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Elements that have similar properties are usually in the same group or family of the periodic table. Within the same group elements have similar electron configurations because the have the same number of valence electrons. Valence electrons are the outermost electrons that are lost, gained or shared with other atoms through chemical reactions. Elements with the same number of valence electrons tend to react in the same ways and produce the same types of compounds.
Here are a few examples:
The Group 1 elements, called Alkali Metals, have one valence electron. The lose this electron to form +1 ion, which gives them the stable electron configuration of a Noble Gas. This results in similarity of compouds - LiCl, NaCl, KCl and so forth.
Similarly, the Group 2 or Alkaline Earth Elements form compounds such as CaCl2 and MgF2 because they lose their two valence electrons.
Group 17 or 7A, the Halogens, have seven valence electrons and have a strong tendency to gain one electon. This gives them a complete octet, or eight electrons in the outer level, like a noble gas. They're all diatomic (have the formulas F2, Cl2, Br2, I2) and react to form similar compounds such as NaF and NaCl.
The Noble Gases, in Group 18 or 8A, are unreactive. They have the stability of a complete octet of eight valence electrons.
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