By changing the electron configuration around these two elements, their chemical properties have been drastically changed. Figure 3 shows the formation of sodium and chloride ions. We refer to this attraction as an ionic bond. The opposite charges on these ions create an attraction that will hold them together. Ions that have a net negative charge are called anions (think of the term anion as an acronym standing for a negative ion). Chlorine picked up one electron and now its outer electron shell is also full and in the process has become an ion with a -1 charge (one more electron than protons). Ions that have a net positive charge are called cations. An ion is an atom that has a net + or – charge. However, the result of losing one electron leaves sodium with one more proton than electron and, therefore, it is now an ion with an electrical charge of +1. Sodium now has 8 electrons in its outer shell and is stable. When these two elements react, sodium gives the one electron in its outer shell to chlorine. Chlorine, on the other hand, has an atomic number of 17 and has 7 electrons in its outer shell. Sodium has an atomic number of 11, hence, sodium has one electron in its outer electron shell. To explain how ionic bonds form we will use common table salt, NaCl, as an example. Hence, H 2 (same element) is a molecule, and H 2O (different elements) is both a molecule and a compound. Molecules composed of more than one type of element can also be called compounds. Molecules are two or more atoms held together by the sharing of electrons (described below). In some cases this involves the formation of molecules. The processes that result in the filling of the outer electron shells result in the formation of chemical bonds. The sections below describe some of the important processes by which atoms become stable. The atoms of all of the other elements have vacancies in their outer electron shells and will react with other atoms to fill their outer shells. Chemists often refer to the “rule of 8” which states that if there are 8 electrons in the outer electron shell, the element is stable. These elements are all gasses and they are all stable, meaning that they do not react with other elements. helium, neon, argon, etc) have 8 electrons in their outer shells (with the exception of helium), hence their outer electron shells are full. Close examination of the periodic table will show that the atoms of all of the elements in the last column of the table (i.e.
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