Main Group Elements: Density - Online Tutor, Practice Problems & Exam Prep

Understanding the density of main group elements is crucial in chemistry, particularly when considering their state of matter at room temperature (25 °C) and standard pressure (1 atmosphere). Generally, solids exhibit higher density than liquids, which in turn are denser than gases. However, water is a notable exception due to the expansion of its bonds during freezing caused by hydrogen bonding. The main group elements can be categorized by their state: solids (most elements), liquids (bromine), and gases (highlighted in red). Some elements, like hydrogen ($H_2$), nitrogen ($N_2$), and oxygen ($O_2$), are diatomic, existing naturally as pairs. Others, such as phosphorus ($P_4$), sulfur ($S_8$), and selenium (${\mathrm{Se}}_8$), are polyatomic, stable in larger groupings. When examining density trends, density decreases as you move up a group, but there is no consistent trend across periods. Exceptions like potassium and calcium have lower densities than expected due to their larger volumes, which impacts the density calculation, where density equals mass divided by volume. Remembering these patterns and exceptions is essential for understanding the behavior of elements under different conditions.

Phases

Hey everyone, So in this video we're going to take a look at the main group elements in terms of their density when looking at their phases. Now recall that at standard room temperature, which is around 25°C, and standard pressure which is 1 atmosphere, the elements can exist under three states of matter. Now we're going to say in terms of density, solids are more dense than liquids and liquids are more dense than gases.

Now of course in chemistry there are exceptions. We know that hydrogen liquid what will water liquid is more dense than water as a solid and it has to do with the bond expanding as it freezes because of hydrogen bonding. So water is an exception to this trend. But remember, generally speaking, solids are more dense than liquids which are more dense than gases.

Now here, if we take a look at the periodic table, First of all, we're only concerned with periods, one to six of the periodic table. The 7th row is made-up of a lot of slab created elements, heavy elements. They're too unstable, too unpredictable, so we don't worry about them. Next, we're going to see that there's a lot of solids in terms of this periodic table, the ones that are in Gray.

Next we have liquids and in terms of main group elements, only bromine is the one we're concerned with when it comes to standard temperature, standard pressure. Mercury is. The other one with Mercury is a transition metal. We're not worried about transition metals in terms of this topic. And then gases. We can see those in red represent our gases.

Now in addition to this, these elements exist in different forms within nature. Some of them exist as diatomic elements. These are elements that are stable in pairs when found in nature and for us to remember the diatomic molecules we say have No Fear of ice cold beer. Now this does not condone any type of irresponsible drinking. This is just a helpful memory tool to help you recall the diatomic elements.

So have hydrogen H₂ no N₂. Fear F₂ of is oxygen O₂ ice stands for in place of iodine, so that's I₂ Cold C is chlorine Cl₂ and B or B is bromine Br₂. These are your diatomic elements. Now if we take a look at this memory tool again, ice. What's the phase of ice? Ice represents a solid iodine under standard conditions. Is a solid? Then beer? What's the phase of beer? Beers. A liquid and bromine under standard conditions is a liquid. So this memory tool help you remember the diatomic elements and also help you to remember the phases.

All the other elements within this memory tool exist as gases. Now in addition to this, we have polyatomic elements. These are elements that are stable in numbers greater than two. So if we come here at the periodic table, we have phosphorus which naturally exists as P₄, sulfur which is S₈ and then selenium which is Se₈. So these are the natural forms in which they exist in nature.

The other elements which we didn't talk about, the main group elements, they exist together. Well, they exist by themselves in nature as monoatomic elements, so that's why we didn't refer to them here. Alright, so keep this in mind when it comes to density, the first thing we can look at is phases. So we're going to say here, solids generally are more dense than liquids, which are more dense than gases.

Main Group Elements: Density Example

Here in this example question it says which of the following exist as a diatomic liquid at room temperature. Alright so remember diatomic means that exist in nature as a pair. So have No Fear of ice cold beer. Bromine exist as a diatomic species.

Tellurium does not. Sulfur exists as a polyatomic element, it exists as assay so that wouldn't count. Chlorine is also diatomic as well as iodine. So the answer is going to be either AD or E.

Now remember in that say memory tool, have No Fear of ice cold beer. Beer exists as a liquid. B also stands in for bromine. Bromine here is our diatomic liquid at room temperature right? So remember that like handy memory tool to help us remember your diatomic elements and also you can rely on it to help you remember the phases of these different diatomic elements.

