The colorful red and blue model in the opening image represents a water molecule. The molecule’s one oxygen atom is colored red, and its two hydrogen atoms are colored blue. Can you guess why? The red color represents negative electric charge, and the blue color represents positive electric charge. The colors show that water is a polar compound.
What Are Polar Compounds?
Polar compounds, such as water, are compounds that have a partial negative charge on one side of each molecule and a partial positive charge on the other side. All polar compounds contain polar bonds (although not all compounds that contain polar bonds are polar.) In a polar bond, two atoms share electrons unequally. One atom attracts the shared electrons more strongly, so it has a partial negative charge. The other atom attracts the shared electrons less strongly, so it is has a partial positive charge. In a water molecule, the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do. This explains why the oxygen side of the water molecule has a partial negative charge and the hydrogen side of the molecule has a partial positive charge.
Q: If a molecule is polar, how might this affect its interactions with nearby molecules of the same compound?
A: Opposite charges on different molecules of the same compound might cause the molecules to be attracted to each other.
Hydrogen Bonding
Because of water’s polarity, individual water molecules are attracted to one another. You can see this in the Figure below. The positively charged hydrogen side of one water molecule is attracted to the negatively charged oxygen side of a nearby water molecule. This force of attraction is called a hydrogen bond.
Hydrogen bonds are intermolecular forces (“between-molecule”), rather than intramolecular (“within-molecule”) forces. They occur not only between water molecules, but between any polar molecules containing an H−N, H−O, or H−F bond. The positive hydrogen atoms are attracted to negative atoms (nitrogen, hydrogen, or fluorine) in nearby molecules. These bonds are extremely polar because of the high electronegativity difference between the atoms. This strong polarity causes very strong dipole-dipole interactions between molecules, called hydrogen bonding. Hydrogen bonds are weaker than chemical bonds. For example, they are much weaker than the covalent bonds holding atoms together within molecules of covalent compounds.
Hydrogen Bonds and Changes of State
Changes of state from solid to liquid and from liquid to gas occur when matter gains energy. The energy allows individual molecules to separate and move apart from one another. It takes more energy to bring about these changes of state for polar molecules. Although hydrogen bonds are weak, they add to the energy needed for molecules to move apart from one another, so it takes higher temperatures for these changes of state to occur in polar compounds. This explains why polar compounds have relatively high melting and boiling points. The Table below compares melting and boiling points for some polar and nonpolar covalent compounds.
| Name of Compound (Chemical Formula) | Polar or Nonpolar? | Melting Point(°C) | Boiling Point (°C) |
|---|---|---|---|
| Methane (CH4) | nonpolar | -182 | -162 |
| Ethylene (C2H2) | nonpolar | -169 | -104 |
| Ammonia (NH3) | polar | -78 | -33 |
| Water (H2O) | polar | 0 | 100 |
Q: Which compound in the Table above do you think is more polar, ammonia or water?
A: Water is more polar than ammonia. Its strong polarity explains why its melting and boiling points are high even for a polar covalent compound.
Summary
- Polar covalent compounds have molecules with a partial negative charge on one side and a partial positive charge on the other side. This occurs because the compounds contain polar bonds. In a polar bond, one atom attracts the shared electrons more strongly than the other atom does.
- In some polar molecules that contain hydrogen atoms, the partial positive charge of the hydrogen atoms of one molecule are attracted to the partial negative charge of an atom of a nearby molecule. This force of attraction is called a hydrogen bond.
- Hydrogen bonds are relatively weak, but they add to the energy needed for molecules to move apart from each other when matter changes state from a solid to a liquid or from a liquid to a gas. This explains why polar covalent compounds have relatively high melting and boiling points.
Review
- What are polar covalent compounds?
- Define hydrogen bond.
- Explain why hydrogen bonds increase melting and boiling points of polar covalent compounds such as water.
Explore More
Watch the video about hydrogen bonds and then answer the questions below.
- What is electronegativity?
- In water molecules, why do shared electrons spend more time orbiting the oxygen atom than the hydrogen atoms?
- Hydrogen forms hydrogen bonds with oxygen. What is another element besides oxygen that may be involved in hydrogen bonds?
- Water has the properties of cohesion and adhesion. Define these two properties, and explain why they occur in water.
- Why are hydrogen bonds extremely important in biology?
