Main Group Elements: Boiling & Melting Points - Online Tutor, Practice Problems & Exam Prep

• Boiling Point (BP) and Melting Point (MP) are defined as the temperatures at which a substance changes from liquid to gas and solid to liquid, respectively.
• In groups 1A to 4A of the periodic table, both BP and MP generally increase as you move from left to right across a period and from bottom to top within a group.
• Group 2A is excluded from the general trend due to irregularities and discrepancies in BP and MP values.
• In groups 3A, certain elements lack consistent melting point data due to variations in bonding types and atomic sizes.
• From groups 5A to 8A, both BP and MP tend to decrease when moving from left to right across a period and from bottom to top within a group.
• Hydrogen, typically placed in group 1A, can also be categorized in group 7A due to its similar bonding characteristics and electron configuration to halogens, aligning with the BP and MP trends of group 7A.

Boiling Point and Melting Point

Here we're going to say that boiling point and melting point, abbreviated BP and MP respectively, are the energy used to change an element from a liquid to gas when we're talking about boiling point and from a solid to a liquid when we're talking about melting point. Now here we're going to have two overall trends that we're looking at and that's periodic, periodic trend one and periodic trend 2. To follow these trends, we're going to split our periodic table into two.

In the first part, we're paying attention to groups 1A to 4A. Again, we're only concerned with our main group elements, so we're not going to pay attention to our transition metals here. We're going to say going from group 1A to 4A, the boiling point and melting point tend to increase moving from the from left to right across a period and going up a group. So as we're heading towards this carbon, this is the overall trend. We'd expect our melting point to increase and our boiling point to increase.

Now let's look at the numbers here. We have lithium, it's boiling point is 1347 and that's in degrees Celsius. Its melting point is 180°C. Now here if we're looking, we can see that we're going from 670-1688, 760-6881 to 1347. We can see the general trend as we're going up this group is this increasing and this is a general trend that we typically see also in groups 1A and 4A. Now notice melting point as we're going up, we can see those are generally increasing as well.

If you are pretty observant, you'll see that, hey, why didn't you list any boiling point and melting points for group 2A? Well, that's because within that group, because of numerous reasons and again we don't need to go delve into them directly, we're going to have huge discrepancies. There's not going to be an overall trade that's easy to see. We're going to see that it can increase as we go up and then decrease and then go back up again. The pattern is not uniform. So we just take away our boiling point and melting points from this group.

Now here as we go to three A, we see the overall trend. We'd also see that in these last four here, they don't have any melting points present because again there are exceptions here that are arising because of differences in bonding type as we're going from metallic to covalent network bonding as atomic size is changing. So there's a lot of reasons why there isn't a uniform pattern for melting point for these four. So we just ignore those as well. That's why the overall trend, when we pay attention to everything else, is increasing.

Now if we look to group 5A to 8A, there's something you'll also notice. Typically we see hydrogen here in Group 1A, but hydrogen can also exist or act as a halogen. It's one electron away from being just like helium. It has similar bonding characteristics of the halogens when it comes to melting point and boiling point. We can put it as part of group 7A because it's overall pattern in terms of boiling point and melting point matches up. We're going to say periodic trend 2 going from 5A, to 8A. Boiling point and melting point tend to decrease moving from left to right across a period and going up a group.

Here we have exceptions with these. Here their pattern is all over the place, so we don't worry about the overall trend for them. But once we take a look at all the other elements, we can see that the overall trend is as we go up a group, we're going to have a decrease in boiling point and melting point. And as we go from flooring to hydrogen, you see that that continues. We see that this overall decrease in boiling point and melting point. So that's why we're allowed to place hydrogen here, right?

So from these two images, there's a lot that's being shown to us. But just remember, from groups 1A to 4A, our boiling point and melting point overall is increasing as we go up a group to the right. From group 5888-A, our overall trend for boiling point and melting point is that they decrease as we head towards the top right. So keep that in mind when asked to compare or observe the differences in boiling point and melting point for our main group elements.

Main Group Elements: Boiling & Melting Points Example

Under standard conditions, which element will be most easily vaporized? Now when we say most easily vaporized, that means which one becomes a gas most easily and between boiling point and melting point. This means we're talking about boiling point because that's where we go from a liquid to a gas. The one that's most easily changed into a gas would be the one with the lowest boiling point.

Now if we take a look at our options here, we have carbon, silicone, potassium rubidium and cesium. If you look at your periodic table, remember that carbon and silicone both can do covalent network bonding. This results in higher than normal boiling points and melting points, so those would not be our answers.

If we look at potassium orbidium and Cessium looking on the periodic table, you'll see that they're all in Group 1A. So we have potassium, rubidium, and cesium. And remember, for groups 1A to 4A, as we go up into the right of the periodic table, our boiling point increases. Here we're looking for the lowest boiling point, so that means the one lowest down, which would be cesium. So here our final answer would be option E.

Which of the following Group 5A elements would be expected to have the highest melting point?

• A. Nitrogen, N
• B. Phosphorus, P
• C. Oxygen, O
• D. Antimony, Sb
• E. Bismuth, Bi