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Slide Notes
(This lecture is meant to be an approx. 10 min review of intermolecular forces for AP Chemistry students in their first week of class.)
*Today's Objective*
You will be able to describe the causes and strengths of intermolecular forces.
*Elicit prior knowledge discussion*
What do you see in this picture? Where have you seen something like this before? What's causing the water to behave this way?
*Lecture*
Intermolecular forces (IMFs) are attractions between particles like molecules. They are caused by attraction of positive and negative charges caused by the movement of electrons. They are different from bonds, which are INTRAmolecular forces (forces inside molecules).
We will come back to this picture at the end of the presentation and explain it.
Image CC BY-NC-ND 2.0. Erwin, N. Spring rain [Image]. Retrieved from https://flic.kr/p/U5io6o
*Today's Objective*
You will be able to describe the causes and strengths of intermolecular forces.
*Elicit prior knowledge discussion*
What do you see in this picture? Where have you seen something like this before? What's causing the water to behave this way?
*Lecture*
Intermolecular forces (IMFs) are attractions between particles like molecules. They are caused by attraction of positive and negative charges caused by the movement of electrons. They are different from bonds, which are INTRAmolecular forces (forces inside molecules).
We will come back to this picture at the end of the presentation and explain it.
Image CC BY-NC-ND 2.0. Erwin, N. Spring rain [Image]. Retrieved from https://flic.kr/p/U5io6o
Intermolecular Forces (IMFs)
Published on Jul 19, 2017
Very basic review of intermolecular forces for AP Chemistry first week of school.
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PRESENTATION OUTLINE
Big Idea 2: Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them.
(This lecture is meant to be an approx. 10 min review of intermolecular forces for AP Chemistry students in their first week of class.)
*Today's Objective*
You will be able to describe the causes and strengths of intermolecular forces.
*Elicit prior knowledge discussion*
What do you see in this picture? Where have you seen something like this before? What's causing the water to behave this way?
*Lecture*
Intermolecular forces (IMFs) are attractions between particles like molecules. They are caused by attraction of positive and negative charges caused by the movement of electrons. They are different from bonds, which are INTRAmolecular forces (forces inside molecules).
We will come back to this picture at the end of the presentation and explain it.
Image CC BY-NC-ND 2.0. Erwin, N. Spring rain [Image]. Retrieved from https://flic.kr/p/U5io6o
*Today's Objective*
You will be able to describe the causes and strengths of intermolecular forces.
*Elicit prior knowledge discussion*
What do you see in this picture? Where have you seen something like this before? What's causing the water to behave this way?
*Lecture*
Intermolecular forces (IMFs) are attractions between particles like molecules. They are caused by attraction of positive and negative charges caused by the movement of electrons. They are different from bonds, which are INTRAmolecular forces (forces inside molecules).
We will come back to this picture at the end of the presentation and explain it.
Image CC BY-NC-ND 2.0. Erwin, N. Spring rain [Image]. Retrieved from https://flic.kr/p/U5io6o
Full charges make strong attractions
*Lecture*
You're familiar with ionic bonds, which form between a metal and nonmetal. An ionic bond is made when electrons are transferred between atoms. The resulting ions are very strongly attracted because of the + and - charges they have. Ionic bonds are a very strong kind of intermolecular force because we can imagine the ions as separate particles.
*Discussion questions*
Can NaCl melt? What happens when sodium chloride is put into water?
[Extended explanation if needed]
The picture show the formation of sodium chloride, table salt. In the picture, sodium gave the red electron to chlorine. As a result, sodium has more protons than electrons and has an overall positive charge. Chlorine has one more electron than proton and has a -1 charge. The two atoms are now called ions. They attract together in an ionic bond.
Ionic compounds are solid because the attractions between + and - charges is so strong. They have high melting points. The ions must move very quickly to break the attraction that holds them in the ionic lattice. Water can dissolve them because water has partial charges called a dipole, so it attracts to the ions. The ions in sodium chloride are very attracted to water. This is called an ion-dipole attraction.
Image adapted from OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3A207_Ionic_Bonding-01.jpg
You're familiar with ionic bonds, which form between a metal and nonmetal. An ionic bond is made when electrons are transferred between atoms. The resulting ions are very strongly attracted because of the + and - charges they have. Ionic bonds are a very strong kind of intermolecular force because we can imagine the ions as separate particles.
