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      • Periodic Table
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    • Unit 1: Beginning Chemistry >
      • Lesson 1: The Atom >
        • What is an Atom?
        • The Structure of Atoms
        • The Periodic Table
        • Modeling Atoms
      • Lesson 2: Chemical Bonding >
        • Why Do Atoms Form Bonds?
        • Ionic Bonding
        • Covalent Bonding
        • Intermolecular Attraction
      • Lesson 3: Chemical Nomenclature >
        • Octet Rule
        • Ionic Compounds
        • Covalent Molecules
      • Lesson 4: Molecular Molecules >
        • Modeling Ionic Compounds
        • Modeling Covalent Molecules
      • Lesson 5: States of Matter >
        • States of Matter
        • Phase Changes
      • Lesson 6: Density >
        • What is Density?
        • Calculating Density
      • Lesson 7: Thermodynamics >
        • Temperature
        • Heat
        • Gas Laws
      • Lesson 8: Solution Chemistry >
        • Diffusion
        • Solutions and Molarity
        • Semi-Permeable Membranes
      • Lesson 9: Thermal Expansion >
        • Thermal Expansion
    • Unit 2: Earth Science >
      • Lesson 10: Earth at a Glance >
        • Perspective
        • Maps
      • Lesson 11: Layers of the Earth >
        • Layers of the Earth
      • Lesson 12: Plate Tectonics >
        • Plate Tectonics
      • Lesson 13: Rocks and Minerals >
        • Rocks and Minerals
      • Lesson 14: Particle Sorting >
        • Differentiation
        • Deposition of Sediment
      • Lesson 15: The Atmosphere >
        • Composition of the Atmosphere
        • Layers of the Atmosphere
        • Change Over Time
        • Atmospheric Disruption
    • Unit 3: The Cell >
      • Lesson 16: Life >
        • What is Life?
        • Structural Hierarchy of Living Things
      • Lesson 17: Biochemistry >
        • Intro to Biochemistry
        • Water
        • Micromolecules
        • Energy, Carbohydrates, Lipids
        • Protein and Nucleic Acid
      • Lesson 18: Cells >
        • What are Cells?
        • Microscopy
        • Plant and Animal Cells
      • Lesson 19: Membrane Transport >
        • A Special Environment
        • The Structure of Membranes
        • Membrane Transport
      • Lesson 20: Energy and Cell Respiration >
        • Energy in Biology
        • Energy Diagrams
        • Glycolysis and Anaerobic Respiration
        • Aerobic Cellular Respiration
      • Lesson 21: Photosynthesis >
        • Plants Get Energy From The Sun
        • Photosynthesis Process
        • Energy, Ecosystems, and the Environment
    • Unit 4: Anatomy and Physiology >
      • Lesson 22: The Human Body >
        • What Are Bodies Made Of?
        • What Do Bodies Do?
      • Lesson 23: The Nervous System >
        • The Nervous System
        • Neuronal Communication
        • The Central Nervous System
      • Lesson 24: The Endocrine System >
        • The Endocrine System
        • Hormones
        • Hormones, Puberty, and Reproduction
      • Lesson 25: The Integumentary System >
        • The Integumentary System
      • Lesson 26: The Musculoskeletal System >
        • The Skeletal System
        • The Muscular System
        • Anatomy Of The Musculoskeletal System
      • Lesson 27: The Cardiovascular System >
        • Blood and Blood Vessels
        • The Heart
      • Lesson 28: The Respiratory System >
        • The Respiratory System
      • Lesson 29: The Digestive System >
        • The Digestive System
        • Nutrition
      • Lesson 30: The Excretory System >
        • The Excretory System
      • Lesson 31: The Immune System >
        • Disease and Infection
        • Immunity
    • Units 5-6 Coming Soon
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    • Unit 1 >
      • EIS >
        • Lesson 1: Atoms
        • Lesson 2: Chemical Bonding
        • Lesson 3: Chemical Nomenclature
        • Lesson 4: Molecular Models
        • Lesson 5: States of Matter
        • Lesson 6: Density
        • Lesson 7: Thermodynamics
        • Lesson 8: Solution Chemistry
        • Lesson 9: Thermal Expansion
      • Unit 1: Project
      • Unit 1: Exam Review
      • Unit 1: Exam
    • Unit 2 >
      • EIS >
        • Lesson 10: Earth at a Glance
        • Lesson 11: Layers of the Earth
        • Lesson 12: Plate Tectonics
        • Lesson 13: Rocks and Minerals
        • Lesson 14: Particle Sorting
        • Lesson 15: The Atmosphere
      • Unit 2: Project
      • Unit 2: Exam Review
      • Unit 2: Exam
    • Unit 3 >
      • EIS >
        • Lesson 16: Life
        • Lesson 17: Biochemsitry
        • Lesson 18: Cells
        • Lesson 19: Membrane Transport
        • Lesson 20: Energy and Cell Respiration
        • Lesson 21: Photosynthesis
      • Unit 3: Project
      • Unit 3: Exam Review
      • Unit 3: Exam
    • Unit 4 >
      • EIS >
        • 22: The Human Body
        • 23: The Nervous System
        • 24: The Endocrine System
        • 25: The Integumentary System
        • 26: The Musculoskeletal System
        • 27: The Cardiovascular System
        • 28: The Respiratory System
        • 29: The Digestive System
        • 30: The Excretory System
        • 31: The Immune System
      • Unit 4 Project
      • Unit 4 Exam Review
      • Unit 4 Exam
    • Units 5-6 Coming Soon
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  • Home
  • About Us
  • Classroom
