Predicting electron-pair geometry and molecular structure: ammonium
Two of the top 50 chemicals produced in the United States, ammonium nitrate and ammonium sulfate, both used as fertilizers, contain the ammonium ion. Predict the electron-pair geometry and molecular structure of the
cation.
Solution
We write the Lewis structure of
as:
We can see that
contains four bonds from the nitrogen atom to hydrogen atoms and no lone pairs. We expect the four regions of high electron density to arrange themselves so that they point to the corners of a tetrahedron with the central nitrogen atom in the middle (
[link] ). Therefore, the electron pair geometry of
is tetrahedral, and the molecular structure is also tetrahedral (
[link] ).
The ammonium ion displays a tetrahedral electron-pair geometry as well as a tetrahedral molecular structure.
Check your learning
Identify a molecule with trigonal bipyramidal molecular structure.
Answer:
Any molecule with five electron pairs around the central atoms including no lone pairs will be trigonal bipyramidal. PF
5 is a common example.
The next several examples illustrate the effect of lone pairs of electrons on molecular structure.
Predicting electron-pair geometry and molecular structure: lone pairs on the central atom
Predict the electron-pair geometry and molecular structure of a water molecule.
Solution
The Lewis structure of H
2 O indicates that there are four regions of high electron density around the oxygen atom: two lone pairs and two chemical bonds:
We predict that these four regions are arranged in a tetrahedral fashion (
[link] ), as indicated in
[link] . Thus, the electron-pair geometry is tetrahedral and the molecular structure is bent with an angle slightly less than 109.5°. In fact, the bond angle is 104.5°.
(a) H
2 O has four regions of electron density around the central atom, so it has a tetrahedral electron-pair geometry. (b) Two of the electron regions are lone pairs, so the molecular structure is bent.
Check your learning
The hydronium ion, H
3 O
+ , forms when acids are dissolved in water. Predict the electron-pair geometry and molecular structure of this cation.
Answer:
electron pair geometry: tetrahedral; molecular structure: trigonal pyramidal
Predicting electron-pair geometry and molecular structure: sf
4
Sulfur tetrafluoride, SF
4 , is extremely valuable for the preparation of fluorine-containing compounds used as herbicides (i.e., SF
4 is used as a fluorinating agent). Predict the electron-pair geometry and molecular structure of a SF
4 molecule.
Solution
The Lewis structure of SF
4 indicates five regions of electron density around the sulfur atom: one lone pair and four bonding pairs:
We expect these five regions to adopt a trigonal bipyramidal electron-pair geometry. To minimize lone pair repulsions, the lone pair occupies one of the equatorial positions. The molecular structure (
[link] ) is that of a seesaw (
[link] ).
(a) SF4 has a trigonal bipyramidal arrangement of the five regions of electron density. (b) One of the regions is a lone pair, which results in a seesaw-shaped molecular structure.
Check your learning
Predict the electron pair geometry and molecular structure for molecules of XeF
2 .
Answer:
The electron-pair geometry is trigonal bipyramidal. The molecular structure is linear.
