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Jump to one of the following Lewis structures' parts:   Choose One Introduction to Lewis Structures Part 1- Basics Part 2- Multiple Bonding and Polyatomic Ions Part 3- Resonance Part 4- Hypervalency Part 5- Formal Charge I Part 6- Formal Charge II Part 7- Electron Deficiency Part 8- Oxoacids Part 9- Quizzes

Let's try something different. How about PCl₅?

Hmmm. Any problems or concerns? I hope so!

Until now, we've only had a maximum of eight electrons about the central atom (octet rule). What about this one, phosphorus pentachloride? WIth five P-Cl bonds, that would put ten electrons abot the central atom. Either I'm lying or there's something new to work on. This is a real compound. It's stinky and it's used in certain industrial processes. So, we have to do something to be able to explain the ten electrons about the central atom.

Notice that phosphorus is in period 3. Remember that the first time we talk about d electrons is for principle quantum number 3 (s, p, and d). If the central atom is in period 3 or greater, there are d orbitals that can be used when bonding. Valence bond theory is one way to explain the use of d orbitals when there are more than eight electrons about the central atom.

There is still a lot of disagreement among chemistry educators about this concept of "hypervalency" (more than eight electrons about the central atom) and when to use it. I've altered the way I teach it. Now, I only use it when there are either more than four atoms about the central atom, or when distributing the rest of the electron pair about the central atom, you end up with more than eight about it.

Now it's time to try PCl₅.

Pull up the Periodic Table if you need one.

Here's a current list of the rules. Rule #5 has been modified for hypervalent compounds.

PCl5
Your turn- enter your answer in the first box and hit "Verify" to see how you did.
1.	Total number of valence electrons.
2. Draw the skeleton structure. 3. Connect the atoms by making bonds. 4. Satisfy the octet rule for the surrounding atoms. 5. Satisfy the octet rule for the central atom.

There was no difference between step 4 and step 5 because all the electrons had been distributed after the Octet Rule was satisfied for the surrounding chlorine atoms. You should see almost from the beginning that phosphorus will need to be hypervalent since there are five surrounding chlorine atoms which gives 10 electrons about phosphorus. It's not vital that you place the 5 chlorine atoms in a specific geometric pattern about the central phosphorus atom. Just make sure the 5 P-Cl bonds are clearly indicated.

Now try xenon tetrafluoride, XeF₄. Work up the Lewis structure through satisfying the octet rule for the surrounding atoms.

XeF4
Your turn- enter your answer in the first box and hit "Verify" to see how you did.
1.	Total number of valence electrons.
2. Draw the skeleton structure. 3. Connect the atoms by making bonds. 4. Satisfy the octet rule for the surrounding atoms.

Normally rule 5, satisfy the octet rule for the central atom, would also be taken care of at this point. After all, Xe has achieved its octet by sharing 4 pairs of electrons with 4 different fluorine atoms.

You were keeping track of the electrons you used, right? You do realize that you've only used 32 of the 36 available valence electrons, right? Just like you couldn't conjure up electrons for the central atom when you were short (and thus the concept of multiple bonds between the central and surrounding atom(s)), you can't simply look the other way and sweep the extra electrons under the rug in the hopes that nobody was watching.

This is the section on hypervalency and xenon is in period 5. The final 2 pairs of electrons that are left over in the Lewis structure for XeF₄ get placed about the central atom- Xe. And, Xe now has 12 electrons about it. It has 2 lone pairs and it is sharing 8 electrons in the Xe-F bonds. Here's what the structure looks like-

Just as in previous cases, it's not vital that you place the extra pairs of electrons in exactly the correct position. There isn't one correct placement for them. Just make sure you've got the right bonds to Xe and the 2 lone pairs in distinct positions about Xe.