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Periodic Table of the Elements |
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With so many elements already found and the possibility of more being discovered,
chemists needed a way to organize them.
Many systems were tried in order to make some sort of pattern in their properties to match the table.
The modern periodic table, based on atomic number and electron configuration, was created primarily by a Russian chemist, Dmitri Ivanovich Mendeleev, and a German physicist, Julius Lothar Meyer, both working independently.
They both created similar periodic tables only a few months apart in 1869.
Mendeleev created the first periodic table based on atomic weight. He observed that many elements had similar properties, and that they occur periodically, hence the name, periodic table. From this, he made the periodic law. |
His periodic law states that the chemical and physical properties of the elements vary in a periodic way with their atomic weights.
The modern one states that the properties vary with atomic number, not weight.
For example, the elements lithium, sodium, potassium, rubidium, and cesium have similar chemical properties.
The elements that immediate follow them, beryllium, magnesium, calcium, strontium, and barium, also have similar chemical properties.
Elements in Mendeleev's table were arranged in rows called periods. The columns were called groups. Elements of each group had similar properties. By Mendeleev's theory, they should have been perfectly arranged by increasing atomic weight. |
Since Mendeleev's table was based on atomic weight, some things didn't match perfectly.
For example, tellurium and iodine caused Mendeleev some problems.
The atomic mass of tellurium was greater than iodine according to the best estimates of that time,
but they didn't fit the groups that Mendeleev devised.
In fact, they should be switched to follow the groups he devised.
The reason for this discrepancy is the fact that atomic number (the number of protons in an atom), not atomic weight, determines the order of the elements in the table, the basis of the modern periodic table. |
