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Course: AP®︎/College Chemistry > Unit 6
Lesson 1: Types of chemical bondsMolecular solids
Molecular solids are composed of discrete molecules held together by intermolecular forces. Because these interactions are relatively weak, molecular solids tend to be soft and have low to moderate melting points. Molecular solids are also poor conductors of electricity because their valence electrons are tightly held within each individual molecule. Created by Sal Khan.
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In the last part of the video (6:00onwards) Sal asks to compare the melting points of molecular Iodine (I2) to that of molecular Chlorine (Cl2)
The video states that because Iodine has a large atomic radius and is therefore a larger atom, it is more polarizable with greater dispersion forces and therefore has a higher melting point that the chlorine molecule....
But if we compare the atomic radii of Cl vs. I, because I is in a lower period (has more shells) than Cl, we can assume that Iodine's electrostatic force is lower than that of Cl (because it has a greater atomic radius that is inversely related to electrostatic force); thereby suggesting that Iodine should have a lower melting point than the molecular Chlorine...
I don't understand Sal's explanation of why Iodine's melting point is indeed higher than Chlorine's. How does it contrast to the logic/answer I deduced? How/why am I looking at it incorrectly?
Thank you!
Please note that - I haven't studied yet intermolecular forces - because I am following the Chemistry syllabus and no videos on these topics have been shown yet (already posted a tip suggesting something be done to address this as I am surely not the only one that was lost by the sequential order and content of this video).(12 votes)
You note you haven’t got to intermolecular forces yet, it may be a good idea to wait until you get to those videos then maybe revisit this question.
You mentioned electrostatic force so Coulomb’s law but that’s relevant to ionic compounds, but this video is dealing with covalently bonded molecules.
When you get to intermolecular forces you’ll learn about dispersion forces which are stronger with more electrons and larger atoms. As iodine atoms are significantly larger and have significantly more electrons than Cl atoms, this is why I2 has a higher melting and boiling point than Cl2.(11 votes)
Why is I more polar just because it is larger? I remember previous videos mentions the electronegativity of elements decreases as they go from top to bottom; thus, Cl should be more polar than I. Right?(3 votes)
Electronegativity and polarizability are are different effects at play here. Electronegativity describes how well an atom attracts a pair of shared electrons in a covalent bond to itself. While polarizability describes essentially how well charges can be distorted on an atom in the presence of an external electric field. Electronegativity is used more when dealing with dipole-dipole intermolecular forces, while polarizability is used when dealing with London dispersion forces.
In a sample of molecular I2, every atom has the same electronegativity value because they are all the same element. Every molecule of I2 has no dipole moment because both atoms of iodine pull on the electrons in the covalent bond equally as much making each molecule non-polar. Same goes for a sample of molecular Cl2. So even though chlorine is more electronegative than iodine, it doesn't matter because both samples are nonpolar because they are composed of the same respective chemical elements.
Polarizability views the atoms as spheres with negative electric charges on the surface of the sphere from the electrons. Having a larger atomic radius in the case of iodine because of its higher shell number means the electrons are more easily able to be pushed around the atom by outside electric fields. These outside electric fields coming form the electrons of other I2 molecules. Since the electrons are more easily able to move to one side of an I2 molecule because of the repulsion from another I2's electrons this creates a temporary dipole moment in the molecule. This type of temporary dipole moment between non polar molecules (all molecules though really) is what we call London dispersion forces. A bigger atomic radii in iodine as compared to chlorine is what causes this increased polarizability, hence greater London dispersion force and a higher melting point. Hope that helps.(8 votes)
Sal only uses London Dispersion forces in his examples. But it is possible to have molecular solids held together with other IMFs, right? For example, solid H2O (ice), which consists only of non-metals, is held together with hydrogen bonds...? Right?(4 votes)- Yes. It has both dispersion and hydrogen bonds.(4 votes)
Why are molecular solids insoluble in water?(2 votes)- Well that's not strictly true, there are examples of molecular solids which are soluble in water. The simplest being sucrose, otherwise known as table sugar, and caffeine which are found in beverages composed mostly of water. Really what determines whether a solid will dissolve in water or not is if it is polar.
Chemicals dissolve in other chemicals based on their polarity and generally follows the rule 'like dissolves like'. Where polar solids dissolve in polar liquids, nonpolar solids dissolve in nonpolar liquids, but nonpolar solids do not dissolve in polar liquids (also polar solids do not dissolve in nonpolar liquids). Water is polar molecule and will generally only allow other polar molecules to dissolve into it. So a molecular solid like solid iodine, I2, will not dissolve in water because iodine is nonpolar, but sucrose will dissolve in water because sucrose is sufficiently polar (it has a lot of polar -OH groups on it).
