Equatorial and Axial Positions - Online Tutor, Practice Problems & Exam Prep

In molecular geometry, the arrangement of electron groups around a central atom affects the compound's stability and energy. For molecules with five or six electron groups, the positions of these groups are categorized as equatorial or axial. Equatorial positions are around the central atom's equator, while axial positions are above or below this plane. Generally, the more electronegative elements prefer the axial positions due to energy considerations, leading to greater stability. Lone pairs in six electron group systems are most stable in the axial positions, whereas in five electron group systems, they are most stable in the equatorial positions. A mnemonic to remember this is associating six o'clock with axial positions for six electron groups and five o'clock with equatorial positions for five electron groups, aiding in the visualization of the most stable molecular structure.

Equatorial and Axial Positions Concept 1

Covalent compounds with five or six electron groups have Equatorial and axial positions for their surrounding elements. Now when we say the Equatorial position, we're going to say the Equatorial position. These are your surrounding elements position around the equator of a compound. So here we take a look at these two illustrations. We can say that the equator of this sphere is right here. And here we have 5 electron groups. Three of them are along the equator. Here we have 6 electron groups and four of them are along the equator of this sphere.

Now if we say axial or apical position, this is basically a surrounding element's position above or below the Equatorial position. So remember we have our equator here, so our axial positions are above it or below it and here above it or below it. Now these arrangements themselves, they increase repulsion between elements. This in turn causes a decrease in energy for the compounds. And just remember, in chemistry, if we decrease our energy, that's a good thing that leads to greater stability.

Now a rule of thumb is we're going to say that the more electronegative element tends to prefer the actual position over the Equatorial position. So this again ties into the whole idea of energy and stability. So just remember these fine points when we're talking about 5 electron group molecules and 6 electron group molecules.

Equatorial and Axial Positions Example 1

Based on your knowledge of axial and Equatorial positions, draw the most likely structure of PF₂Cl₃. All right. So remember when it comes to drawing Lewis structures, we place the least electronegative element in the center. In this case, it'd be phosphorus. Phosphorus has attached to it 5 bonding groups, 2 fluorines and three chlorines, so it's a 5 electron group system. Now phosphorus and group five base, so this makes sense.

Remember that three of them would be along the equator and two of them would be in the axial position. Now remember in terms of stability and energy, we want the more electronegative element to be in the axial position. Fluorine is more electronegative than chlorine, so fluorine we place in the axial positions. So remember fluorine's in Group 7A, so it only has seven valence electrons, halogens like chlorine and fluorine.

When they're not in the center, they only make single bonds as surrounding elements, and then we draw the chlorines. Chlorines are in Group 7A as well, so they have 7 valence electrons. Then he were drawing them here there along the equator. So this would be the most stable, most likely structure of PF₂Cl₃ where we have a 5 electron system and therefore we have Equatorial axial positions.

Fluorine is more electronegative than chlorine. So remember fluorine would go into the axial positions and chlorine would go into the Equatorial positions.

Equatorial and Axial Positions Concept 2

Lone pair positions are extremely important when trying to draw the most accurate molecule. Here we're going to say that lone pairs will orient themselves in order to decrease the interactions between surrounding elements. Now here when we're drawing different structures, we're paying more attention to those with six electron groups and those with five electron groups.

We're going to say here that with six electron group systems, lone pairs are most stable in the axial position. Within 5 electron group systems, lone pairs are most stable in the equatorial position. Now you might ask, how do I remember this? Well, we have a memory tool for that, and the memory tool is. Just remember, it's a lock as long as you remember the hands of the clock.

So if we take a look here at the first clock, we have the hands giving us a time of 6:00. Now let's just imagine that that circle in the middle, that dark circle, represents our central element. And here this would represent our equator. We're going to say we said when it deals with six electron groups, they are in the axial positions. So remember, axial positions are straight up and straight down, so above and below the equator of our structure. And if we look, they're pointing at 6:00, so we're going to say 6:00 is for six electron groups. The hands are pointing straight up and straight down, just like axial positions point straight up and straight down.

For five electron group systems. Again, we imagine that we have our equator. Here, both the hands are not pointing straight up and straight down. So they wouldn't represent axial positions, they represent equatorial positions. So here we'd say that 5:00 is for five electron groups, right? So just remember, you can remember 6:00 straight up and straight down. Those are actual positions. And you remember lone pairs are most stable in the axial positions. If it's at 5:00, that means both hands are not straight up and straight down. So they couldn't be actual positions, they would be equatorial positions.

So in those cases, lone pairs should be oriented in the equatorial positions to draw the best possible structure. So keep that in mind when we deal with five electron group systems and six electron group systems.

Equatorial and Axial Positions Example 2

For this example question, it says determine the molecular geometry of the following ion. So here we're dealing with SCl₃^-. All right, so here we're going to place sulfur as our central element. Sulfur itself is in Group 6A, so it has six valence electrons, and here it's going to use three out of the six to make connections to our three chlorines. Chlorines are in Group 7A, so they each have 7 valence electrons. They use one of their electrons to make a single bond to our sulfur atom and we're going to say this is what our structure would look will look like.

So the chlorines have used all their electrons available sulfur is only used three out of its six total electrons. So here we're going to draw its remaining three. Now we're going to go back and clean this up based on our understanding of five electron systems versus 6 electron systems. So right now, let's just draw the basic framework of our structure. So there goes 5 valence electrons, and here's the 6th. -1 means we gained an outside electron. The chlorine's no longer need an extra electron. They're all making the right amount of bonds that they want. So that extra electron that we're gaining, because it's -1, it's going to go to the sulfur. And because our structure has a charge, we need to draw it in brackets with the charge on the outside.

