SCIENCE-10TH CBSE ANSWER KEY

PERIODIC CLASSIFICATION OF ELEMENTS(20-26)
Question 20:How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer: In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same. Elements across a period show an increase in the number of valence electrons.
Question 21:In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21, and 38. Which of these have physical and chemical properties resembling calcium?
Answer: The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).
Question-22What are the merits and demerits of newland’s classification?
Solution:Merits
(i) This classification worked well for lighter elements only up to Ca.
(ii) This classification gave us a relation between the properties of the elements and their atomic masses.
(iii) It was shown by this classification for the first time that there exists a periodicity in the properties of the elements.
Demerits
(i) This classification failed when the heavier elements beyond Ca were arranged according to Newland's law of octaves.
(ii) At the time of this law, noble gases were unknown. When noble gases were discovered, neon (Ne) between F and Na, and argon (Ar) between Cl and K, it becomes the ninth element and not the eighth which has the similar properties.
Question-23X and Y are two elements having which have similar properties and obey Newland’s land of octaves. How many elements are there in-between X and Y?
Solution:Six.
Question-24What is achievement of Dobereiner’s law of traids?
Solution:The law recognised the relation between atomic weight of an element and its chemical properties for the first time.
Question-25Lithium, Sodium and potassium were put in the same group on the basis of their similar properties.
i) What is the similarity in their properties?
 Solution:i) Li, Na and K are very reactive alkali metals, which react with water with the formation of an alkali and hydrogen gas.
Question-26State two reasons for rejecting law of octaves.
Solution:(i) The law did not extend properly beyond the element calcium.
(ii) The law did not provide any specific place for hydrogen.
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