Chemical Bonds - Online Tutor, Practice Problems & Exam Prep

Chemical bonds are categorized into intramolecular bonds, which exist within a molecule, and intermolecular bonds, which occur between different molecules. Intramolecular bonds include ionic bonds, where electrons are transferred between atoms with opposite charges, and covalent bonds, where electrons are shared. Covalent bonds can be polar or nonpolar, depending on the electronegativity difference. Intermolecular forces include hydrogen bonds, dipole-dipole interactions, and van der Waals forces, which arise from temporary dipoles. Understanding these concepts is crucial for grasping molecular interactions and properties.

Chemical Bonds Introduction

We're going to quickly cover chemical bonds. So you guys already know that chemical bonds are how individual atoms interact with one another and how they link and connect to other atoms. And really, chemical bonds fall under one of two categories, either intramolecular bonds or intermolecular bonds. Now, intramolecular bonds are bonds that exist within a molecule, whereas intermolecular bonds exist between different molecules. And so these two words sound similar, and what helps me distinguish between them is that intramolecular bonds are trapped within a molecule, whereas intermolecular bonds are not trapped, and instead, they exist between different molecules. So, in our example below, we're going to label the chemical bonds appropriately. And notice we have two different HCl molecules down below, and this bond here between the H and the Cl, the chlorine atoms, this is trapped within the molecule. So this must be our intramolecular bond. Now, the other bond shown here with the dotted line that exists between two different HCl molecules is not trapped, so this is an intermolecular bond. In our next video, we're going to talk about specific intramolecular bonds, and that includes ionic and covalent bonds. So I'll see you guys in that video.

Ionic & Covalent Bonds

So you guys already know that ionic and covalent bonds are types of intramolecular bonds that occur within a molecule. And recall that ionic bonds are interactions between atoms that have opposite charges due to losing and gaining electrons, but not sharing electrons. And recall that covalent bonds occur when 2 atoms share a pair of electrons, and those electrons can either be shared equally or unequally. If the electrons are shared equally between 2 atoms, then this forms a nonpolar covalent bond, a nonpolar covalent bond. Whereas, if the electrons are shared unequally, then this forms a polar covalent bond. And what determines the polarity of a bond is actually the difference in electronegativity, electronegativity between 2 atoms.

And so, in our example below, we're going to consider the 3 types of bonds. In the left column, we have ionic bonds. So, we know that ionic bonds do not share electrons, so we can put no in here. And in fact, what they do is they transfer electrons. So there's a transfer of electrons, but no sharing. Now, covalent bonds, regardless of the type, we know that there is sharing of electrons, so we can put yes in here. Now, for polar covalent bonds, we know that there is unequal sharing of electrons, whereas for nonpolar covalent bonds, we know that there is equal sharing of electrons. Now, for the electronegativity of the atoms, in ionic bonds, the atoms differ greatly in electronegativity, and there's actually a transfer of electrons. Now, for polar covalent bonds, there's also a significant difference, and in nonpolar covalent bonds, the atoms have either the same electronegativity or a very, very similar electronegativity.

And so, for examples, recall that the classic example of an ionic bond is table salt or sodium chloride, where the sodium has one less electron and the chlorine has one additional electron than proton, and so that gives them their charges. And the opposite charges are what keep this molecule intact or allow them to create an ionic bond. Now over here for polar, we know that water molecules are polar molecules because they have an oxygen and hydrogen. Oxygens are super electronegative, whereas hydrogens are not very electronegative, and so oxygens are going to have a partial negative charge and the hydrogens are going to have a partial positive charge, and that will occur on both hydrogens. And, over here with this molecule, the carbon dioxide, there are also polar bonds. So, you can see the carbon here is going to have a partial positive charge, whereas the 2 oxygens are going to have partial negative charges. But because the polarity is going in opposite directions, so you have electron density being pulled in opposite directions, the overall molecule, even though it contains polar covalent bonds, the overall molecule is nonpolar. So for a molecule to be considered polar, you also have to consider its shape. And so this is a nonpolar molecule that contains polar bonds. And then for nonpolar covalent bonds, we have this hydrogen gas molecule where we have 2 hydrogen atoms that have the same electronegativity, and so that creates a nonpolar bond where they share electrons equally.

So in our next video, we're going to recap intermolecular bonds. So, I'll see you guys in that video.

Intermolecular Forces

Recall that non-covalent intermolecular forces include hydrogen bonds, dipole-dipole interactions, and van der Waals forces. Recall that hydrogen bonds are interactions that involve a hydrogen atom, as well as electronegative atoms such as nitrogen, oxygen, or fluorine. Now a dipole itself simply explains a shift in the electron density that results due to electronegativity differences between two atoms. So, if you have a dipole on one molecule where there's an electron density shift, and then you have another dipole on a different molecule, that can create a dipole-dipole interaction between the two molecules, and so, that's what we see here. Now, van der Waals forces are forces that exist between all molecules because they result from instantaneous dipoles that can exist at any moment in any molecule.

