Video Transcript
An ammonium ion, NH4+, contains one
coordinate covalent bond. Which of the following displayed
formulas correctly shows the structure of an ammonium ion?
In the ammonium ion, which has the
formula NH4+, nitrogen is the donor atom as far as the formation of the coordinate
covalent bond is concerned. We’re being asked to identify the
displayed formula of the ammonium ion in this question. The displayed formula shows all
bonds and all atoms in the structure.
All of our possible structures here
show the correct number and type of atom for the ammonium ion. Each possible structure contains
one nitrogen atom and four hydrogen atoms present. All of our possible structures have
the positive charge located in the right place.
When writing displayed formulas, we
use a straight line or a dash to represent normal covalent bonds. And we should use an arrow to
represent coordinate covalent bonds. In this case, the arrow must show
the correct direction of donation of the lone pair, that is, from the nitrogen atom
to the hydrogen atom.
Remember that the nitrogen atom is
the donor atom and the hydrogen atom is the acceptor atom in this bond. The only structure here that is
using the correct notation for a displayed formula is structure (E). It’s the correct answer.
Let’s see why the other answers are
not correct. In structure (A), we see a dotted
line used to represent one of the bonds. A dotted line is often used to
indicate that a bond is not a full bond; it’s only a partial bond. An example of this would be a
hydrogen bond formed between two molecules. Although ammonia molecules can form
into molecular hydrogen bonds, this dotted line does not represent the coordinate
covalent bond in the ammonium ion. This structure is therefore an
incorrect answer.
In structure (B), we see one of the
covalent bonds represented as a wedge shape. A bold wedged line indicates that
the bond is protruding out from the plane of the drawing surface. It’s pointing out of the page. This structure does not represent
the correct three-dimensional shape of this molecule. And it does not show the coordinate
covalent bond either. It’s not the correct answer.
Structure (C) includes a wavy line
as one of the covalent bonds. A wavy line is used when the true
stereochemistry or direction of the bond is unknown. This is not a displayed formula,
and it doesn’t contain our coordinate covalent bond. So it’s not the correct answer.
Although structure (D) is very
close to the correct answer, there is no coordinate covalent bond depicted by an
arrow shown at all. It shows four ordinary covalent
bonds, and it’s therefore not the correct answer. The correct answer is therefore
structure (E).
