The Foundations of Chemistry

TETRAHEDRAL ELECTRONIC GEOMETRY: ABU 3

SPECIES (THREE LONE PAIRS OF ELECTRONS ON A)

Each Group VIIA element has seven electrons in its highest occupied energy level. The

Group VIIA elements form molecules such as HXF, HXCl, ClXCl, and IXI by sharing

one of their electrons with another atom. The other atom contributes one electron to the

bonding. Lewis dot formulas for these molecules are shown in the margin. Any diatomic

molecule must be linear. Neither VSEPR theory nor VB theory adds anything to what

we already know about the molecular geometry of such molecules.

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330 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

Problem-Solving Tip:Some Molecules Have Two Central Atoms and

Lone Pairs

Methanol, CH 3 OH, is a simple molecule that has more than one central atom andtwo

lone pairs. It is the simplest member of a family of organic molecules called alcohols;all

alcohols contain the atom grouping CXOXH. The Lewis formula for methanol is

Again, we consider the arrangements around two central atoms in sequence. The

carbon atom (outlined in red) has four regions of high electron density. VSEPR theory

tells us that this atom has tetrahedral electronicgeometry; the resulting atomic arrange-

ment around the C atom has four atoms (O and three H) at the corners of this tetrahedral

arrangement, so the molecular geometry about the C atom is tetrahedral. The oxygen

atom (outlined in blue) has four regions of high electron density, so it, too, has tetrahe-

dral electronicgeometry. Thus, the CXOXH arrangement is angular and there are two

unshared pairs on O. The VB interpretation is that both atoms are sp^3 hybridized. The

CXO bond is formed by overlap of a half-filled sp^3 hybrid orbital on the C atom with

a half-filled sp^3 hybrid orbital on the O atom. Each covalent bond to an H is formed by

the overlap of a half-filled sp^3 hybrid orbital with the half-filled 1sorbital on an H atom.

Each unshared pair of electrons on O is in an sp^3 hybrid orbital.

H

H

HOC H

C O

H H

H

H

H

H

H

C O

H

We represent the halogen as X.

In latter case, either halogen may be
considered the “A” atom of AB.

HX, H X X 2 , X X