Hydrogen Bonds. Hydrogen bonds form when hydrogen atoms covalently bonded to nitrogen N , oxygen O , or fluorine F in the form of covalent compounds such as ammonia NH 3 , water H 2 O and hydrogen fluoride gas HF.
In these molecules, the hydrogen atoms do not pull as strongly on the shared electrons as the N, O, or F atoms. Therefore, the molecules are polar; the hydrogen atoms become positively charged and are able to form hydrogen bonds to negative ions or negatively charged parts of other molecules such as the N, O, and F atoms that become negatively charged in these compounds.
Hydrogen bonds are not true bonds like covalent bonds or ionic bonds. Hydrogen bonds are attractions of electrostatic force caused by the difference in charge between slightly positive hydrogen ions and other, slightly negative ions. These attractions are much weaker than true ionic or covalent bonds, but they are strong enough to result in some interesting properties.
In the case of water, hydrogen bonds form between neighboring hydrogen and oxygen atoms of adjacent water molecules. The attraction between individual water molecules creates a bond known as a hydrogen bond. See Fig. A molecule of water has two hydrogen atoms. Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules. Every water molecule can be hydrogen bonded with up to three other water molecules See Fig. However, because hydrogen bonds are weaker than covalent bonds, in liquid water they form, break, and reform easily.
Thus, the exact number of hydrogen bonds formed per molecule varies. Molecules of pure substances are attracted to themselves. This sticking together of like substances is called cohesion.
Depending on how attracted molecules of the same substance are to one another, the substance will be more or less cohesive. Hydrogen bonds cause water to be exceptionally attracted to each other. Therefore, water is very cohesive. Our experience with water, however usually involves water touching something else or being acted upon by gravity.
In space, water is able to form perfectly round spheres because the attraction of water to itself pulls the water into the shape with the least amount of surface area compared to the volume — a sphere. A European Space Agency astronaut Pedro Duque of Spain watches a water bubble float between him and the camera, showing his image refracted, on the International Space Station. B A large water sphere made on a 5 cm diameter wire loop by U.
Weird Science. Adhesion is similar to cohesion, but it involves unlike i. At room temperature anywhere from zero degree centigrade to degrees centigrade , water is found in a liquid state.
This is because of the tiny, weak hydrogen bonds which, in their billions, hold water molecules together for small fractions of a second. Water molecules are constantly on the move. If they are moving fast enough they become a gas. A gas is a physical state of matter where the molecules are far apart and moving very quickly.
But, because of the hydrogen bonds, as water molecules come together they stick to one another for a small, but significant amount of time. This slows them down, and holds them closer to one another. They become a liquid; a different state of matter where the molecules are closer and slower than in a gas. Molecular water, therefore is a liquid at room temperature, a fact that is profoundly significant for all living things on this planet.
Everything dissolves in water. Stone, iron, pots, pans, plates, sugar, salt, and coffee beans all dissolve in water. Things which dissolve are called solutes and the liquid in which they dissolve is called a solvent.
In the liquid state, the hydrogen bonds of water can break and reform as the molecules flow from one place to another. When water is cooled, the molecules begin to slow down. Eventually, when water is frozen to ice, the hydrogen bonds become permanent and form a very specific network.
Figure 3. When water freezes to ice, the hydrogen bonding network becomes permanent. Each oxygen atom has an approximately tetrahedral geometry — two covalent bonds and two hydrogen bonds. The bent shape of the molecules leads to gaps in the hydrogen bonding network of ice. Ice has the very unusual property that its solid state is less dense than its liquid state. Ice floats in liquid water. Virtually all other substances are denser in the solid state than in the liquid state.
Hydrogen bonds play a very important biological role in the physical structures of proteins and nucleic acids.
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