Video: Selecting the Substance Whose Molecules Have an Unpaired Electron

Which of the following substances is a gas in which each molecule has an unpaired electron? [A] C(graphite) [B] MgO [C] N₂H₄ [D] CO [E] NO₂

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Video Transcript

Which of the following substances is a gas in which each molecule has an unpaired electron? A) C, graphite. B) MgO. C) N₂H₄. D) CO. Or E) NO₂.

This question is tasking us with identifying a substance based on a number of criteria. The substance should be a gas. It should form molecules, rather than be an ionic substance for instance. And it should have an unpaired electron. Since we want to find a substance that has an unpaired electron, it may be helpful to draw the Lewis structures for our answer choices so we can visually see if there’s an unpaired electron in their structure.

But before we do that, let’s go through our answer choices and see if we can eliminate any of them based on our other criteria. Our first answer choice is carbon in the form of graphite. Graphite is made of a large network of carbons, and each carbon is bonded to three other carbons. Since carbon can form a maximum of four covalent bonds, each carbon and graphite has an additional delocalized electron that is spread out throughout the whole structure.

Graphite is actually something most people interact with on a regular basis. Since it’s the substance that’s found in pencil lead. From experience, we know that this is a solid. And we’re looking for a substance that’s a gas, so answer choice A can’t be the correct answer.

Answer choice B is MgO, which is called magnesium oxide. Magnesium is a metal that’s found in group two of the periodic table. And oxygen is a non-metal found in group 16, sometimes called group six of the periodic table. When a metal and a non-metal come together to form a substance, there’s typically such a big electronegativity difference between the non-metal and the metal that the non-metal takes the electrons from the metal. This will form positively and negatively charged ions, which makes this substance bound with ionic bonds, not covalent bonds characteristic of molecules. Since we want a substance that is a molecule, answer choice B is not the correct answer.

Our next answer choice is N₂H₄, which is called dinitrogen tetrahydride. Dinitrogen tetrahydride certainly forms molecules, so it’s a contender for a correct answer choice. Our next answer choice is CO, which is carbon monoxide. Carbon monoxide is definitely a gas, and it forms molecules, so it’s also a contender for the answer. Our final answer choice is NO₂, which is called nitrogen dioxide. Nitrogen dioxide also exists as a gas. And it forms molecules, so it’s also a contender for the correct answer.

Now that we’ve eliminated some answer choices, let’s draw some Lewis structures for our remaining ones to determine which one has an unpaired electron. Let’s start off with N₂H₄. The first step in drawing this Lewis structure will be to determine the number of valence electrons in this molecule, which we can do by using the periodic table. Nitrogen has five valence electrons, and hydrogen has one, so we have a total of 14 valence electrons in this structure. Next, we’ll place our atoms and connect them with single bonds. We want to put nitrogen in the center here because hydrogen can only accept one more electron before its outer shell is full.

Placing these bonds have used up 10 of our total valence electrons, so we have four valence electrons remaining to place in our structure. And these electrons should be used to fill the octets of both of our nitrogens. Now, our structure for N₂H₄ is complete. And looking at it, we have some single bonds and some lone pairs, but no unpaired electrons. So, answer choice C is not the correct answer.

Let’s move on to carbon monoxide. Carbon has four valence electrons and oxygen has six, so we have 10 total valence electrons to place in our structure. Now, we’ll connect our atoms with single bonds, which has used up two of our electrons. So, we have eight electrons remaining to fill carbon and oxygen’s octet. If we place these eight electrons, we’ll run out of electrons to place in our structure before both of our atoms have a full octet.

We can remedy this by removing one of the lone pairs and using it to create a double bond. This has helped, but carbon still doesn’t have a full octet, so we can do the same thing again, removing a lone pair to create another bond between the carbon and the oxygen. Now, both carbon and oxygen have eight total electrons in the structure, so they have a full octet. So. looking at our finished structure, we have a triple bond and some lone pairs, but no unpaired electrons. So, answer choice D is not the correct answer choice either.

So, now, let’s take a look at NO₂. Nitrogen has five valence electrons. And the oxygens both have six, which gives us a total of 17 valence electrons to place in our structure. Next, we’ll place our atoms and connect them with single bonds. Here, the nitrogen goes in the center because it has fewer valence electrons in its outer shell, meaning that can accept more bonds than the oxygen can. Placing these bonds used up four of our electrons, so we have 13 electrons remaining to place in our structure.

If we place these, we’ll run out of electrons before nitrogen has a full octet. We can attempt to remedy this by removing one of the lone pairs from oxygen and using it to create a double bond. Now, nitrogen has seven electrons, not the eight that it needs to have a full octet. If we tried to fix this by removing another lone pair from oxygen to create another bond between nitrogen and oxygen, it would cause nitrogen to have nine electrons, which would be too many. So, this structure must be okay.

If we look at our finished structure, there’s an unpaired electron on the nitrogen. So, NO₂ must be the correct answer. One thing you might have noticed is that we didn’t actually need to draw Lewis structures for all of these molecules. The one that had an unpaired electron was the one that had an odd number of valence electrons. So, we simply could have determined the number of valence electrons for each molecule. And the one that had the odd number would be the one with the unpaired electron. Either way, NO₂, or nitrogen dioxide, is the substance that’s a gas in which each molecule has an unpaired electron.

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