Video Transcript
Complete the following. Structural isomers are molecules
with blank. (A) Different molecular formulas,
(B) the same structural formula, (C) different structural formulas, (D) the same
displayed formula, (E) the same skeletal formula.
In this question, we want to
identify the missing words that will correctly complete the statement. The statement describes structural
isomers. Firstly, for molecules to be
considered isomers, they must have the same molecular formula but a different
arrangement of atoms in space. Knowing this allows us to eliminate
answer choice (A) because by definition isomers must have the same molecular
formula.
Structural isomerism is just one
type of isomerism possible among molecules with the same molecular formula. We know that a molecular formula is
a chemical formula expressing the exact number and type of atoms of each element in
a molecule. For example, the molecular formula
C4H10 describes a molecule that is composed of exactly four carbon atoms and 10
hydrogen atoms. The displayed formulas of two
structural isomers of C4H10 are shown above.
The displayed formulas show all of
the atoms and bonds in the molecules, and covalent bonds between atoms are
represented by lines. We can see that the displayed
formulas of butane and 2-methylpropane are different. Butane is a straight-chain
alkane. All four carbon atoms are connected
in one continuous chain. In contrast, 2-methylpropane is a
branched-chain alkane, in which a methyl side chain is bonded to the second carbon
atom in a three-carbon chain. From this example, we can see that
structural isomers have the same molecular formula, but different displayed formulas
because the atoms are connected in different ways. Therefore, we can now eliminate
answer choice (D).
Now, we can simplify the displayed
formula of each molecule by drawing structural formulas. While the displayed formulas show
all of the atoms and bonds and bonds are drawn as lines, structural formulas
represent the bonding in a molecule by writing atoms in the order in which they are
connected. The two structural formulas shown
here are showing only the bonds between carbon atoms. Some structural formulas, such as
the one shown here for butane, can be simplified even further by omitting the
connecting lines between carbon atoms and using only text. We can clearly see that the
structural formulas of butane and 2-methylpropane are different. This is because the atoms are
connected in a different way.
So, when defining structural
isomers, we can say that they are molecules with the same molecular formula that
differ by how their atoms are connected. And because we’ve determined that
structural isomers have different structural formulas, we can eliminate answer
choice (B) and select answer choice (C) as our correct answer. Structural isomers are molecules
with different structural formulas.