*Discussion questions*
Can NaCl melt? What happens when sodium chloride is put into water?
[Extended explanation if needed]
The picture show the formation of sodium chloride, table salt. In the picture, sodium gave the red electron to chlorine. As a result, sodium has more protons than electrons and has an overall positive charge. Chlorine has one more electron than proton and has a -1 charge. The two atoms are now called ions. They attract together in an ionic bond.
Ionic compounds are solid because the attractions between + and - charges is so strong. They have high melting points. The ions must move very quickly to break the attraction that holds them in the ionic lattice. Water can dissolve them because water has partial charges called a dipole, so it attracts to the ions. The ions in sodium chloride are very attracted to water. This is called an ion-dipole attraction.
Image adapted from OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3A207_Ionic_Bonding-01.jpg
van der Waals forces are weaker and depend on polarity (partial charges)
*Lecture*
Van der Waals forces is the name given to attractions between partially charged particles.
The picture shows two kinds of particles - red water molecules and a green chlorine molecule. They are attracted to each other because of the partial charges in the molecules.
Water has charges because of the electronegativity difference between O and H. Because of water's bent shape, the O end of the molecule is partially - and the H end is partially +. This makes a dipole and the molecule is called polar. Dipole-dipole attractions are weaker attractions than ion-ion; therefore, water is a liquid at room temperature instead of a solid.
Cl2 has no electronegativity difference between the atoms in it. One end of the molecule is near a partially + H and the other end is near partially - O. Some electrons in its cloud move to be near the positive H. As a result, the chlorine gets an induced dipole with weak partial charges. The top end is partially - and the bottom end is partially +. Cl2 attracts to other Cl2 very weakly. This is why Cl2 is a gas instead of a liquid or sold.
*Discussion Questions*
Is H2S polar or nonpolar?
Is Br2 polar or nonpolar?
How do hydrogen bonds fit into this idea?
*Lecture*
Wrapping up, think back to the image at the beginning. Water forms droplets because the water molecules are attracted to each other. They form dipoles with partial charges. The leaf itself must not be polar, so it doesn't attract to the water as much as the water attracts to itself.
[Extended explanation]
As you can see, oxygen (dark red) holds the electrons more strongly than hydrogen (light red). The oxygen end of the molecule is more negative so it has a delta-negative charge symbol. This is a partial charge, so a delta is used. Hydrogen is electron deficient, so it has a partial positive charge - delta +.
Water is more attracted to itself than it is to chlorine. Dipole-dipole attractions between H2O and H2O are stronger than dipole-induced dipole attractions like when Cl2 is dissolved in H2O. Only small numbers of chlorine molecules could attract to water molecules.
Image By Riccardo Rovinetti (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3AForze_di_Debye.png
Van der Waals forces is the name given to attractions between partially charged particles.
The picture shows two kinds of particles - red water molecules and a green chlorine molecule. They are attracted to each other because of the partial charges in the molecules.
Water has charges because of the electronegativity difference between O and H. Because of water's bent shape, the O end of the molecule is partially - and the H end is partially +. This makes a dipole and the molecule is called polar. Dipole-dipole attractions are weaker attractions than ion-ion; therefore, water is a liquid at room temperature instead of a solid.
Cl2 has no electronegativity difference between the atoms in it. One end of the molecule is near a partially + H and the other end is near partially - O. Some electrons in its cloud move to be near the positive H. As a result, the chlorine gets an induced dipole with weak partial charges. The top end is partially - and the bottom end is partially +. Cl2 attracts to other Cl2 very weakly. This is why Cl2 is a gas instead of a liquid or sold.
*Discussion Questions*
Is H2S polar or nonpolar?
Is Br2 polar or nonpolar?
How do hydrogen bonds fit into this idea?
*Lecture*
Wrapping up, think back to the image at the beginning. Water forms droplets because the water molecules are attracted to each other. They form dipoles with partial charges. The leaf itself must not be polar, so it doesn't attract to the water as much as the water attracts to itself.
[Extended explanation]
As you can see, oxygen (dark red) holds the electrons more strongly than hydrogen (light red). The oxygen end of the molecule is more negative so it has a delta-negative charge symbol. This is a partial charge, so a delta is used. Hydrogen is electron deficient, so it has a partial positive charge - delta +.
