Chemists? help please?

Question

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?

Answer 1

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.

Answer 2

Ok, to start off, a valence electron is the electrons in the outer shell of an atom. The first shell holds only 2, and for Helium, since it has only 2, those are its valence electrons. Basically, each atom wants to be like to Noble Gases, with a full outer shell of 8 electrons. The Noble Gases are starting with Helium and moving down that group. To determine the quantity of valence electrons an element has, one must look at the periodic table group (vertical column) in which the element is categorized. With the exception of groups 3–12 (transition metals), the number within the unit's place identifies how many valence electrons are contained within the elements listed under that particular column.

Group 1 (I) (alkali metals) 1
Group 2 (II) (alkaline earth metals) 2
Groups 3-12 (transition metals) 1 or 2*
Group 13 (III) (boron group) 3
Group 14 (IV) (carbon group) 4
Group 15 (V) (nitrogen group) 5
Group 16 (VI) (chalcogens) 6
Group 17 (VII) (halogens) 7
Group 18 (VIII or 0) (noble gases) 8**

Valence elctrons aren't generally useful for transition metals. The amount of valence electrons basically govern how that element will bond with other elements. Like I said before, all elements want to be "happy" and have a full outer shell. For all purposes now, assume that elements can only have 8 electrons in their outer shell. So take Flourine for example, it has 7 valence electrons. Therefore, it can only bond with another element and "take" one of its atoms. Thats why they can't "pair up." I took AP Chemistry last year, so if you have any other questions, feel free to email me.

Answer 3

The shell model is not the best way to explain this. If you look at the electron configuration for phosphorus, it is:

1s2 2s2 2p6 3s2 3p3

Since it takes more energy to pair electrons in an orbital, the system will prefer to put one electron in each orbital before it pairs them up. Therefore, the electrons in the 3p orbitals will be in one orbital each.

Alkali metals have one unpaired electron because they all have the same outer electron configuration - s1. Li is [He]2s1, Na is [Ne]3s1, K is [Ar]4s1, etc. And alkaline earth metals do not have unpaired electrons, as their outer electron configuration is s2.

Answer 4

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