Formation of Compounds - Part III

3. Other Bonds
There are two other very simple type of bonds found in nature. One is called metallic bond, where the valence electrons of metal atoms are shared by more than one neighboring atom. This type of bond occurs in metals only. The metal atoms are held together by a “sea” of electrons floating around. Metals consist of a lattice of positive ions through which a cloud of electrons moves. The positive ions will tend to repel one another, but are held together by the negatively charged electron cloud. The mobile electrons can transfer thermal vibration from one part of the structure to another, i.e. metals can conduct heat. They are good conductors of electricity also. Metals are malleable and ductile because the positive ions in a metal are not held by rigid bonds; they are capable of sliding past one another if the metal is deformed.

Another type of bond is called the Van der Waal’s bonds, which is very weak and is usually found between gaseous compounds. In solids there is one example of a graphite crystal. Each planar hexagonal structure in a graphite crystal is attracted to the other by Van der Waal’s forces or bonds. The residual electrical charges left inside a molecule get attracted to neighboring molecule’s positive charge and vice-versa.

Cohesion and Adhesion

Cohesion and adhesion phenomena are two effects seen mainly where liquids are in the picture. For example : formation of water droplets is because of forces acting between the water molecules. Such forces are cohesive forces. A cohesive force is a force that holds together the like atoms, ions, or molecules of a single body.

Attraction between unlike surfaces is due to adhesion. For example : If you put water in a test tube, watch the level of water. The meniscus is upwardly curved. This is because the water molecules on the top surface are experiencing an attractive force with the glass test tube. An adhesive force, is a force that holds two separate bodies together. Instead of water, if you take mercury in a test tube, you will see that the mercury level has an inverted hump. This means that the cohesive forces within the mercury atoms is higher than the adhesive forces between the mercury atoms and glass.

A number of phenomena can be explained in terms of adhesion and cohesion. For example, surface tension in liquids results from cohesion, and capillary action results from a combination of adhesion and cohesion. The hardness of a diamond is due to the strong cohesive forces between the carbon atoms of which it is made. Friction between two solid bodies partly depends upon adhesion.

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