Water is more attracted to itself than it is to chlorine. Dipole-dipole attractions between H2O and H2O are stronger than dipole-induced dipole attractions like when Cl2 is dissolved in H2O. Only small numbers of chlorine molecules could attract to water molecules.
Image By Riccardo Rovinetti (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3AForze_di_Debye.png
van der Waals forces are weaker and depend on polarity (partial charges)
*Lecture*
Van der Waals forces is the name given to attractions between partially charged particles.
The picture shows two kinds of particles - red water molecules and a green chlorine molecule. They are attracted to each other because of the partial charges in the molecules.
Water has charges because of the electronegativity difference between O and H. Because of water's bent shape, the O end of the molecule is partially - and the H end is partially +. This makes a dipole and the molecule is called polar. Dipole-dipole attractions are weaker attractions than ion-ion; therefore, water is a liquid at room temperature instead of a solid.
Cl2 has no electronegativity difference between the atoms in it. One end of the molecule is near a partially + H and the other end is near partially - O. Some electrons in its cloud move to be near the positive H. As a result, the chlorine gets an induced dipole with weak partial charges. The top end is partially - and the bottom end is partially +. Cl2 attracts to other Cl2 very weakly. This is why Cl2 is a gas instead of a liquid or sold.
*Discussion Questions*
Is H2S polar or nonpolar?
Is Br2 polar or nonpolar?
How do hydrogen bonds fit into this idea?
*Lecture*
Wrapping up, think back to the image at the beginning. Water forms droplets because the water molecules are attracted to each other. They form dipoles with partial charges. The leaf itself must not be polar, so it doesn't attract to the water as much as the water attracts to itself.
[Extended explanation]
As you can see, oxygen (dark red) holds the electrons more strongly than hydrogen (light red). The oxygen end of the molecule is more negative so it has a delta-negative charge symbol. This is a partial charge, so a delta is used. Hydrogen is electron deficient, so it has a partial positive charge - delta +.
Water is more attracted to itself than it is to chlorine. Dipole-dipole attractions between H2O and H2O are stronger than dipole-induced dipole attractions like when Cl2 is dissolved in H2O. Only small numbers of chlorine molecules could attract to water molecules.
Image By Riccardo Rovinetti (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3AForze_di_Debye.png
Van der Waals forces is the name given to attractions between partially charged particles.
The picture shows two kinds of particles - red water molecules and a green chlorine molecule. They are attracted to each other because of the partial charges in the molecules.
Water has charges because of the electronegativity difference between O and H. Because of water's bent shape, the O end of the molecule is partially - and the H end is partially +. This makes a dipole and the molecule is called polar. Dipole-dipole attractions are weaker attractions than ion-ion; therefore, water is a liquid at room temperature instead of a solid.
Cl2 has no electronegativity difference between the atoms in it. One end of the molecule is near a partially + H and the other end is near partially - O. Some electrons in its cloud move to be near the positive H. As a result, the chlorine gets an induced dipole with weak partial charges. The top end is partially - and the bottom end is partially +. Cl2 attracts to other Cl2 very weakly. This is why Cl2 is a gas instead of a liquid or sold.
*Discussion Questions*
Is H2S polar or nonpolar?
Is Br2 polar or nonpolar?
How do hydrogen bonds fit into this idea?
*Lecture*
Wrapping up, think back to the image at the beginning. Water forms droplets because the water molecules are attracted to each other. They form dipoles with partial charges. The leaf itself must not be polar, so it doesn't attract to the water as much as the water attracts to itself.
[Extended explanation]
As you can see, oxygen (dark red) holds the electrons more strongly than hydrogen (light red). The oxygen end of the molecule is more negative so it has a delta-negative charge symbol. This is a partial charge, so a delta is used. Hydrogen is electron deficient, so it has a partial positive charge - delta +.
Water is more attracted to itself than it is to chlorine. Dipole-dipole attractions between H2O and H2O are stronger than dipole-induced dipole attractions like when Cl2 is dissolved in H2O. Only small numbers of chlorine molecules could attract to water molecules.
Image By Riccardo Rovinetti (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3AForze_di_Debye.png
