Early attempts at the classification of elements mendeleeve’s periodic table

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05 Periodic table

MENDELEEV’S PERIODIC TABLE

  • Dmitri Ivanovich Mendeléev was a Russian chemist. He is considered to be the most important contributor to the early development of the periodic table.
  • Newlands Octave Law was rejected and in 1869, Mendeleev Periodic table came into the picture. Mendeleev periodic table was the most important among many periodic tables that were made.
  • Only 63 elements were known when Mendeleev conducted experiments. He took 63 cards and wrote the properties of one element on each card.
  • He grouped the elements with similar properties and pinned it on the wall. He studied the properties of every element and found that the properties of elements were related to atomic mass in a periodic way.
  • He observed that elements were arranged in the increasing order of atomic mass and there was the periodic occurrence of elements with similar properties.
  • So, he arranged the elements such that elements with similar properties fell into same vertical columns of the periodic table.
  • The arrangement of elements is on the basis of their atomic masses and similarity of their chemical properties.
  • Base on this observation, Mendeléev gave a periodic law in 1869.
  • According to Mendeleev’s periodic law: The properties of elements are a periodic function of their masses. The original periodic table of Mendeleev consists of seven horizontal rows called periods and eight vertical columns called groups. Mendeleev was guided by two factors:
  • Increasing atomic masses, and Grouping together of elements having similar properties.
  • Mendeleev considered formulae of hydrides and oxides as one of the basic criteria for categorization, among chemical properties.

MERITS OF MENDELEEV’S CLASSIFICATION OF ELEMENTS:

  • Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time. Mendeleev’s periodic table left proper gaps for the elements that had not been discovered at that time, like gallium (Ga), scandium (Sc) and germanium (Ge)
  • Mendeleev’s periodic table could predict the properties of several elements on the basis of their positions in the periodic table. The properties of the then undiscovered elements like gallium, scandium and germanium were predicted in this way.
  • Mendeleev’s periodic table could accommodate noble gases when they were discovered. They were placed in a new group without disturbing the existence order of other elements.

ANOMALIES (OR LIMITATIONS) OF MENDELEEV’S CLASSIFICATION OF ELEMENTS

  • Isotopes are the elements having similar chemical properties but different atomic masses. Mendeléev gave no explanation about the position of isotopes
  • No proper explanation about the wrong order of atomic masses of some elements. For example, on arrangement of elements on the basis of chemical properties; the element cobalt having higher atomic mass of 58.9 comes first and nickel element with slightly lower atomic mass of 58.7 comes later. Mendeleev’s Periodic law could not explain this abnormal situation of wrong of atomic masses.
  • No fixed position was assigned to hydrogen on the periodic table.

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