Q24 of 60 Page 1

(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.

(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 together. That’s why we classify elements.


(b) Mendeleev used atomic mass and similarities in chemical properties to classify the elements.


(c) Mendeleev predicted existence of some elements in the periodic table which were not discovered at that time, so he left place for those elements in the periodic table.


(d) Nobel gases are inert and are present in extremely low concentration in the atmosphere.


That’s why Mendeleev didn’t placed Nobel 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.


(d) Since Mendeleev used atomic mass and similar properties as a criteria 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 same slot. This was a contradiction and therefore, the position of the isotopes of an element was not clear in the Mendeleev’s periodic table.


More from this chapter

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22

Explain the following terms related to spherical lenses:

(i) optical center


(ii) centers of curvature


(iii) principal axis


(iv) aperture


(v) principal focus


(vi) focal length


(b) A converging lens has focal length of 12 cm. Calculate at what distance should the object be placed from the lens so that it forms an image at 48 cm on the other side of the lens.

 23

(a) List the parts of the human eye into it. Explain how they perform this function.

(b) Write the function of retina in human eye.


(c) Do you know that the corneal-impairment can be cured by replacing the defective cornea with the cornea of the donated eyes? How and why should we organize groups to motivate the community members to donate their eyes after death?

 25

In an experiment to study the properties of ethanoic acid, a student takes about 3 ml of ethanoic acid in a dry test tube. He adds are equal amount of distilled water to it and shakes the test tube well. After sometime he is likely to observe that

 26

A student takes about 2 mL ethanoic acid in a dry test tube and adds a pinch of sodium hydrogen carbonate to it. He reports the following observations:

I. Immediately a colourless and colourless gas evolves with a brisk effervescence.


II. The gas turns lime water milky when passed through it.


III. The gas bums with an explosion when a burning splinter is brought near it.


IV. The gas extinguishes the burning splinter that is brought near it.


The correct observations are