The octahedral structure is next in order from
Chemical Structures.
The octahedral structure is characterized by six regions of electron density
around the central atom. A region of electron density is either a bonding pair
of electrons or a lone pair (non-bonding pair) of electrons. There are three molecular
geometries that we will discuss which are distinguished by the number of bonding pairs
and lone pairs of electrons. All three molecular structures have the same octahedral
electronic geometry. All the regions of electron density occupy sp3d2
hybrid orbitals.
The octahedral structures are discussed in Chapter 10 of your textbook.
By this time, I am assuming that you are comfortable with the terms,
"molecular geometry" and "electronic geometry." If you do not understand these terms,
go back to your textbook to review their definitions.
AB6 Molecular Structure
In the geometry shown below, the central atom is white, six other atoms are red,
and the six bonds are shown in blue. All six regions of electron density
are bonding electron pairs. The molecular geometry is octahedral and the
electronic geometry is octahedral.
Place the cursor over the molecule, press and hold the left button down, and move the mouse.
You should be able to spin the molecule in any direction, and examine the orientation of
the atoms. (If the image looks weird, like it is only partially shown or it looks like a double image,
click the refresh button of your browser. It is at the top in the toolbar.)
As you rotate the molecule with your mouse, convince yourself that the six
red atoms are geometrically equivalent. (Ignore possible chemical differences.)
As you continue to rotate the molecule, can you identify
a geometric plane which contains four red atoms and the central white atom?
There are actually three separate geometric planes which contain three separate sets of
four red atoms each. Each of the three planes contain the central white atom.
You can actually think of the red atoms as being along the three axes of an xyz
Cartesian coordinate system. The three planes then are the xy, xz, and the yz planes
in the Cartesian coordinate system.
Can you identify each of the three geometric planes in the 3D applet?
If you can not find them, be sure to ask your instructor for help.
SF6 is an example of a molecule with octahedral molecular and electronic structures.
Does the central atom, S, obey the octet rule? Why or why not?
AB5U Molecular Structure
In this structure, there are five red atoms and one lone (non-bonding) electron pair around
the central white atom. There are still six regions of electron density
around the central atom. Thus, the electronic geometry is octahedral.
Does it matter which of the six positions the lone electron pair occupies?
The molecular geometry is square based pyramid.
Identify the geometric plane that contains four red atoms and the central white atom.
The geometric plane that you identified forms a square and is the base for a pyramid when
combined with the five red atom. Thus the name square based pyramid.
Convince yourself that the four red atoms in the square base of the pyramid are geometrically equivalent to each other.
Also, convince yourself, that the fifth red atom at the apex of the pyramid is unique among the five red atoms.
Does the geometric plane with the four red atoms and the white central atom
correspond in any way with one of the geometric planes
that you identified above with the octahedral molecular structure?
AB4U2 Molecular Structure
Now we have four red atoms and two lone (non-bonding) electron pairs around the central white atom.
Since there are still six total regions of electron density around the central atom,
the electronic geometry is still octahedral.
The two lone electron pairs occupy positions around the central atom that are
opposite each other. The molecular geometry is square planar.
A geometric plane is formed with the four red atoms and the central white atom,
the four red atoms form a square. Thus the name square planar.
Does this geometric plane correspond in any way with one of the geometric planes
that you identified above with the octahedral molecular structure?
URL: http://www.rretc.com/chemistry/octahedral.html
Copyright (c) 2006 with Michael Keyes (michael.keyes@rretc.com)
Feel free to use for single-user, educational purposes only. All other purposes, please email.