So, down below in our example, we're going to label each of these diagrams with the appropriate intermolecular force. And so here, with these carbonyl groups on these different molecules, notice that there is a dipole moment going in this direction towards the oxygen in both molecules to create a partial negative charge on the oxygens and a partial positive charge on the carbons. And so, there's an attraction or an interaction between the partial positive carbon here and the partial negative oxygen on separate molecules, and so that is our dipole-dipole interaction. Now, over here on the right, we have water molecules, which are also polar molecules and have their dipoles. And so, each of these applies to all of these water molecules. And so, there's going to be a specific type of dipole-dipole interaction called a hydrogen bond between the hydrogen atoms on one molecule and the oxygen atoms on a different molecule, and so that is what a hydrogen bond is. And so notice that a single water molecule here can form up to four hydrogen bonds. So here, we will put hydrogen bonds.

Now, for our last example, we have these two nonpolar molecules, these two nonpolar hydrogen gas molecules, and so, overall, they don't have any partial charges because they have a similar electronegativity and there's no shift in electron density. There are no dipoles. However, at any single instant, there could actually be an instantaneous dipole where one of these hydrogens could have a partial negative, and the other could have a partial positive charge. And so, this is where our van der Waals forces come into play, existing between all molecules. This is a good summary of our intermolecular forces, and I'll see you guys in our practice videos.

Which of the following is classified as a nonpolar molecule?

• A.
• B.
• C.
• D.

Identify the types of chemical bonds present in scenarios #1 & 2 below:

So this practice problem wants us to identify the types of chemical bonds that are present in scenarios number 12 below. And so in scenario number 1 here with the pink background, we have this methane molecule. And each of these lines represents a covalent bond. And more specifically, these are non-polar covalent bonds. And all that means is that electrons are being shared equally between the carbons and the hydrogens, because the difference in electronegativity between carbon and hydrogen is really, really small. So, carbon and hydrogen have about the same electronegativity. They share electrons because of that, and that makes them nonpolar. So all carbon-hydrogen bonds are going to be nonpolar, and that's important to remember as we move forward in our course. Now, comparing that to this ammonia molecule in scenario number 2, each of these lines also represent covalent bonds, but these are polar covalent bonds. And that's because the electrons between the nitrogen and the hydrogens are being shared unequally. They are not being shared equally. And so, that's because the difference in electronegativity between nitrogen and hydrogen is very large, and nitrogen pulls a lot harder on the electrons than hydrogen does. And so, that's going to give the nitrogen a partial negative charge and each of these hydrogens a partial positive charge. Carbonyl groups are also polar groups. So the oxygen is really electronegative, pulls on electrons, so it's going to have a partial negative charge, and that's going to leave the carbon down here with a partial positive charge. Now, between the hydrogen of the ammonia and the oxygen on the carbonyl group, there's going to be an electrostatic charge between these partial charges. And so, there's going to be what's known as a hydrogen bond forming between them. So, it's a type of dipole-dipole bond, and so we can add hydrogen bond to our list. Now, the last type of bond, or the next type of bond that we'll see here are the carbon-carbon bonds. These two are carbons, and so there's a carbon-carbon bond. We have another carbon-carbon bond shown over here. And then each of these carbons at the end here have three hydrogens attached to them, so there's more carbon-hydrogen bonds, which we've already said are nonpolar bonds. These are also present, nonpolar covalent bonds. And then lastly, the type of bond that's present between all molecules is the Van der Waals forces. And so, we can also add that to our list. And so, this concludes our practice problem, and I'll see you guys in our next video.

Here’s what students ask on this topic:

What is the difference between intramolecular and intermolecular bonds?

Intramolecular bonds are the forces that hold atoms together within a molecule. These include ionic bonds, where electrons are transferred between atoms with opposite charges, and covalent bonds, where electrons are shared between atoms. Covalent bonds can be further classified as polar or nonpolar based on the electronegativity difference between the atoms. Intermolecular bonds, on the other hand, are forces that occur between different molecules. These include hydrogen bonds, dipole-dipole interactions, and van der Waals forces. Understanding the distinction between these types of bonds is crucial for grasping molecular interactions and properties.

How do ionic and covalent bonds differ?

Ionic bonds occur when electrons are transferred from one atom to another, resulting in the formation of ions with opposite charges that attract each other. This typically happens between atoms with a large difference in electronegativity. Covalent bonds, however, involve the sharing of electrons between atoms. If the electrons are shared equally, the bond is nonpolar covalent; if shared unequally, it is polar covalent. The type of bond formed depends on the difference in electronegativity between the atoms involved.

What are hydrogen bonds and how do they form?

Hydrogen bonds are a type of intermolecular force that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine, interacts with another electronegative atom. This creates a dipole-dipole interaction where the hydrogen atom has a partial positive charge and the electronegative atom has a partial negative charge. Hydrogen bonds are crucial in many biological processes, including the structure of DNA and the properties of water.

What are van der Waals forces?

Van der Waals forces are weak intermolecular forces that arise from temporary dipoles in molecules. These forces exist between all molecules, regardless of their polarity. They result from the momentary distribution of electron density that creates a temporary dipole, which induces a dipole in a neighboring molecule. Although weak individually, van der Waals forces can collectively have a significant effect on the physical properties of substances, such as boiling and melting points.

How does electronegativity affect bond polarity?

Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond. When two atoms with different electronegativities form a bond, the electrons are not shared equally, resulting in a polar covalent bond. The atom with higher electronegativity will have a partial negative charge, while the other atom will have a partial positive charge. If the electronegativity difference is very large, the bond may become ionic. In contrast, if the electronegativities are similar, the bond will be nonpolar covalent, with equal sharing of electrons.