Which of the following diatomic molecules would be expected to have the greatest density?

• A. Chlorine
• B. Selenium
• C. Bromine
• D. Iodine
• E. Argon

Periodic Trend

Hey everyone. So here we're going to take a look at the periodic trend when it comes to the density of main Group elements. Here we're going to say periodic trends are used when asked to compare elements of the same phase or when they're in the same group. Now the periodic trend is density tends to decrease going up a group, but across a period there is no uniform trend.

So if we take a look here at this periodic table, again the 7th row, the seventh period is eliminated because it's 2 variables. A lot of those elements are too heavy or man-made, so they're very unstable. So we're paying attention to rows one to six. Now here going up a group, our density decreases generally speaking. But as we go from left to right either direction, it's too much variability. So we see that there's no uniform trend when we're looking across a period.

Now here, you might notice we have purple for calcium and potassium here because they also represent some type of difference in the overall trend. Here we're going to say they represent an exception. We're going to say that potassium and calcium have lower densities than sodium and magnesium because they have higher volumes than expected. So here as we're going up, we should expect our densities to decrease. So we should have expected potassium and calcium to have a higher density than sodium and magnesium, but that's not the case.

All you need to know on your part is that potassium and calcium just have larger than normal volumes. This affects their overall density because remember, density equals massvolume. Anything beyond that is beyond the scope of this course. So don't worry about that, all right. So just keep in mind when we're looking up a group, for the most part our density is decreasing and remember the exception that we have here.

Main Group Elements: Density Example

Without any given values, predict which the following would possess the greatest density. So here we have the periodic table. And remember, the general trend is as we head up a group, our density decreases. There is no uniform trend when we're going across a period though, so we won't pay attention to that.

Now here we have lithium, we have barium, we have magnesium, beryllium and sodium. We can see that four of them are grouped closely together and then on the very bottom we have barium. Remember, the general trend is as we head up a group, our density decreases.

Since barium is the furthest down, we'd expect barium to have the greatest density out of the options that are given. That means that option B, barium would be our final answer.

List the following elements in order of decreasing density under standard conditions:

a) Magnesium, Mg

b) Bromine, Br

c) Calcium, Ca

d) Barium, Ba

e) Hydrogen, H

• A. d > a > c > b > e
• B. d > c > a > e > b
• C. e > b > c > a > d
• D. e > c > b > d > a
• E. a > c > b > e > c

Here’s what students ask on this topic:

What element has the lowest density out of all the elements?

The element with the lowest density of all the elements is hydrogen. Hydrogen's density is just 0.08988 grams per liter at standard temperature and pressure (0°C and 100 kPa), which is less than one-thousandth the density of water. This extremely low density is due to hydrogen's small atomic mass (it has only one proton and one electron) and the fact that it exists as a diatomic gas (H₂) under normal conditions, meaning its molecules are composed of two hydrogen atoms. This makes hydrogen the lightest and least dense element on the periodic table. Its low density is one of the reasons why hydrogen is used in applications where weight is a critical factor, such as in lighter-than-air balloons and modern fuel cells.

What are the most dense elements?

The most dense elements are typically found at the bottom of the periodic table, as density tends to increase with atomic number due to the greater mass of the nuclei. Osmium and iridium are the two densest naturally occurring elements. Osmium has a density of about 22.59 grams per cubic centimeter, while iridium's density is approximately 22.56 grams per cubic centimeter. These high densities are due to their tightly packed atomic structures and high atomic weights.

Platinum, gold, and rhenium are also among the densest elements, with densities of 21.45, 19.32, and 21.02 grams per cubic centimeter, respectively. These heavy metals have a significant presence of electrons and protons, contributing to their mass and, consequently, their density. It's important to note that the density of an element can vary depending on its allotrope (different physical forms of the same element) and the conditions under which it is measured, such as temperature and pressure.

What are the three most dense elements?

The three most dense elements are osmium, iridium, and platinum. Osmium holds the title for the densest naturally occurring element, with a density of about 22.59 grams per cubic centimeter. Iridium is a close second, with a density of approximately 22.56 grams per cubic centimeter. Platinum comes in third, with a density of around 21.45 grams per cubic centimeter. These densities can vary slightly depending on the sample and its conditions. These elements are all transition metals, found in the middle of the periodic table, and are known for their high melting points, hardness, and, notably, their high densities. They are often used in applications that require materials with extreme durability and resistance to corrosion, such as in high-end electronics, industrial catalysts, and jewelry.