Hope that helps.(5 votes)
As stated in the video, Carbon Dioxide has a solid state on earth, commonly known as dry ice, and goes through sublimation, so there is no liquid state; however, isn't it possible to have a liquid form at a higher pressure? I know changing the pressure affects molecules' properties, like how the boiling point of water is lower at higher altitudes, causing lower atmospheric pressure, so wouldn't increasing the pressure raise the boiling point making liquid Carbon Dioxide possible? Thanks(2 votes)- Whether a substance has a liquid phase at normal atmospheric conditions or not depends on the triple point of that substance. The triple point is the exact pressure and temperature in which a chemical exists in all three physical states at once; solid, liquid, and gas. It’s also the minimum pressure required for a substance to exist as a liquid. For carbon dioxide its triple point is -57.57 °C and 5.11 atm.
So at normal atmospheric conditions of ~1 atm, there simply isn’t enough pressure for carbon dioxide to exist as a liquid to it sublimates (and deposits) instead. So to have liquid carbon dioxide, you would have to subject it to a pressure of at least 5.11 atm in a vessel.
Hope that helps.(2 votes)
What temperatures are considered to be "low" or "high" melting points?(2 votes)- High and low melting temperatures are admittedly a vague description with no single value. Tungsten for example has a melting point of 3422°C and would safely be consider a high melting point by almost everyone. But zinc has a melting point of 419.53°C, which if looking at other metals is comparatively low, but when compared to most organic molecules is much higher. As low as zinc is, if we consider handling molten zinc, then most people would consider that a high temperature because we’re most accustomed to weather temperatures. Saying high or low is a quantitative description which can be subjective.
Hope that helps.(2 votes)
- Are molecular solids crystalline or amorphous? Why is that?(2 votes)
- Crystalline means there's a repeating pattern to your solid and forms a crystal lattice, while amorphous means there is no pattern and they do not form a defined geometric shape. Molecular solids would belong to the crystalline group which form lattices weaker than that of lattices in ionic solids. This is because ionic crystals are held together by strong electrostatic attractions while molecular solids are held together by comparatively weak intermolecular forces. Hope that helps.(3 votes)
- Isn't Boron a nonmetal too? Sal didn't include it under nonmetals when marking the nonmetals on the periodic table.(2 votes)
- How do you distinguish between covalent network solids and molecular solids when looking at the chemical representation? like SiC and SiO or something like that?(2 votes)
this question isnt related to the video but its part of my syllabus and i dont understand so can someone explain this statement to me:
"Molecular ions are compounds comprising atoms that have been covalently bonded together but contain unequal numbers of protons and electrons. Common examples include hydroxide (OH^–), nitrate (NO3^–) and sulfate (SO4^2–).
If there is more than one molecular ion in the compound, then brackets are required to make sure the reader can understand which ions are involved. This is because you need to put a bracket around the molecular ion to show how many of the whole molecular ions there are in the compound.
ionic compounds can also form between two or more ionic molecules. The formula of an ionic compound involving a molecular ion can be worked out in the same way as that of an ionic compound formed when a metal and non-metal react."
thanks(1 vote)
6:00
Video transcript
- [Instructor] So let's talk a little bit about molecular solids. So just as a little bit of review, we've talked about ionic solids where ions form these lattices. So those might be the positive
ions right over there, and then you have your negative ions. And the negative is
attracted to the positive. The positive is attracted to the negative. And I'm just showing a
two-dimensional version of it, but it forms a three-dimensional lattice. So that's an ionic solid. We have also seen metallic solid where you have metals that all contribute some valence electrons
to the sea of electrons. So what you end up having is essentially these positive cations that are in this sea of electrons. And we've talked about those properties, very good at conducting
electricity, malleable, et cetera. Now, what we're gonna do
is talk about what happens when you have nonmetals. So the nonmetals, you can see
in yellow right over here, also includes hydrogen. Now, of course, noble
gases are also nonmetals, but they're not reactives. So we're gonna talk about
the reactive nonmetals. They can form molecules with each other. For example, one iodine can bond to another iodine with covalent bonds. So you could have a molecule like I2. You have things like carbon dioxide. Each carbon can bond to two oxygens. These are each molecules formed due to covalent bonds between nonmetals. Now, when we talk about molecular solids, we're talking about putting
a bunch of these together. So let's say putting a bunch
of iodine molecules together, and the intermolecular forces at a sufficiently low temperature are sufficient to hold together
those molecules as a solid. So what do I mean by that? Let's look at a few examples. This right over here is a
picture of solid iodine, and the way it's made up is you
have these iodine molecules. Now, each of these iodine
molecules are formed by a covalent bond
