Periodic Relationships Among the Elements

The periodic table places the most reactive metals in Group 1A and the most reactive nonmetals in Group 7A. When these elements are mixed, we predict a vigorous reaction to ensue, as evidenced by the formation of sodium chloride from sodium and chlorine. The models show Na metal, CI2 molecules, and NaCl. 

Gallium melts in a person s hand (body temperature is about 37 C). 

In the nineteenth centnry, when chemists had only a vague idea of atoms and molecules and did not know of the existence of electrons and protons, they devised the periodic table using their knowledge of atomic masses. Accurate measurements of the atomic masses of many elements had already been made. Arranging elements according to their atomic masses in a periodic table seemed logical to those chemists, who felt that chemical behavior should somehow be related to atomic mass. 

In 1864 the English chemist John Newlands noticed that when the elements were arranged in order of atomic mass, every eighth element had similar properties. Newlands referred to this peculiar relationship as the law of octaves. However, this law turned out to be inadequate for elements beyond calcium, and Newlands s woik was not accepted by the scientific community. 

With a few exceptions, Moseley found that atomic number increases in the same order as atomic mass. For example, calcium is the twentieth element in order of increasing atomic mass, and it has an atomic number of 20. The discrepancies that had puzzled earlier scientists now made sense. The atomic number of argon is 18 and that of potassium is 19, so potassium should follow argon in the periodic table. 

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