Covalent bonds can be non-polar or polar and react to electrostatic charges. This document may be freely reproduced and distributed for non-profit educational purposes. Skip to main content. Search form Search. Join The Community Request new password. Main menu About this Site Table of Contents. Types of Covalent Bonds: Polar and Nonpolar. In unit two, we compared atoms to puppies and electrons to bones in our analogy of how bonding works.
Now one puppy has two electron bones and one puppy has none. Because the electron bones in our analogy have a negative charge, the puppy thief becomes negatively charged due to the additional bone. The puppy that lost its electron bone becomes positively charged. Because the puppy who lost his bone has the opposite charge of the thief puppy, the puppies are held together by electrostatic forces, just like sodium and chloride ions! In our analogy, each puppy again starts out with an electron bone.
Some covalently bonded molecules, like chlorine gas Cl2 , equally share their electrons like two equally strong puppies each holding both bones. Other covalently bonded molecules, like hydrogen fluoride gas HF , do not share electrons equally. The fluorine atom acts as a slightly stronger puppy that pulls a bit harder on the shared electrons see Fig. Even though the electrons in hydrogen fluoride are shared, the fluorine side of a water molecule pulls harder on the negatively charged shared electrons and becomes negatively charged.
The hydrogen atom has a slightly positively charge because it cannot hold as tightly to the negative electron bones. Covalent molecules with this type of uneven charge distribution are polar.
Point out that the water is able to stay together in these arcs because water molecules are very attracted to each other. Project the animation Polar Water Molecule. Project the animation Polar water together. Students will record their observations and answer questions about the activity on the activity sheet. Look at the teacher version of the activity sheet to find the questions and answers. Project the image Attractions on different levels. Students may be confused about the bonds within a water molecule and the attractions between water molecules.
Explain to students that the interaction between the oxygen of one water molecule and the hydrogen of another is different than the sharing of electrons between the oxygen and the hydrogens within the water molecule itself. Point out to students that attractions between positive and negative works on three different levels. Remind students that water molecules are very polar. Students should say that they will need the same small amount of water and alcohol.
These liquids should be placed at the same time on a surface like a brown paper towel so that students can tell when each liquid evaporates.
The dark spot on the paper towel made by the alcohol will turn lighter faster than the dark spot made by the water. This indicates that the alcohol evaporates more quickly than the water. Read more about counting molecules in the teacher background section. Note : This test is fine for middle school students but there is something about the test that does not make it completely fair.
There are many more water molecules in a drop of water than alcohol molecules in a drop of alcohol. The test would be more fair if the same number of water and alcohol molecules are placed on the paper towel. Determining the number of particles in a sample is a basic concept in chemistry, but is beyond the scope of a middle school chemistry unit.
Even if the same number of water and alcohol molecules were used in this activity, the alcohol would evaporate faster. Project the image Water and Alcohol Molecules. Tell students that understanding about polarity can help explain why water evaporates more slowly than alcohol. Remind students that the oxygen-hydrogen O—H bonds in water make it a polar molecule.
This polarity makes water molecules attracted to each other. Explain that the oxygen-hydrogen O—H bond in the alcohol molecule is also polar.
But, the carbon-hydrogen C—H bonds in the rest of the alcohol molecule are nonpolar. In these bonds, the electrons are shared more or less evenly. Because there are both polar and nonpolar areas on the alcohol molecule, they are somewhat less attracted to each other than water molecules are to each other.
This makes it easier for alcohol molecules to come apart and move into the air as a gas. This is why alcohol evaporates faster than water. You know that water and alcohol have different characteristics because of the molecules they are made of and how these molecules interact with each other.
Project the image Water and Alcohol Boiling. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 5. Project the video Water Balloon.
Project the video Water Fountain. Explain Show molecular model animations that illustrate why water molecules are attracted to each other. First Frame of the Animation Electrons are shared between atoms in a covalent bond. Remind students how the shared electrons in a water molecule are attracted to the protons in both the oxygen and the hydrogen atoms.
These attractions hold the atoms together. Water molecules are neutral. Ionic and covalent bonds are the most important in all of chemistry. With ionic bonds, atoms give or take electrons. With covalent bonds, they have to share them. Now think about a magnet. So do batteries. So does the Earth. When things are different at each end, we call them polar. Some molecules have positive and negative ends too, and when they do, we call them polar.
If they don't, we call them non-polar. Things that are polar can attract and repel each other opposite charges attract, alike charges repel. The two magnets in the image above will attract because their opposite poles are near.
Reverse one of them and they will repel each other. So why do soaps and detergents clean our dishes and our clothes? Soaps are chemically similar to cell membranes. When soap is added to water, it forms structures called "micelles. When a soap micelle encounters oil or grease, these non-polar materials are forced to the inside of the micelle to get away from the polar water and polar heads of the micelle, where they are trapped.
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