between two iodine atoms. Now, the reason why it's a solid is there's enough dispersion forces. We talked about these
London dispersion forces that are formed by temporary dipoles inducing dipoles in neighboring molecules. For example, just by random chance, for a moment, you might
have more electrons on this end of this iodine molecule, creating a partially negative charge. And then that means some of the electrons on this end of this
neighboring iodine molecule might be repulsed by that negative charge, so it forms a partially positive charge. And so you have a temporary dipole inducing a dipole in the
neighboring molecule, and then they'll be
attracted to each other, and we've talked about that
as London dispersion forces. And at a sufficiently low temperature, that can keep them altogether as a solid. Now, it's important to point out, I keep saying sufficiently low temperature because these molecular solids, because they are only held together not by the covalent bonds, the covalent bonds hold
together each of the molecules, but the molecules are held together by these fairly weak dispersion forces. They tend to have relatively
low melting points. For example, solid iodine right over here has a melting point, has a melting point of 113.7 degrees Celsius. And I know what you're saying. That's not that low. That's higher than the
temperature at which water boils. It would be quite
uncomfortable for any of us to be experiencing 113.7 degrees Celsius. But this is relatively low
when you talk about solids. Think about the temperatures it requires to melt, say, table salt. We've talked about that. Think about the temperatures
it takes to melt iron. There, you're talking
about hundreds of degrees, in certain solids, thousands
of degrees Celsius. This is much lower. And so as a general principle, molecular solids tend to have
relatively low melting points. Now, how good you think they're gonna be as conductors of electricity? Pause the video and think about that. Well, in order to be
conductors of electricity, somehow charge needs to
move through the solid. And unlike metallic solids, you don't have the sea of electrons that can just move around, so these tend to be bad
conductors of electricity. If you wanna see another
example of a molecular solid, this right over here is
solid carbon dioxide, often known as dry ice. What you see here is each of
these molecules, each carbon, is bonded to two oxygens. It has a double-bond with
each of those oxygens. These are covalent bonds that form each of these molecules. But what keeps all of the
molecules attracted to each other is, once again, those dispersion forces. And these forces between
the molecules are so weak that solid carbon dioxide
doesn't even really melt. It doesn't even go to a liquid state. If you heat it up enough to overcome these intermolecular forces, these dispersion forces, it will sublime, which means it goes directly
from a solid to a gas state, and it does that at a
very low temperature. It sublimes at negative 78.5 degrees Celsius. And if you've ever handled a dry ice, which I don't recommend
you doing without gloves because it will hurt your
skin if you do touch it, I actually did that recently
at my son's birthday party, we were playing around with dry ice, you don't mess around with this thing because it is so incredibly cold. And at that temperature,
it will go from a solid. It won't even melt to a liquid state. It will go straight to a gas state. Now, the last thing I wanna do is think about why
different molecular solids will have different melting points. So let's compare, for
example, molecular iodine to molecular chlorine. Each of these can form molecular solid. We looked at iodine a few minutes ago. Which of these would you think
would form molecular solids with higher melting points? Pause the video and think about that. Well, as we talked about
it, each of these molecules, they're formed by covalent
bonds between two atoms, and what keeps the solid together are these dispersion forces. In an earlier videos, when we first talked
about dispersion forces, we talked about temporary
dipoles and induced dipoles, and they were likely to
form between heavier atoms and molecules because they
have larger electron clouds and are more polarizable. So if you compare molecular
iodine to molecular chlorine, you can see that iodine is
clearly made up of larger atoms and is therefore a larger molecule, which is more polarizable. So it's larger, which means it's more
polarizable, generally speaking, polarizable, which means it has stronger, generally speaking, dispersion forces, stronger dispersion forces. Now, just as a reminder,
these dispersion forces are between molecules. Each molecule has a covalent
bond between two iodines, and then the dispersion forces
are between the molecules. But because it has
stronger dispersion forces, we would expect that a
molecular solid formed by iodine is gonna have a higher melting point than a molecular solid formed by chlorine. And I actually do have the numbers here. So the melting point of a molecular solid formed by iodine, we've already talked about that, that's 113.7 degrees Celsius, while the melting point
of a molecular solid formed by molecular chlorine has a melting point of
negative 101.5 degrees Celsius, which is very cold, and so iodine has a higher melting point because of the stronger dispersion forces. Now, as I said, those dispersion forces are still not that strong. This is still not that
high of a temperature compared to melting points
of other types of solids we have looked at in the past.