    • Resources >
      • Study Skills
      • Periodic Table
      • Common Ion Sheet
      • The Scientific Method
      • Doing Background Research
    • Introduction and Course Philosophy
    • Unit 1: Beginning Chemistry >
      • Lesson 1: The Atom >
        • What is an Atom?
        • The Structure of Atoms
        • The Periodic Table
        • Modeling Atoms
      • Lesson 2: Chemical Bonding >
        • Why Do Atoms Form Bonds?
        • Ionic Bonding
        • Covalent Bonding
        • Intermolecular Attraction
      • Lesson 3: Chemical Nomenclature >
        • Octet Rule
        • Ionic Compounds
        • Covalent Molecules
      • Lesson 4: Molecular Molecules >
        • Modeling Ionic Compounds
        • Modeling Covalent Molecules
      • Lesson 5: States of Matter >
        • States of Matter
        • Phase Changes
      • Lesson 6: Density >
        • What is Density?
        • Calculating Density
      • Lesson 7: Thermodynamics >
        • Temperature
        • Heat
        • Gas Laws
      • Lesson 8: Solution Chemistry >
        • Diffusion
        • Solutions and Molarity
        • Semi-Permeable Membranes
      • Lesson 9: Thermal Expansion >
        • Thermal Expansion
    • Unit 2: Earth Science >
      • Lesson 10: Earth at a Glance >
        • Perspective
        • Maps
      • Lesson 11: Layers of the Earth >
        • Layers of the Earth
      • Lesson 12: Plate Tectonics >
        • Plate Tectonics
      • Lesson 13: Rocks and Minerals >
        • Rocks and Minerals
      • Lesson 14: Particle Sorting >
        • Differentiation
        • Deposition of Sediment
      • Lesson 15: The Atmosphere >
        • Composition of the Atmosphere
        • Layers of the Atmosphere
        • Change Over Time
        • Atmospheric Disruption
    • Unit 3: The Cell >
      • Lesson 16: Life >
        • What is Life?
        • Structural Hierarchy of Living Things
      • Lesson 17: Biochemistry >
        • Intro to Biochemistry
        • Water
        • Micromolecules
        • Energy, Carbohydrates, Lipids
        • Protein and Nucleic Acid
      • Lesson 18: Cells >
        • What are Cells?
        • Microscopy
        • Plant and Animal Cells
      • Lesson 19: Membrane Transport >
        • A Special Environment
        • The Structure of Membranes
        • Membrane Transport
      • Lesson 20: Energy and Cell Respiration >
        • Energy in Biology
        • Energy Diagrams
        • Glycolysis and Anaerobic Respiration
        • Aerobic Cellular Respiration
      • Lesson 21: Photosynthesis >
        • Plants Get Energy From The Sun
        • Photosynthesis Process
        • Energy, Ecosystems, and the Environment
    • Unit 4: Anatomy and Physiology >
      • Lesson 22: The Human Body >
        • What Are Bodies Made Of?
        • What Do Bodies Do?
      • Lesson 23: The Nervous System >
        • The Nervous System
        • Neuronal Communication
        • The Central Nervous System
      • Lesson 24: The Endocrine System >
        • The Endocrine System
        • Hormones
        • Hormones, Puberty, and Reproduction
      • Lesson 25: The Integumentary System >
        • The Integumentary System
      • Lesson 26: The Musculoskeletal System >
        • The Skeletal System
        • The Muscular System
        • Anatomy Of The Musculoskeletal System
      • Lesson 27: The Cardiovascular System >
        • Blood and Blood Vessels
        • The Heart
      • Lesson 28: The Respiratory System >
        • The Respiratory System
      • Lesson 29: The Digestive System >
        • The Digestive System
        • Nutrition
      • Lesson 30: The Excretory System >
        • The Excretory System
      • Lesson 31: The Immune System >
        • Disease and Infection
        • Immunity
    • Units 5-6 Coming Soon
  • Workbench
    • Unit 1 >
      • EIS >
        • Lesson 1: Atoms
        • Lesson 2: Chemical Bonding
        • Lesson 3: Chemical Nomenclature
        • Lesson 4: Molecular Models
        • Lesson 5: States of Matter
        • Lesson 6: Density
        • Lesson 7: Thermodynamics
        • Lesson 8: Solution Chemistry
        • Lesson 9: Thermal Expansion
      • Unit 1: Project
      • Unit 1: Exam Review
      • Unit 1: Exam
    • Unit 2 >
      • EIS >
        • Lesson 10: Earth at a Glance
        • Lesson 11: Layers of the Earth
        • Lesson 12: Plate Tectonics
        • Lesson 13: Rocks and Minerals
        • Lesson 14: Particle Sorting
        • Lesson 15: The Atmosphere
      • Unit 2: Project
      • Unit 2: Exam Review
      • Unit 2: Exam
    • Unit 3 >
      • EIS >
        • Lesson 16: Life
        • Lesson 17: Biochemsitry
        • Lesson 18: Cells
        • Lesson 19: Membrane Transport
        • Lesson 20: Energy and Cell Respiration
        • Lesson 21: Photosynthesis
      • Unit 3: Project
      • Unit 3: Exam Review
      • Unit 3: Exam
    • Unit 4 >
      • EIS >
        • 22: The Human Body
        • 23: The Nervous System
        • 24: The Endocrine System
        • 25: The Integumentary System
        • 26: The Musculoskeletal System
        • 27: The Cardiovascular System
        • 28: The Respiratory System
        • 29: The Digestive System
        • 30: The Excretory System
        • 31: The Immune System
      • Unit 4 Project
      • Unit 4 Exam Review
      • Unit 4 Exam
    • Units 5-6 Coming Soon
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Intermolecular Attraction

