a. The electron structure of an atom also determines the chemical properties of an atom. So far we have talked only about filling up the K-shell. In order to fill up the Lshell,eight electrons are required. Let us look at an oxygen atom; this has eight protons, eight neutrons, and, therefore, eight electrons revolving about the nucleus. Two of the eight electrons will be found in the K-shell. This means that six are in the L-shell in a normal oxygen atom. In order to fill the L-shell and make it complete, two more electrons would be needed. Now most atoms would like to fill up their outer shell; but for oxygen, this would mean that it would have a total negative charge of -2. It is possible that the oxygen atom could borrow enough electrons from hydrogen atoms to complete the L-shell. Two hydrogen atoms would be needed. The oxygen atom would have a net charge of –2 and each hydrogen atom would then be left with a positive, +1, charge. Since positive attracts negative, this group of particles would tend to hold together. This form of chemical combination is known as ionic bonding. This hydrogenoxygengroup is called a molecule of water (see Figure 1-5).

Figure 1-5. Descriptive pictures of molecules.

b. Notice that the total charge of the oxygen plus the hydrogens is zero. A molecule of water is quite stable; the hydrogen and oxygen do not separate very easily. Sometimes this molecule is called H20 and now the reason is obvious. When the atoms of different elements combine in this way, the substance formed is called a compound. Here the compound, water, is made up of H20 molecules. A compound is defined as the chemical combination of two or more elements.

c. Often in the process of forming compounds, large amounts of energy are given off. Hydrogen gas may explode violently and, when it does, it is simply combining with oxygen to form water.

d. Another type of molecule encountered can be formed from hydrogen. There is no reason why one hydrogen atom cannot unite with another. In this case, there would be two electrons orbiting two hydrogen nuclei. Here the K-shell would be complete at least part of the time for each hydrogen atom (see Figure 1-5). This sort of molecule is called a diatomic molecule. Hydrogen gas is composed of diatomic molecules of hydrogen. Thus, a molecule is the smallest particle of any substance, element, or compound as it normally exists in nature.

e. It is therefore apparent that the electron structures of atoms are important things. They determine the way in which compounds are made and the world we live in is composed mostly of compounds. Also, the electronic structure is intimately involved in the production of visible light and other electromagnetic radiation.

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