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      • 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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  • 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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Covalent Bonding

Sharing is Caring

As a refresher, in order for atoms to bond with each other, they need to either donate or share valence electrons to attain a stable configuration (like an octet). Donating electrons forms an ionic bond; sharing electrons forms a covalent bond. When atoms don’t bond with each other to attain a more stable configuration, they are sad. When atoms do bond with each other to attain a more stable configuration, they are happy!
As you learned, an ionic bond will only form between a metal and a nonmetal. In the same token, a covalent bond will only form between two nonmetals.

Most nonmetals have a lot of electrons in their valence shells, so it takes A LOT of energy to make one of them a cation. Chlorine (Cl), for example, would need to lose seven electrons to look like neon (Ne), whereas it only needs one more to look like argon (Ar), and it’s happy as either noble gas. Since you need a cation for an ionic bond, it just doesn’t happen with two nonmetals: it would take way to much energy to give up enough electrons for one of the nonmetals to become a cation. So, instead of completely selling off their electrons, our nonmetal friends just share with each other, and everybody wins. Sharing is caring!
​


Just like NaCl is a good model for ionic bonding, water has become the poster child of covalent bonds. A water molecule, or H₂O, looks like this:
​

Picture

​In this picture, you can see that oxygen
thinks it has 8 electrons in its valence shell (a stable octet), but it actually only owns 6 of these electrons. Likewise, hydrogen is super happy with its 2 electrons, but really it only owns 1. It’s kind of like a timeshare for electrons.


(A little tip on the octet rule, which says that atoms want 8 electrons: since it's more of a guideline, it may be more useful to remember that noble gases are perfectly stable. Atoms will bond until they look like noble gases.)

The real trick with covalent bonding is trying to figure out an arrangement that makes everybody happy. Atoms, of course, don't even have to think about it. They will automatically go to the most stable configuration—the best way of sharing—when they come in contact with another atom. But, for us mere mortals, we have to solve the puzzle. You'll have a lot of practice with this when we talk about molecular models. For now, our real question is: Why do covalent bonds form? What holds them together?

Why Do Covalent Bonds Form?

Much of this question can be answered with electrostatic attraction and stability, which you learned when we talked about the basics of chemical bonding. With covalent bonds, though, it goes a bit further. A lot of people have trouble wrapping their heads around the concept that I'm about to explain, so don't freak out if you're one of these people. It is just to show you that the universe makes sense, and to illustrate the point that science doesn't just happen because scientists said so. It all comes back to the fundamental laws of the universe. As long as you understand this point, you're in good shape. In other words, put down your pens and stop taking notes, but do keep reading.

Remember when I said that electrons are sort of like particles and sort of like waves? Well, they're actually mostly like waves. Yes, like a wave in the ocean, or like a sound wave, or like a heat wave, or like a wave of light. (Mostly. They're 3-D, and these other kinds of waves aren't. But the concept still stands. Hehe. Get it? Stands. Oh, wait, I haven’t told you that they’re called standing waves yet. Inside joke, inside joke.)

Electron waves can add together in one of two ways: to make one big wave, or to make no wave at all.
​

Picture
This is called interference. It is a property of all waves.
So, when electron waves get close together, they can make one big wave—which is a bond—or no wave at all—an anti-bond. As it turns out, it takes less energy to make a bond, because the nuclei are now both attracted to the electrons instead of repelling each other (the electrons shield them from the high-energy, repulsive positive charges). Hydrogen gas (H₂) is the simplest example of this, because only 2 electrons are brought in, which is exactly how many you need for a bond.
​

Picture
Hydrogen gas anti-bond
Picture
Hydrogen gas bond

​But, we know that helium doesn’t form He₂: it stays all by itself. Why?

​Well, atoms will form a bond if they can—but, an atom can only create a certain number of bonds. After that, its only choice is to make anti-bonds. It’s like how you would rather buy things than sell them, but at some point you’ll run out of money if you only ever buy things. If too many electrons are brought into the picture, they will form an anti-bond, even though it would be easier to make a bond. Eventually, the number of anti-bonds cancels out the number of bonds, and you get no bonds. And guess what? This number of bonds and anti-bonds that atoms can form goes right along with what you've already learned about the octet rule.
​

Picture
Bond anti-bond
Okay, that's enough quantum physics. Cool, no? 

Polarity: Sharing is Not Always Equal

Now that you’ve got the basics (and some of the advanced stuff) about why bonds form, it’s time to add one little qualifier: Not all bonds involve equal sharing. This idea is really important when we talk about the stuff that molecules do, like bond with each other (yep, molecules bond, too) in intermolecular bonds, and react with each other in chemical reactions. 

Polarity is the unequal distribution of negative charge in a molecule because of unequal sharing of electrons. This happens because not all elements like sharing as much as others. Elements that don’t like to share are considered electronegative, or electron hogs. (They are negative because they hoard all of the electron density, making them negatively charged. And because that’s mean, making them “negative” like “bad.”) Electronegativity basically describes how much an element “wants” electrons (or how much is doesn’t want to share).

Sometimes, one atom attracts more strongly and pulls the electrons to one end of the bond, creating an area of negative charge around that atom (remember: Electrons are negative). As a result, the now “bare” atom on the other end of the bond has a (partial) positive charge. It’s like tug-of-war, and the more electronegative element always wins. A bond is termed polar if this “tug-of-war” occurs.
​

Picture
Polar Bond

This pulls electron density away from the bond, which makes that bond weaker and more likely to break in a chemical reaction. It also puts a partial positive charge on one side of the molecule and a partial negative charge on the other side of the molecule, which allows electrostatic interactions to occur.


Electronegativity is measured on a relative scale, with fluorine being the highest at 4.0. Other electronegativities are provided below. The closer you get to fluorine on the Periodic Table, the more electronegative you are.
​

ELECTRONEGATIVITY

Picture

​The polarity of a molecule depends on both the polarity of its bonds and the shape of the molecule as a whole: That is, if one element if pulling electrons towards one end of the molecule, it will be polar, but if electrons are being pulled in opposite directions, the effect will cancel out, as shown below.
Picture
This video provides a great overview of polarity:
​

Summary

You should understand:
  • That atoms share electrons in a covalent bond.
  • That covalent bonds form between two nonmetals.
  • That all of the atoms you will be working with (except hydrogen and helium) follow the octet rule. This means that they "want" to have 8 electrons in their valence shell. Hydrogen and helium want 2. (The always-true rule is that atoms “want” to look like noble gases).
  • That any bonded molecule that (semi-permanently) stays a bonded molecule is more stable than the atoms would be separately. Otherwise, they wouldn't bother to form a bond.
  • That electronegative elements will pull electron density toward them and out of a bond, making that portion of the molecule relatively more negative.

This video provides a great overview of these points, as well as other topics we’ve addressed so far in chemical bonding.
​


​Understand these points, and you will be well prepared to understand the other chemistry we will deal with in this course. Covalent bonding is a very important concept in molecular chemistry. If any of the things in this recap were news to you, it may be worth going back and taking a second look at this and past lessons.

Learning Activity

Picture
Next: INTERMOLECULE ATTRACTION

Content contributors: Emma Moulton, Eli Levine, Emily Zhang
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