Molecules Attracting Molecules

So far, we have been talking about intramolecular attraction, or attraction within a molecule. But, obviously, molecules don't tend to come in a party of one. When you look at a chair, you aren't looking at a molecule, you're looking at a chair. So, then, we need to talk about another type of interaction if we want to really understand the universe. This is called intermolecular attraction and is what holds a group of molecules together.

There are several types of intermolecular bonds that can form. Some are pretty strong. Some are pretty fluid, more like molecules dancing around each other, coming into attraction for a brief moment, and then flitting away to dance with other molecules. It’s poetic. Like a song. A song that sounds like this one:
​


​The relative strength of these intermolecular attractions determines many of the physical and chemical properties of elements and molecules, including some pretty fundamental ones, like whether they are solids, liquids, or gases. So let’s take a deeper dive into what these different types of intermolecular attractions are.

Ionic Bonds

Ionic bonds are a kind of weird mashup between an intramolecular bond and an intermolecular bond, so I'll include it here, but it's really nothing new. They are very strong, because they are the result of electrostatic interactions of complete, permanent charges: That is, completely and forever giving up or receiving electrons. This is in contrast to the partial or temporary charges that we’ll learn about in some of the other intermolecular bond types. Because these bonds are so strong, they tend to result in solid compounds.

Dipole-Dipole Interactions. Take a look at this picture of a water molecule:
Picture
Water is polar. This means that it has a negative end and a positive end. The simplest way to recognize this is by, going back to the picture, seeing that one end of the molecule has free electrons (those dots). As you know, electrons are negative. So, the end of water with free electrons must also be negative. The other end of the molecule is bonded to hydrogen. Hydrogen has protons, and protons are positive. So, that end of water must be positive (at least compared to the other end).

The other, more important reason that water is polar is because oxygen is more electronegative than hydrogen. As you’ve learned, electronegativity means that oxygen is a greedy electron hog. It will pull the electrons away from hydrogen and towards itself. And, again, since electrons are negative, this makes the oxygen atom slightly more negative than the hydrogen atoms. This creates a partial charge or dipole.

