Tetrahedral
If you are following the structures in order from
Chemical Structures, you should be accustomed to the
interactive java applets by now. As we continue to put more atoms around
the central atom, the structures become more complex and their
geometry less intuitive. Woe be to the students who ignore this warning.
The tetrahedral structure is characterized by four 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. The four regions of
electron density occupy four sp3 hybrid orbitals. There are four molecular
geometries possible which are distinguished by the number of bonding pairs
and lone pairs of electrons. All four molecular structures have the same tetrahedral
electronic geometry.
The tetrahedral structures are described in Chapter 10 in your textbook.
Make sure 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.
AB4 Molecular Structure
In the 3D applet shown below, the central atom is white, the four other atoms are red,
and the four bonds are shown in blue. All four regions of electron density
are bonding electron pairs. The molecular geometry is tetrahedral and the
electronic geometry is tetrahedral.
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.)
Convince yourself of several things. First, there is no combination of four atoms
which lie in the same plane. Recall that the trigonal planar
structure had four atoms in one plane.
Second, all four of the bonds are equivalent. Try to rotate the molecule and
convince yourself that you can not identify a red atom that is different from
the other three red atoms. (Assume that there are no chemical differences
between the red atoms, that is, they present the same element.)
Most students do not find that the
geometric angle formed by any red atom , the white central atom, and a second red atom as intuitive.
Indeed, each red atom is 109.5 degrees from all the other red atoms.
If you feel like you're having a difficult time understanding the tetrahedral
structure, first don't be discouraged. Many students find it difficult at first.
Second, be sure to ask your instructor for help.
Two noteworthy examples of the above structure is methane, CH4, and
the ammonium ion, NH4+.
Convince yourself that the central atoms in both these molecules obey the octet rule.
AB3U Molecular Structure
In this structure, there are three red atoms and one non-bonding electron pair around
the central white atom. There are still four regions of electron density
around the central atom. The molecular geometry is trigonal pyramidal and
the electronic geometry is tetrahedral.
Rotate this molecular structure with the mouse.
Can you identify a configuration where all four atoms
(one white and three red) lie in a single geometric plane?
What is the difference between the trigonal pyramidal molecular structure and
the trigonal planar structure?
Are all three red atoms geometrically equivalent to each other?
(Ignore possible chemical differences.)
A noteworthy example of the AB3U molecular structure is ammonia, NH3.
Does the nitrogen in NH3 obey the octet rule?
Do the central atoms in the examples given for the
trigonal planar molecular geometry obey the octet rule?
How does the presence or absence of lone electron pairs on the
central atom affect the molecular geometry?
Be sure you can explain the differences in geometric geometries between NH3 and BH3.
AB2U2 Molecular Structure
Now we have two red atoms and two non-bonding electron pairs around the central white atom.
Since there are still four total regions of electron density around the central atom,
the electronic geometry is still tetrahedral. The molecular geometry is angular (sometimes called bent).
Rotate this molecular structure with the mouse.
Can you see the geometric plane in which the two red atoms and the white atom lie?
Can you see a different geometric plane in which the two pairs of non-bonding electrons
and the central white atom lie?
Convince yourself that the two planes are perpendicular to each other.
A noteworthy example of the angular molecular geometry is water, H2O.
What is the difference between the angular molecular geometry and
the linear molecular geometry?
Does the oxygen atom in H2O obey the octet rule?
Do the central atoms in the examples given for the
linear molecular geometry obey the octet rule?
How does the presence or absence of lone electron pairs on the
central atom affect the molecular geometry?
Be sure you can explain the differences in geometric geometries between H2O and BeCl2.
ABU3 Molecular Structure
This structure is composed of one red atom and three non-bonding electron pairs around the central white atom.
The electronic structure is again tetrahedral. The molecular geometry is linear.
What are the differences and similarities between the ABU3
and AB3U molecular structures discussed above.
Do both structures have the same electronic geometry?
Do both structures have the same molecular geometry?
Does the central atom in both structures obey the octet rule?
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Copyright (c) 2006 with Michael Keyes (michael.keyes@rretc.com)
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