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 like 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

To answer this question, you should recall the formation of the periodic table. You should be aware of the symbols of various elements in the periodic table. The modern periodic table is formulated only according to atomic number.

Complete step by step answer:
A. Due to discovery of different elements, it was difficult to organize the information and properties of the elements, so there was a strong need to find some pattern in the properties of different elements so that they can be grouped. That's why we classify elements.
B. Mendeleev used atomic mass and similarities in chemical properties to classify the elements.
C. Mendeleev predicted the existence of some elements in the periodic table which were not discovered at that time, so he left the place for those elements in the periodic table.
D. Noble gases are inert and are present in extremely low concentration in the atmosphere. That's why Mendeleev didn't place Noble gases in the periodic table. When they were discovered, they were placed in a different group so that they don't create any difference in the periodic table.
E. Since Mendeleev used atomic mass and similar properties as a criterion to classify elements but $Cl - 35$ and $Cl - 37$ have different atomic masses, so they should be placed at different places but the two isotopes have similar chemical properties, so they should be placed in the same slot. This was a contradiction and therefore, the position of the isotopes of an element was not clear in Mendeleev's periodic table. The isotopes of chlorine, $Cl - 35$ and $Cl - 37$ are placed in the same slot because they have similar chemical properties and same atomic number.

Note:
There are 4 blocks in the periodic table: s-block, p-block, d-block and f-block. All of the s-block elements are metals. Usually, they are shiny, silvery, good conductors of heat and electricity, and lose their valence electrons easily. The p-block due to its ability to accommodate a large number of electrons has the biggest variety of elements and is the only block that contains three types of elements: metals, nonmetals and metalloids. d-block elements have properties that are between s-block elements and p-block elements properties. It consists of transition metals. The f-block elements are mostly radioactive. The block names were created to represent the quality of the spectroscopic lines of the atomic orbitals.