Since water has a negative end and a positive end, and opposites attract, the positive end of one water molecule is attracted to the negative end of another water molecule, and the two molecules stick together! Amazing!
Picture
When a molecule becomes charged because of electronegativity, it is called a dipole moment (see the word pole, as in polar, in there?). As such, the interaction in which the negative end of a polar molecule sticks to the positive end of another—like with water molecules—is called a dipole-dipole interaction. It can occur between any polar molecules, not just water. In fact, the attraction in water is actually a special kind of dipole-dipole interaction called hydrogen bonding. You'll hear this phrase a lot later on because it is important to life.
​
Most substances that have this interaction are usually gasses or liquids at room temperature.

Note: this type of interaction is somewhat similar to an ionic bond in that charges are interacting to hold stuff together. But, it is important to differentiate between the two. In an ionic bond, the whole molecule is charged, and it interacts with another molecule with a whole (opposite) charge. In this case only part of the molecule is charged, so it can interact with other molecules of the same type.


Hydrogen Bonding. As mentioned, hydrogen bonding is a special case of dipole-dipole interaction. However, hydrogen bonds are much stronger than dipole-dipole because the hydrogen atom is virtually left “naked,” because it doesn’t have electrons “protecting” it—it doesn’t have unshared electrons, because hydrogen doesn’t have or need an octet. This makes it extra positive, compared to things like carbon, which can participate in dipole-dipole interactions, but not hydrogen bonding.

As you may have guessed from the name, hydrogen bonding always involves hydrogen. Its negative counterpart will always be a very electronegative atom, like N, O, or F (reminder: the closer you are to fluorine on the periodic table, the more electronegative you are), because these are the biggest electron hogs.


London Dispersion Forces. London dispersion forces are very weak and occur between neutral (uncharged), nonpolar molecules. They, like the other types of intermolecular attraction, rely on positive and negative charges interacting. However, unlike the other types of intermolecular attraction, these charges are not permanent (hence why London dispersion forces are weak). To understand this, we must take a step back to the structure of the atom.

Electrons are not stuck in place. This is one of the fall-backs of drawing models on paper-- we can't see the motion. But, in reality, motion does occur, and, in fact, it occurs quite a lot, and quite fast. Electrons are always moving. ALWAYS.

What does this mean in terms of intermolecular attraction? It means that electrons, just by chance, can end up on one side of a molecule. They won't stay there forever, but for this fleeting moment, the molecule is polar― one side is negative, leaving the other positive and stripped of electrons. So, in this fleeting moment, it can stick to another molecule, which also happens to have a dipole at this moment in time.

This video shows London dispersion forces pretty well:
​

​The odds that all the electrons in a molecule would just happen to end up on one side of it may seem pretty small and insignificant. Really, what are the chances that, say, 40 electrons in a molecule end up on the same side of the molecule at the same time, and in close proximity to another molecule which also just happens to have 40 electrons on the same side of the molecule? Fat chance, right? This is just a guess, but the odds are probably greater that you will win the lottery, buy an NFL team, and watch them win the Super Bowl 5 years in a row. (Okay, maybe it's not
that rare, but you get the idea).


So, how can this possibly be a significant source of intermolecular attraction? Well, for one thing, not all of the electrons have to be on the same side of the molecule for London dispersion forces to work. Just a majority of them. This is a lot more likely. For another thing, there are billions upon trillions of molecules in even a relatively small volume of a substance. (For example, if molecules were the size of cupcakes, 18 mL of water would fill approximately 12 billion trillion Olympic-sized swimming pools). This means that even though it might be rare for any two given molecules to be held together by London dispersion forces, they’re actually not that uncommon if you look at the entire volume of substance. Alone they are weak, but together they are mighty. In fact, it's what allows a gecko to climb up glass:
​

​These forces are most often found among gasses, and they’re sometimes found in liquids.

Summary

You should understand:
  • The relative strengths of ionic bonds, hydrogen bonding, dipole-dipole interactions, and London dispersion forces, and how this relates to whether a charge/dipole is permanent, temporary, complete, or partial.
  • Which forces are most commonly found in solids, liquids, and gasses, and how that relates to the strength of the interaction. We’ll also talk about this more specifically later on.

Learning Activity

Picture
Lesson 3: chemical nomenclature
lesson 2 "exploration in science"

Content contributors: Rebecca Deng, Emma Moulton, Eli Levine, Emily Zhang
© COPYRIGHT 2020. ALL RIGHTS RESERVED.