Video: Identifying the Linear Covalent Molecule in a Set of Chemical Formulas

Which of the following corresponds to a linear covalent molecule? [A] SO₂ [B] KBr [C] CF₄ [D] HCl [E] C₆H₆


Video Transcript

Which of the following corresponds to a linear covalent molecule? A) SO₂, B) KBr, C) CF₄, D) HCl, E) C₆H₆.

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 not correct.

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