A Simple Method to Determine the Number of Lone Pairs on a Hypervalent Central Atom

p-block elements

So I do not need to do as much review, I give my Junior-level Inorganic Chemistry class Review Quizzes. Literally, I give them the quiz a week ahead of time and, with very minor changes, give them the same quiz. I have a tutor session online the night before the quiz and, not surprisingly, they have questions over the quiz. Thus, it becomes a not-too-subtle way for me to get an extra hour of discussion over general chemistry material for those who need it.

In the chapter on Molecular Structure and Bonding, hypervalent compound structure is a chronic challenge, not just for General Chemistry students, but for my Inorg-Chem students. The discussion led to a simpler, quicker way of determining the number of lone pairs on the hypervalent central atom. It is an extension and a twist on Counting Bonds method of determining the formal charge on atoms. It can be extended to any molecule comprised exclusively of single bonds and zero formal charges.

The discussion is given below in bullet format suitable for lecture notes or PowerPoint.

  • Hypervalent compounds:
    • More than octet of electrons
    • Noble gas compounds are by definition hypervalent.
      • They start with 8 electrons.
      • Bonding shares more electrons.
    • There are also hypervalent compounds that have a central atom of Group V, VI and VII and n = 3 or higher with a lot of halides (particularly F).
  • To have a zero formal charge on the central atom:
    • The even groups (18 and 16) need to have an even # of bonds.
    • The odd groups (17 and 15) need an odd # of bonds.
    • Lone pairs add to bonding electrons to minimize formal charge.
  • (Group # of central atom – # of halides)/2 = # lone pairs

Here is a table illustrating the results across the p-block. (X = halide, l.p. = lone pair, steric # = l.p + X)

Group # # of X # of l.p. Steric # Examples
8 2 3 5 XeF2
8 4 2 6  
8 6 1 7  
7 1 3 4 octet
7 3 2 5  
7 5 1 6 IF5
7 7 0 7  
6 2 2 4 octet
6 4 1 5  
6 6 0 6 SeCl6
5 1 2 3  
5 3 1 4 octet
5 5 0 5  
4 4 0 4 octet
3 3 0 3 BF3*

*: hypovalent

The explanation seems obvious to us as instructors, but for the students this was a revelation. The halides (and you can consider hydrogen a halide in this context) have:

(1) one opening in their valence shell, i.e. one place for an electron from another atom to bind, and 

(2) an electron in the same orbital to donate to make a 2-electron bond to another atom. 

Put more philosophically, "empty is not nothing, it is the capability to be filled". The halide has one electron and one "empty" to interact with the central atom. The glass is both ½ filled and ½ empty.

This method could be expanded upon to deal with multiple-bonded systems, but there are better ways of doing that. However, I have never seen a particularly satisfying method to determine number of lone pairs and by extension the steric number for hypervalent molecules. This method does just that in a simple way that builds chemical intuition, and has the added bonus of being expandable to hypovalent and certain octet structures.