Question Video: Completing a Statement That Describes Structural Isomers | Nagwa Question Video: Completing a Statement That Describes Structural Isomers | Nagwa

Question Video: Completing a Statement That Describes Structural Isomers Chemistry • Third Year of Secondary School

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Complete the following: Structural isomers are molecules with _. [A] different molecular formulas [B] the same structural formula [C] different structural formulas [D] the same displayed formula [E] the same skeletal formula

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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.

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