

Posted on November 15th, 2009 by admin

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Metal Models Three

Chemists? help please?

I have a chemistry problem that I need some help with. I have the awnser(it is a practice prob from my practice book) but i am befuddled on the complete subject now. Help!!! the problem is

"How many unpaired valence electrons are there in a phosphourous atom[P] that has 15 electrons? (use shell models to deduce awnsers.)"

the awnser is

"Three. Two of phosphourous electrons pair up in the first shell and 8 pair up in the second. The remaining 5 occupy the third. Because the third shell has a capacity for 8 electrons, two are forced to pair up leaving 3 unpaired."

Ok...so I dont understand why 2 of the three cant pair up and just leave one by itself after all, alkali metals only have 1 unpaired electron.

And why do alkali earth have 2 unpaired? Cant the remaining two pair up?

I would suggest you read up on LEWIS DOT STRUCTURES.

They are meant to represent the OUTER (valence) shell electron configuration. Start by placing a single dot (electron) to the right of the element letter. That represents one (1) electron. To determine the Lewis Dot Structure for a given element, consider what family it's in. Phosphorus is family 5. (V) That means it has 5 valence electrons. Carbon is in IV...so it has 4 valence e's.
Zenon (Xe) is in famly VIII...it has 8 valence e's. Etc...

Phosphorus has 5 valence electrons. Try this. We're going to "encircle" the element with dots until we reach the number of dots that equals the number of valence electrons. (i.e. for P, we stop at 5...). So, start at the right, place a dot. Underneath..a dot...to the left, a dot...above, a dot. for the 5th one (thru the 8th if that many) place a second dot where you already have one.

So when you are done with P, you should have 2 dots to the right of P, one on bottom, one to left, one on top. Two dots together represent an ELECTRON PAIR. Single dots represent unpaired e's. So P has two paired electrons, and 3 unpaired.

Lewis dot structures are useful for bookkeeping, but don't always tell the whole truth about structure and shape.

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