Trigonal Planar
Chemical Structure
The trigonal planar geometry is characterized by three atoms bonded to a central atom.
Focus on boron, B, as discussed in Chapter 10 of your textbook.
What group and row do you find B in the PeriodicTable?
How does the group number for B compare to Be discussed in
linear structure?
As with the linear structure, there are no lone
electron pairs (non-bonding electron pairs) on the central atom.
There are only a total of three electron pairs around the central atom and they are all bonding electrons.
The molecule is perfectly happy in this state, and it is an exception
to the octet rule that we learned, or will soon learn, in class.
The molecule has both a trigonal planar molecular geometry and a trigonal planar electronic geometry.
Take a look at the geometry below. The central atom is in white,
the three other atoms are in red, and the bonds are shown in blue.
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.)
The white atom represents B, and the three red atoms are commonly halides.
The halides are in Group VII in the Periodic Table. The red atoms may also be
hydrogen, H. Convince yourself that all
four atoms lie in a single geometric plane. Also, the
three atoms in red form an equilateral triangle with the white atom in the center.
This is where the name "trigonal planar" arises.
Name the formulas for three molecules with B as the central atom
that are expected to have the trigonal planar molecular structure?
Orbitals
In the trigonal planar structure, the three chemical bonds are sp2 hybrid orbitals.
How many s orbitals and how many p orbitals on the
central atom are used to create the three sp2 hybrid orbitals?
Convince yourself that there is one p orbital on the central atom that does not
participate in forming the sp2 hybrid orbitals.
Below is the same chemical structure as shown above, except the unhybridized p orbital is also shown.
Rotate the structure with your mouse.
How would you describe the orientation of the unhybridized p orbital relative to the geometric plane
in which all four atoms lie?
Is the unhybridized p orbital occupied or unoccupied by electrons?
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