Which of the following molecules
and ions does not contain a dative covalent bond? A) CO, B) HNO₃, C) BF4 minus, D) CS₂, or
E) NH4 plus.
First, let’s remind ourselves what
we mean by a dative covalent bond. In a normal covalent bond, two
atoms are held together by sharing a pair of electrons. One electron in this pair comes
from one atom, and the other belongs to the other atom. In a dative covalent bond, we still
have a pair of electrons shared between two atoms, forming our bond. But in this case, both electrons
originated from the same atom. So what we need to do to answer
this question is to look at each of the molecules and ions on our list and draw some
dot-and-cross diagrams to show the origin of each of the electrons in the bonds.
Remember though that the question
is asking us which of these does not contain a dative covalent bond. This means that four out of the
five molecules and ions we’re given should contain a dative bond. Sometimes you might hear these
referred to as coordinate bonds.
Let’s start with CO, carbon
monoxide. When drawing our dot-and-cross
diagrams, we only really need to focus on the valence electrons. So here we have our molecule with a
carbon and an oxygen atom. And now we just have to place the
electrons. The carbon atom in our CO molecule
contains four valence electrons, while our oxygen contains six valence
You may have seen carbon monoxide
drawn like this, with a triple bond between the carbon and the oxygen. A triple bond suggests that there
should be three pairs of electrons in the shared area down the middle. We will denote the electrons from
carbon as dots and those from oxygen as crosses for this diagram. We can easily draw in two normal
covalent bonds with two pairs of electrons and one electron in each pair coming from
the oxygen and one electron in each pair coming from the carbon.
We now have two electrons left to
place in carbon and four left to place in oxygen. If we were to create one more
covalent bond with one electron from carbon and one for oxygen, both the carbon and
oxygen would be left with an odd number of electrons, which is not ideal. So this is definitely not the
case. What we actually end up with is one
lone pair on the carbon, one lone pair on the oxygen, and then a shared lone pair
donated by the oxygen. And this creates our dative
In this question, we’re looking for
a molecule or ion that does not contain a dative covalent bond, which means that
carbon monoxide is not the correct answer.
Let’s move on to HNO₃. You may have seen the structure of
HNO₃ drawn in two different ways, one of which showing the delocalised nature of the
bonding between the nitrogen and the two oxygen atoms. Let’s try to convert these into a
dot-and-cross structure. It’s easiest to draw this in a
dot-and-cross diagram as the nondelocalised structure.
Again, we’re only going to be
interested in the valence electrons. Each oxygen atom is going to
contribute six valence electrons. Each nitrogen will contribute five
and hydrogen just the one. The OH bond is easy to depict. In this diagram, we’ll use dots for
the electrons in hydrogen and nitrogen and crosses for the oxygen.
This first oxygen is fairly
straightforward to fill in, with one covalent bond to the hydrogen, one to the
nitrogen, and two lone pairs. Now let’s focus on the
nitrogen. If we draw the nitrogen oxygen
double bond in with our dots and crosses, we are left with two electrons in nitrogen
unaccounted for. This bottom oxygen has four, which
we can easily display as two lone pairs. So we’re left with six electrons on
our last oxygen and two unaccounted for electrons on our nitrogen.
If we were to arrange these so that
they were all in the same shell, we would have a full shell of eight electrons. We can do this by arranging the six
oxygen electrons into lone pairs and using the two electrons in nitrogen to form a
dative covalent bond. So just like carbon monoxide, this
molecule does contain a dative covalent bond and therefore is not the correct answer
to our question.
Remember of course that, in
reality, this molecule exists with some delocalisation. So this dot-and-cross diagram isn’t
Now let’s look at BF4 minus. Boron can contribute three valence
electrons and fluorine seven electrons each. Electrons from boron will be
denoted by a dot and those from fluorine with a cross. Don’t forget though that this is
BF4 minus. So we’re going to have an extra
electron somewhere to account for the charge.
Once we’ve drawn in three
boron-to-fluorine covalent bonds, the boron has already run out of valence
electrons. This means it has no electrons to
contribute to the final BF bond. We can consider this last fluorine
as fluorine minus. BF4 minus of course is formed by the
reaction of BF₃ plus an F⁻. This is a Lewis acid–Lewis base
reaction. So our F⁻ is going to have eight
valence electrons. This means it is able to donate two
electrons to the boron–fluorine bond. And this is our dative covalent
We can add in the rest of the
fluorine lone pairs for completeness. So this ion does contain a dative
covalent bond so is not the correct answer to our question.
Now let’s look at CS₂. You can think of CS₂ as having a
similar structure to CO₂ since both oxygen and sulphur are in the same group of the
periodic table so have the same number of valence electrons. Each carbon is going to contribute
four valence electrons and each sulphur six. Here electrons from carbon will be
denoted by dots and those from sulphur with crosses.
Let’s first put in the two covalent
bonds between carbon and sulphur each side. After drawing in these electrons,
we can see that carbon’s four initial valence electrons have now been used up. And its outer shell contains eight
electrons. So this satisfies the octet
Sulphur, meanwhile, still has four
electrons to place. And we can arrange these simply as
two lone pairs on each sulphur atom. As you can see, in this molecule,
there are no dative covalent bonds, just normal covalent bonds. So this is a correct answer to our
question. But let’s check the last answer
just to be safe.
In this diagram, nitrogen will have
five valence electrons and hydrogen one electron each. However, again, we have an ion in
this case. Here it’s a cation. So we’re going to be missing one
electron to give it the overall charge. NH4 plus, the ammonium ion, is usually
formed when ammonia reacts with H⁺ from an acid. In a curly arrow diagram, we would
draw this with an arrow from the nitrogen lone pair going towards the H⁺. And this can be seen in our
We have three normal covalent bonds
between nitrogen and three of the hydrogens. Nitrogen still has two electrons
unaccounted for. And these are the lone pair that we
saw in our curly arrow diagram. These two final electrons in the
nitrogen lone pair go on to form a dative covalent bond with our H⁺. So here we have the dot-and-cross
structure for the ammonium ion. And we can see it does contain a
dative covalent bond. So this is not a correct
From these molecules and ions, the
only one which does not contain a dative covalent bond is CS₂.