Periodic Classification of Elements cbse 10 chemistry notes

9/15/2015 CBSE



1.Why inert gases have zero valencies?





Ans: It is because Inert gases have 2 or 8 electrons in valence shell.


2. How is valency of an element determined?





Ans: The valency of an element measures its ability to combine with other
elements. The valency is determined by knowing place in periodic table. . All
the elements in group VIII have eight electrons in their outer shells, and thus
have a valency of zero (highly stable). Elements in group I just have one
valence electron in their outer shells and thus have a valency of one.





3. Why does silicon is classified as Metalloid?





Ans: Silicon�is gray color solid at room temperature with very high melting
point and boiling point�that lose or gain 4 electrons [�3s2, 3p2]
having both metallic and non metallic properties so it is classified as
Metalloid eg. Sio2


4. why could no fixed position be given to hyderogen in Mendleev�s
Periodic table?





Ans: Hydrogen placed in the first column above alkali metals. it is because
hydrogen and alkali metals have similar properties.

However, hydrogen also resembles halogens. Like halogens, hydrogen is a gas, and
exists as a diatomic molecule (H2). It forms covalent compounds like halogens
unlike alkali metals. Hence, it can also be placed above the halogen group.
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Therefore, Mendeleev was not able to explain the position of hydrogen.


5. In modern periodic table, the isotopes of Chlorine Cl-35 and Cl-37
having different atomic masses will be kept in different slots or they would be
assigned same position on the basis of their chemical properties? Give reason in
support of your answer





Ans: It is because�periodic table�is based on the atomic�numbers of the element
and both the�isotopes�of chlorine have the same atomic number (Z = 17).



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