Now we need to determine is this a 5 electron system or a six electron system? Well, if we look, we're going to say that we have 1, 2, 3, 4, 5 electron groups, so this is a 5 electron group system. So think about our memory tool. It's a long, as long as you remember the hands of the clock. So five groups means we're looking at 5:00. Both hands aren't pointing straight up or straight down South. They're not in the axial position, They'd be in the Equatorial position. That means our lone pairs should be oriented in the Equatorial position to draw the best possible structure.

So here we're going to place Sulfur in the center. So going to draw the correct structure over here. Sulfur goes in the center. The 2 lone pairs that it has, we have to draw them in the Equatorial position, and one of the chlorines will also be in the Equatorial position as well. So there it goes. And then the other two are going to be in the axle positions pointing straight up and straight down. OK, so there goes our structure. Now remember draw this one as our extra electron. Remember because the structure has a charge and put it in brackets with the charge on the outside. So this would be the correct way of drawing SCl₃^-, right?

So just remember when you're dealing with the 6th electron group system or five electron group system, it's important to remember where should my lone pairs go. Knowing that gives you the best possible structure.

Draw the most likely shape for the following compound:XeF₄

Draw the most likely shape for the following molecule. So here we have xenon tetrafluoride. Xenon is in group 8A, so it has 8 valence electrons. And remember, fluorines only make single bonds, so they only need 1 of the valence electrons each to make a bond with xenon. So here go our fluorines. Our sticks are fluorines. And then remember, xenon is in group 8A, so it has 4 more electrons we haven't drawn, so we draw them here and we draw them here. Now, if we take a look here, we'd see that this is a 6 electron group system, and the way I've drawn it makes sense. Because it's a 6 electron group system, that means it's most stable when our lone pairs are in the axial positions. So here we have our 4 fluorines in the equatorial position, and in the axial positions above and below the equator will be our 2 lone pairs. Alright. So this would be the best way to draw xenon tetrafluoride.

Draw and determine the geometry for the following molecule:Br₂CO

It's just to draw and determine the geometry for the following molecule, Br2CO. So here, carbon will go in the center since it's the least electronegative. And remember carbon is a group 4A, so it has 4 valence electrons. Halogens, when they're not in the center, only make single bonds. Our bromines are the halogens. They're in group 7A, so they make a single bond with the carbon. Remember about carbon, carbon wants to make 4 bonds. So, in order to do that, it has to form a double bond to the oxygen. Oxygen is in group 6A, so it has 6 valence electrons. It uses 2 of them to form a double bond with carbon, and it keeps its 2 lone pairs next to itself. Here, this is neither a 5 electron group system nor a 6 electron group system. This is the way we draw the structure itself though. So it's always good to keep practicing this. In this particular example, we have carbon connected to 3 surrounding elements or what we call bonding groups.

How many lone pairs reside in the equatorial position of the KrCl₅⁺ ion. 

• A. 0
• B. 2
• C. 1
• D. 3
• E. 4

Here’s what students ask on this topic:

What are the axial and equatorial positions?

Axial and equatorial positions refer to the locations of atoms or groups in a molecule that has a three-dimensional geometric structure, particularly in cyclic structures like cyclohexane. Imagine a cyclohexane molecule as a chair, which is the most stable conformation for this six-membered ring.

In this "chair" form, there are two types of positions for the hydrogen atoms or substituents attached to the carbon atoms of the ring:

1. Axial positions: These are oriented parallel to the axis of the ring, which you can think of as being perpendicular to the imaginary plane that the ring lies in. If you visualize the ring flat on a table, axial substituents would either point straight up or straight down, like the legs and backrest of a chair.
2. Equatorial positions: These are located around the equator of the ring, almost in the plane of the ring itself, or slightly above and below it. They are angled outward and are more spread out, akin to the seat and armrests of a chair.

The difference in these positions is important in chemistry because they can influence the molecule's reactivity and stability, particularly when larger substituents are involved, due to steric hindrance and strain.

How can one determine equatorial and axial positions?

To determine equatorial and axial positions, it's essential to understand the geometry of the molecule in question. Typically, these terms are used in the context of trigonal bipyramidal molecular geometry, such as in the case of phosphorus pentachloride (PCl₅).

In a trigonal bipyramidal structure, there are five positions around the central atom. Three of these positions lie in the same plane and form an equilateral triangle; these are the equatorial positions. They are spaced at 120-degree angles from each other. The remaining two positions are above and below this plane, aligned along the vertical axis; these are the axial positions.

To determine which is which, look at the molecule's 3D structure:

• Equatorial positions: They are in the horizontal plane and tend to have more space around them.
• Axial positions: They are aligned vertically, along the central atom's axis, and typically have less space due to the linear arrangement with the central atom.

The distinction is important because it can affect the molecule's physical and chemical properties, such as dipole moment and reactivity.

What is the difference between axial and equatorial positions?

Axial and equatorial positions refer to the locations of atoms or groups in a molecule with a trigonal bipyramidal geometry, which is common in molecules with five atoms or groups around a central atom.

Axial positions are aligned parallel to the principal axis of the molecule. Imagine a molecule shaped like a seesaw; the axial positions are like the seats of the seesaw, located at the top and bottom, along the imaginary vertical axis passing through the center of the molecule. These positions are often less stable due to increased electron repulsion, as they are 90 degrees relative to more atoms compared to the equatorial positions.

Equatorial positions, on the other hand, lie along the plane perpendicular to the axial positions. In our seesaw analogy, these would be like the handles on the sides of the seesaw. There are three equatorial positions forming a triangle around the central atom in the middle plane of the molecule. These positions are usually more stable because they experience less electron repulsion, being 120 degrees apart from each other and 90 degrees from only two axial positions.