How Can Phosphorus Exhibit a Valence of 5?

[xilc]

I was just reading through this in my bio book, and it was very unspecific... If you read it it claims that the valence of Phosphorus has a valence of 3, due to 3 unpaired valance electrons in the Valence shell... But in some molecules involving phosphorous, it can have 3 single bonds and a double bond... and thus, have a valance of 5... how is this possible?

 

[Pythagorean]

depends on how you define valence. A lot of people judge it by the periodic table, thinking of the oxidation number, but it's technically the active number of bonds.

So phosophorous, theoretically, looking at periodic table, has 5, but 3 are in one shell and 2 are in another. Depending on the context (energy and other interacting matter) the 2 electrons in their own shell will either be freely available to bond, or will be coupled to each other as part of the phosphorous (and thus not available for bonding) leaving the valence to 3.

 

[xilc]

depends on how you define valence. A lot of people judge it by the periodic table, thinking of the oxidation number, but it's technically the active number of bonds.

So phosophorous, theoretically, looking at periodic table, has 5, but 3 are in one shell and 2 are in another. Depending on the context (energy and other interacting matter) the 2 electrons in their own shell will either be freely available to bond, or will be coupled to each other as part of the phosphorous (and thus not available for bonding) leaving the valence to 3.

Yes, I understand that 3 are in one shell and 2 are in another... But the 2 happen to be in the valence (outermost) shell... Do you mean that if they bond the valence will be 5 due to covalent bonding? And if they do not bond they will remain the same? I don't get what you mean by being coupled to each other (keep in mind, just learned this today)??


I have no idea what you meant by the oxidation number thing... What does oxidation have to do with valence ? And I'm not bashing you I just don't know that much about oxidation and wouldn't mind if you helped me out :p

 

[Pythagorean]

they are coupled through their spin: one has spin up, one has spin down and of course, they're in the same shell, but sometimes they can dissociate and become part of the bond. If they stay coupled to the phosphorous atom, they do not participate in the bond.

oxidation number also tells you how filled the outer shell is, but in a slightly different context (i.e. hydrogen has an oxidation state of 1, oxygen is -2). For phosphorous, the maximum oxidation number is 5.

 

[xilc]

they are coupled through their spin: one has spin up, one has spin down and of course, they're in the same shell, but sometimes they can dissociate and become part of the bond. If they stay coupled to the phosphorous atom, they do not participate in the bond.

oxidation number also tells you how filled the outer shell is, but in a slightly different context (i.e. hydrogen has an oxidation state of 1, oxygen is -2). For phosphorous, the maximum oxidation number is 5.

How do I find the oxidation number ?

 

[Ygggdrasil]

I don't really like the concept of valency and oxidation number. While they're useful in certain cases, I mostly try to think about bonding in terms of the number of electrons in the valence shell, so I'll focus on this number in my discussion:

The valence shell is the outermost shell of electrons of an atom, and the amount of electrons that the valence shell can accommodate defines the number of bonds each type of elements can form. For elements in row 2 or higher of the periodic table, the outermost shell consists of one s-orbital and three p-orbitals. Since each of these four orbitals can hold two electrons, this gives us the familiar octet rule -- the idea that each atom is most stable when it has eight electrons surrounding it, filling the valence shell of the atom.

Elements in the third row or higher (such as sulfur or phosphorus) can, however, violate the octet rule and hold more than eight electrons in their valence shell. How can this happen? Where do the extra electrons go if all of the s- and p-orbitals are full?

The third shell actually has an extra set of orbitals, the d-orbitals, which do not normally participate in chemical bonding. In certain situations, however, these d-orbitals join the valence shell and allow the valence shell to hold ten (if one d-orbital participates) or twelve electrons (if two d-orbitals participate). This valence shell expansion through participation of the d-orbitals helps explain why phosphorus can sometimes have a valency of more than three.

Now I will note that, while this explanation is useful for understanding the bonding of atoms like sulfur and phosphorus and is used in many introductory chemistry classes, this explanation fails to explain some of the details of the bonding correctly. More advanced chemistry classes will explain this phenomenon in terms of molecular orbital theory which more correctly explains the bonding of these atoms. Unfortunately, molecular orbital theory is more complicated and I'm not sure I'd be able to write up a good explanation of it.