Which of the following corresponds
to a linear covalent molecule? A) SO₂, B) KBr, C) CF₄, D) HCl, E)
Molecules are formed when two or
more atoms are bonded together in some way. The atoms can be the same, or they
can be different. Chlorine gas, water, and ethanol
all exist as simple molecules. The molecules are held together by
strong covalent bonds.
A single covalent bond is a shared
pair of elections. In a covalent bond, two atoms share
a pair of electrons. The positive nuclei of each atom
are attracted electrostatically to the shared pair of electrons. In the diagram here, the nuclei are
not drawn to scale, nor are other outer shell electrons shown for clarity. A single covalent bond is
frequently represented as a single straight line drawn between the two atoms.
Covalent bonds are usually formed
when nonmetallic elements bond to other nonmetallic elements. This is the case in responses A, C,
D, and E. If we examine a list of possible
compounds, we see that response B, KBr, contains a metal bonded to a nonmetal. This would involve ionic
bonding. In KBr, positively charged
potassium ions are electrostatically attracted to negatively charged bromide ions in
a giant ionic lattice. KBr is an ionic crystal containing
a giant lattice of ions. It is not a simple molecule, so
answer B can be eliminated.
Having identified A, C, D, and E as
candidates for covalent molecules, we now need to identify their molecular
shape. To do this, we consider the
arrangement of electron pairs, that is, bonding pairs and lone pairs, around the
atom that is central to the molecule. We use valence shell electron pair
repulsion theory, which simply states that electron pairs in charge clouds repel
each other due to their negative charge.
In response A, we have sulfur
dioxide. Sulfur is in group 16, otherwise
known as group six, of the periodic table. It is logical to place sulfur at
the center of the molecule with six electrons in its outer shell. Oxygen is also in group 16, also
having six electrons in its outer shell. By sharing two pairs of electrons,
each oxygen atom can form a double covalent bond with sulfur.
Sulfur can accommodate more than
eight electrons in its outer shell. This is known as expansion of the
octet. It is a stable structure. We discover that there is a lone
pair of electrons located on the sulfur atom and two other electron pair domains,
one in each double bond. Double bonds and triple bonds are
treated just like single bond electron domains, as far as the repulsion theory is
concerned. The lone pair repels the bonding
pairs considerably. So the molecule is not linear. The bonds do not lie along a
straight axis. It would be described as V-shaped,
bent line, or nonlinear. Answer A is not correct.
In response C, we have carbon
tetrafluoride. It is logical to place carbon as
the central atom. Carbon is in group 14, or group
four, of the periodic table. So it has four electrons in its
outer shell. Carbon shares four electron pairs
with four fluorine atoms, making a total of four covalent bonds. There are no lone pairs on the
central carbon atom here. The four bonding pairs repel each
other equally and get as far away from each other as possible in 3D space. The shape of this molecule is
described as tetrahedral. So answer C is not correct.
In D, we have hydrogen
chloride. Hydrogen only has one electron in
its outer shell so can only form a single covalent bond. The nuclei of these two atoms and
the single covalent bond are orientated along the axis of a straight line. So the shape of hydrochloride is
described as linear. This is the correct answer.
Let’s also examine response E,
which shows the molecular formula for benzene, C₆H₆. In benzene, each carbon atom forms
single covalent bonds to the carbons on each side of it and to one hydrogen atom in
a cyclic structure. The remaining unpaired electron in
each carbon atom lies in a p-orbital that projects above and below the plane of the
ring or cyclic structure. The p-orbitals overlap in a
sideways orientation above and below the ring.
The electrons are said to be
delocalized. Delocalized rings of electrons
above and below the molecule complete the bonding in benzene. All of the bonds in the benzene
ring are the same, and they are the same length. The structure of benzene is often
abbreviated as a hexagon with a circle in the middle of it. The shape of a benzene molecule is
flat or planar. It is not linear, so answer E is