Question

(a) Why do we classify elements?
(b) What were the two criteria used by Mendeleev in creating his Periodic Table?
(c) Why did Mendeleev leave some gaps in his Periodic Table?
(d) In Mendeleev's Periodic Table, why was there no mention of Noble gases such as Helium, Neon and Argon?
​(e) Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.

Answer 1

(a) The need to classify elements into families with similar properties arose because, lacking this basic tool, the chemist had to deal with each of the 45 or so known elements one at a time, as if there were no rhyme or reason to them.

(b) Two criteria used by Mendeleev in creating his periodic table:
(i)Mendeleev's periodic table was based on the observation that the properties of elements are the function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties get repeated after regular intervals.
(ii) Relative atomic mass and similarity of chemical properties.

(c) As at the time of Mendeleev some elements are not discovered, so he realized this fact and left gaps in his table for such elements and also predicted their properties.

(d) Because at the time of Mendleeve these elements are not discovered, so he did not mention about them.

(e) In the modern periodic table, all isotopes are placed in the same position as in modern periodic table is based on the atomic number not on atomic masses.