Wednesday, November 14, 2012

X chemistry Ch- 5 Periodic Classification of Elements

Q.1. 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
Q.2.Why inert gases have zero valencies?

Ans: It is because Inert gases have 2 or 8 electrons in valence shell.
Q.3. 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.
Q.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.                                                
                                           Therefore, Mendeleev was not able to explain the position of hydrogen.
Q.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).
Q.6. Why was Dobereiner’s system of classification of elements into triads not found to be useful?

Ans: it is because he could not arrange all elements in triads.
Q.7. Oxygen (O, 8) and sulphur (S, 16) belong to group 16 of the periodic table :-
(i) Write the electronic configuration and valency of these two elements?
(ii) Which among these will be more electronegative? Why?

Ans(i) Oxygen (O, 8) = 2,6  ; Vacancy =2  
Sulphur (S, 16) = 2,8,6 ; Vacancy =2  
(ii) Oxygen will be more electronegative due to electro negativity decreases from top to bottom because atomic size increases.
Q.8. How does electronic configuration of atoms change in a period with increase in atomic number?

Ans. On moving across a period from left to right, the atomic number of the elements increases, therefore, the number of electrons in the valence shell increases from 1 to 8, i.e, the first element in the given period will have one electron in its valence shell and the last element in the same period will have eight electrons.
Q.9. What happens to the melting points and boiling points of elements while moving down in a group?

Ans. The melting points and boiling points decrease while moving down in group of metals.
The melting points and boiling points increases while moving down in group of non-metals.
Q10. Why Chlorine (atomic number 17) is more electronegative than sulphur (atomic number 16)

Ans: The nucleus of chlorine has more tendency to attract an extra electron than the nucleus of sulphur because chlorine needs only one 1 electron to complete its shell. Hence, chlorine is more electronegative than sulphur.

Related posts for further Study: Chapter: X chemistry Ch - 5 Periodic Classification of Elements 

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