We know what molecules are - a cluster of atoms held together by covalent bonds. Here we will discuss the difference between ionic compounds and covalent molecules and the Van der Waals forces.
All molecular substances can either be polar or nonpolar. If they are nonpolar, they almost always exist as gases at room temperature. Why? The forces of attraction between their molecules are not strong enough to keep them from scattering into the gas phase at room temperature. Also, polar covalent substances are usually liquids or solids at room temperature, since the attraction between opposite charges tends to hold one molecule next to a nearby molecule. Because of this close framework held by attraction of polar covalent substances, surface tension exists in polar covalent liquids such as water.
In solids, liquids, and gases (under high pressure), molecules are close enough together that new forces of attraction develop between them. These forces, known as Van der Waals forces, are much weaker than those that exist in chemical bonds between atoms. They occur as a result of shifts in the position of electrons in the molecule. Such a shift produces an uneven distribution of charge and causes one region of a molecule to become temporarily negative. This makes another region of the molecule temporarily positive. This effect induces a temporary dipole moment in the molecule. The larger the molecules, the greater the force of attraction between molecules. Such temporary changes in charge distribution may affect other nearby molecules, producing an effect quite similar to the domino effect. For this reason, at low temperatures and high pressures, gases like oxygen and hydrogen can be changed into the liquid or even the solid phase.
Finally, we will look at a different type of structure, which is responsible for the hardness of diamond. Known as network solids, covalent bonds extend from atom to atom in a continuous pattern. In diamond, each carbon atom is at the center of a tetrahedron with other carbon atoms at each corner. Silicon dioxide and silicon carbide are other substances that are network solids. These type of substances are generally very hard and have very high melting points because of their very strong covalent bonds.
At long last, we have finished our quest. If you feel you need to go back over any material, feel free to do so. Other topics that you can look at are biographies of scientists who discovered most of this material and the history line of development of covalent bonds. Thank you very much for visiting our site and we hope you have had a wonderful educational experience here!
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