MadSci Network: Chemistry
Query:

Re: Some atoms can have more than eight outer electrons. Why?

Date: Thu Apr 12 14:04:01 2001
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 986508040.Ch
Message:

There are answers on a number of levels. The Lewis level is easiest:
  • Cl contributes 7 electrons (as the central atom, you count all of its valence electrons).
  • Each F contributes 1 electron (the electrons used to form bonds to Cl), or 4 more electrons.
  • The single negative charge is 1 more electron.
7 + 4 + 1 = 12, or 6 pairs of electrons around the chlorine atom. VSEPR says that they will be arranged in an octahedral geometry. Since ClF4- is square planar, accounting for four of the six octahedral positions, there must be two lone pairs of electrons in the apical positions.

ClF4- is isoelectronic with XeF4; if you search our archives for "XeF4" you should find an answer I wrote on that compound a couple of years ago, which gives references.

Of course, one could consider ClF4- from a hybridization (sp3d2) or molecular-orbital standpoint, but that's another bucket of worms (search for my answer on IF7!) and the Lewis model is so much easier.

If you must assign the lone pairs to orbitals, remember that they must be (hybridized or unhybridized) atomic orbitals on Cl; since they must be orthogonal, the pz and dz2 orbitals will do.
Dan Berger
MadSci Administrator




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