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How does the presence of lone pairs of electrons affect the shape of A) ammonia...

How does the presence of lone pairs of electrons affect the shape of
A) ammonia
B) water molecules?

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In chemistry , a lone pair refers to a pair of
valence electrons that are not shared with
another atom and is sometimes called an
unshared pair or non-bonding pair. Lone pairs
are found in the outermost electron shell of
atoms. They can be identified by using a
Lewis structure . Electron pairs are therefore
considered lone pairs if two electrons are
paired but are not used in chemical bonding .
Thus, the number of lone pair electrons plus
the number of bonding electrons equals the
total number of valence electrons around an
Lone pair is a concept used in valence shell
electron pair repulsion theory (VSEPR theory)
which explains the shapes of molecules. They
are also referred to in the chemistry of Lewis
acids and bases . However, not all non-
bonding pairs of electrons are considered by
chemists to be lone pairs. Examples are the
transition metals where the non-bonding pairs
do not influence molecular geometry and are
said to be stereochemically inactive. In
molecular orbital theory (fully delocalized or
otherwise), the concept of a lone pair is less
distinct, but occupied non-bonding orbitals (or
mostly nonbonding) are frequently regarded as
"lone pairs" as well.
Lone pairs in ammonia (A), water (B), and
hydrogen chloride (C)
A single lone pair can be found with atoms in
the nitrogen group such as nitrogen in
ammonia, two lone pairs can be found with
atoms in the chalcogen group such as oxygen
in water and the halogens can carry three lone
pairs such as in hydrogen chloride.
In VSEPR theory the electron pairs on the
oxygen atom in water form the vertices of a
tetrahedron with the lone pairs on two of the
four vertices. The H–O–H bond angle is
104.5°, less than the 109° predicted for a
tetrahedral angle, and this can be explained
by a repulsive interaction between the lone
Various computational criteria for the
presence of lone pairs have been proposed.
While electron density ρ( r ) itself generally
does not provide useful guidance in this
regard, the laplacian of the electron density is
revealing, and one criterion for the location of
the lone pair is where L ( r ) = – ∇2ρ( r ) is a
local maximum. The minima of the
electrostatic potential V ( r ) is another
proposed criterion. Yet another considers the
electron localization function (ELF).
3 months ago
combine with hydrogen to ammoniumchloride;
4 months ago
AB3E: Ammonia, NH3
The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom (see Figure 8).

Lone pair electrons in ammonia

The domain geometry for a molecule with four electron pairs is tetrahedral, as was seen with CH 4 . In the ammonia molecule, one of the electron pairs is a lone pair rather than a bonding pair. The molecular geometry of NH 3 is called trigonal pyramidal (see Figure 9).

Ammonia has a trigonal pyramidal shape
Figure 9. Ammonia molecule.

Recall that the bond angle in the tetrahedral CH 4 molecule is 109.5°. Again, the replacement of one of the bonded electron pairs with a lone pair compresses the angle slightly. The H-N-H angle is approximately 107°.

AB2E2: Water, H2O
A water molecule consists of two bonding pairs and two lone pairs (see Figure 10).

Lone pair electrons on a water molecule
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