PO4-3 and NO-3

[Guest TjsTjs]

the molecules PO4-3 and NO3-3 both have double bonds

PO4-3 is a tetrahedral, that means all 4 oxygens are sharing a total of 5 electrons with the phosphorus, and P only needs 3 electrons! How is this possible

 

Also, with NO-3 there is one double bond making the Os share 4 electrons with the N which also only needs 3 electrons, why are these electrons in bonds rather than simply being paired off on the oxygens! For example

O-

|

N--O-

||

O

 

Why not

 

O-

|

N--O-

|

O-

[wolfson]

The nitrate ion is distinctly awkward and there isn't any simple way of working out its shape. The problem is that it contains a co-ordinate (dative covalent) bond.

 

Around the central nitrogen there are 4 pairs of shared electrons, and no remaining lone pair. The original lone pair has now become a bonding pair. Two of those pairs make up a double bond. The double bond unit and the two single bonds arrange themselves as far apart as possible in a trigonal planar arrangement - exactly the same as the carbonate ion.

 

The phosphorus and and one oxygen are bonded through a double bond which counts as "one electron pair". Hence the molecule has four electron pairs and is tetrahedral.

 

The Lewis diagram is as follows:

P = 5 e-

O = 6e- x 4 = 24e-

3- charge = 3e-

Total electrons = 32

 

Phosphorus is an atom that may exceed an octet of electrons, P5+ is only slightly larger than S6+ and, hence, like sulfur, forms a tetrahedral anionic (PO4)3-group with oxygen.

 

I hope this explains all if you need more just ask.

[Hades]

TjsTjs said in post #1 :

Also, with NO-3 there is one double bond making the Os share 4 electrons with the N which also only needs 3 electrons, why are these electrons in bonds rather than simply being paired off on the oxygens! For example

O-

|

N--O-

||

O

 

Why not

 

O-

|

N--O-

|

O-

 

 

NO3 applies to the octet rule. However, this structure has resonance, meaning there are multiple proper ways to draw te=he structure. But before i go into that, u ask why these e- are in bonds and not lone pairs? That would disrupt the symmetry, and no atom, let alone molecule wants that.

 

all three oxygens have a total of 8 e- orbiting their outer shell at a designated time. This satisfies the orbital, thus the atom. The center atom, the Nitrogen, is also satisfied with one double bond, and two singles. A double bond is four e-, plus the two single bonds that have a value of 2 e- each. This equates to four.

The same goes with the other oxygens. There is no other way to leave lone pairs on the nitrogen b/c then an oxygen would not have enough e- in its orbital. Now, b/c electrons can not accurately be predicted, and there position is varying, obviously if theyre being shared, we need to show this by using resonance! It is simply all combinations of that same molecule with only a variance in bond type, and location.

[wolfson]

The electronic configuration is responsible for the bond angle, The bonds in a molecule will spread out to be as far apart as possible. This is because the electron pairs in the bonds will repel each other (because they are the same charge). There are 2 types of electron pairs in molecules: bonding electron pairs - the electrons involved in making the bond.

 

Lone pairs - the electrons not involved in bonding but residing on the outer shell of the atoms. Some examples of bonding:

 

BeCl2 This molecule has 2 bonding pairs of electrons, the bond angle is 180o, this shape is called LINEAR.

 

BF3 This molecule has three bonding pairs of electrons, the bond angle is 120o, the shape is called TRIGONAL PLANAR.

 

CH4 This molecule has 4 bonding pairs of electrons, and in three dimensions the furthest apart they can be is 109o, the shape is called TETRAHEDRAL.

 

PCl5 This molecule has five bonding pairs of electrons, the bond angles are shown in the diagram and the shape is called TRIGONAL BIPYRAMIDAL.

 

SF6 This molecule has 6 bonding pairs of electrons, all bond angles are 90o and the shape is called OCTAHEDRAL.

 

NH3 is bonded as “lone pairs”, This molecule has three bonding pairs of electrons like BF3 above. But there is also a lone pair of electrons on the nitrogen. The lone pair takes up space as if it were a bond, so the shape is tetrahedral like CH4 above but because lone pairs repel more than bonding pairs, the bond angle is less than the expected angle of 109o.

[wolfson]

o Degrees or (radians = o x pi / 180, marked as c).

[chemistry]

Remember that NO3 has resonance. The molecule will actually exist as a interspecies btw all of the resonance structures that can be drawn. Thus the pz molecular orbitals will be delocalized.

 

"Why not

 

O-

|

N--O-

|

O-"

 

This structure does not have full octet around the nitrogen atom and thus it will not be a plausible resonance structure.

 

As for phosphorus: Phosphorus can form an expanded octet, that is it disobeys the octet rule. The reason for this can be account for by the atomic bonding theory. Phosphorus can form hybrid spd orbitals by promoting an electron to the d orbital. The stability of the bonding usually suffices for the energy needed to do this.