Video: Assessing and Comparing the Shapes of Molecules

Which of the following has a molecular shape most similar to that of ammonia, NH₃? [A] CCl₄ [B] H₃O⁺ [C] H₂O [D] BF₃ [E] NH₄⁺.

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Video Transcript

Which of the following has a molecular shape most similar to that of ammonia, NH₃? A) CCl₄, B) H₃O⁺, C) H₂O, D) BF₃, or E) NH₄⁺.

To determine the molecular shape of all these molecules, we’ll need to first draw their Lewis structures. Let’s start by drawing the Lewis structure of the one that we need to compare everything to, which is ammonia or NH₃. The first thing that we’ll need to do is determine the number of valence electrons, which we can do by using the periodic table. Nitrogen has five valence electrons. And hydrogen each has one. And there’s three of them in a molecule of ammonia. This gives us a total of eight valence electrons.

Next, we should place our atoms and connect them with single bonds. We should place the atom in the center that can accept the most number of electrons before it fills its outer shell, because this one will be able to form the most number of bonds. Here, hydrogen can only accept one additional electron before it fills its outer shell. So nitrogen should be the atom that goes in the center. In placing these bonds, we used up six electrons of our eight valence electrons already because each single bond contains two electrons. So now, we have two electrons remaining to place in our structure. These electrons should go on nitrogen because hydrogen can only accept one more electron before it has a full shell. And it’s already accepted that electron in forming a bond with nitrogen.

I’ve redrawn this structure, using the more three-dimensional looking lines and wedges sort of notation. Since this molecule has three bonds to other atoms and one lone pair, we would describe its molecular shape as a trigonal pyramid. So this is the shape that we want to compare everything else to. Now let’s take a look at our other answer choices. Now let’s figure out the structure for the first answer choice, CCl₄. Carbon has four valence electrons. And the chlorines each have seven valence electrons. So we have a total of 32 valence electrons in this structure.

Next, we’ll place our atoms and connect them with single bonds. Carbon goes in the center here because it has the fewest number of electrons in its outer shell. Which means it can form the most number of bonds before its shell is full. Placing our bonds used up eight electrons. So we have 24 electrons remaining. If we place all of them around the chlorines, all of our atoms will have a full octet. If we look at our structure, we’ll see that our central atom has bonds to four other atoms. So the shape of this molecule is tetrahedral.

Now, let’s look at our next answer choice, H₃O⁺. Our hydrogens each have one valence electron. And oxygen has six. Since H₃O⁺ is a positive ion, that means it’s lost in electron. So we need to subtract one electron from our total number of valence electrons. This gives us the total of eight valence electrons for our structure. Next, we’ll place our atoms and connect them with single bonds. Oxygen goes in the center because hydrogen can only accept one more electron before it has a full outer shell. Placing our bonds used up six of our valence electrons. So we have two remaining electrons to place in our structure. It should go around the oxygen to fill its octet. Our structure has three bonds to other atoms around the central atom and one lone pair, which makes its molecular shape trigonal pyramid. This matches ammonia. So it’s most likely the correct answer choice. But let’s look at the other ones just in case.

Our next answer choice is H₂O. Hydrogens each have one valence electron. And oxygen has six, which gives us a total of eight valence electrons for this structure. If we place our atoms and connect them with bonds, we’ll use up four of our valence electrons. So we have four electrons left to place in our structure. They should go around the oxygen. If we look at our structure, we’ll see that we have two bonds to other atoms and two lone pairs. So the molecular shape for this molecule is bent.

Our next molecule is BF₃. Boron has three valence electrons. And fluorine has seven. This gives us a total of 24 valence electrons for our structure. Next, we’ll place our atoms, putting boron in the center because it has the fewest number of electrons in its outer shell. So it can form the most number of bonds. This has used up six of our electrons. So we have 18 electrons remaining to be placed. If we place these remaining electrons around the fluorines, we’ll fill the octets for the fluorines but run out of electrons before we can fill boron’s octet. Which is okay, because boron is a common exception to the octet rule. If we look at our structure, there are three bonds to other atoms around the central atom and no lone pairs. So the molecular shape for this molecule is trigonal planar.

Our last answer choice is NH₄⁺. The nitrogen has five electrons. And the hydrogen has one. Since this is a positive ion, it’s lost an electron. So we should subtract one electron from our total. This would give us eight valence electrons. If we place all of the hydrogens around the nitrogen and connect them with single bonds, we’ll use up all of our valence electrons. If we look at our structure, we have four bonds to atoms around the central atom. So the molecular shape for NH₄⁺ is tetrahedral.

If we look at all the structures that we’ve drawn, the one that matches ammonia or NH₃ is H₃O⁺, just like we suspected earlier. Both of them have a trigonal pyramid molecular shape. H₃O⁺ is answer choice B.

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