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CLASS - 10 SCIENCE IMPORTANT QUESTIONS OF MARK 5 FOR BOARD EXAMINATION   

QUESTION NO. 5

(a) How is valency of element determined if its electronic configuration is known ? Determine the valency of an element of atomic no.9 .
(b) Given below are some elements of the modern periodic table. Atomic number of the elements are given in parenthesis:
    A(4), B(9), C(14), D(19), E(20)

 (i) With the help of electronic configuration, find out which one of the above element will have one electron in its outermost shell.
(ii) Which two elements belongs to the same group ? Give reasons for your aswe.
(iii) Which one of the above element belonging to the fourth period has bigger atomic radius and why ?

ANSWER NO.5

(a) An elements valency is determined by the number of electrons in its outer shell. Hence, the number of valence electrons obtained from the electronic configuration of the element gives the valency i.e. the number of electrons lost, gained or shared by the element to attain the noble gas configuration.

 (b)  (i)  A(4)  =  2,2
              B(9)  =  2,7
              C(14)  =  2,8,4
              D(19)  =  2,8,8,1
              E(20)  =  2,8,8,2
Element D will have one electron in its outer shell.
(ii) Element A and E will belong to the same group because according to the modern periodic table the elements present in any one group have the same number of valence electron. Periodic table signify an identical outer shell electronic configuration.
(iii) Element D belongs to the fourth period. Element D has a greater atomic radius,because according to the modern periodic table when move down a group, the atomic radius increase and when move from left to right in a period the atomic radius decreases.

QUESTION NO. 6

(a) What is double displacement reaction ? Explain with an example.
(b) A small amount of quicklime is added to water in a beaker.
(i) Name and define the type of reaction that has taken place.
(ii) Write balanced chemical equation for the reaction for the above reaction and the chemical name of the product formed.
(iii) List two main observations s reaction.

ANSWER NO. 6

(a) DOUBLE DISPLACEMENT REACTION :


 The reaction in which an exchange of ions between the reactants are called double displacement rections.
Na2SO4 + BaCl2 ----------->BaSO4 + 2NaCl
(b) It is combination equation.

(i) DEFINITION :

     In this reaction a single product is formed from two or more reactants.
(ii) CaO + H2O ----------->  Ca(OH)2 + Heat

 In this reaction, calcium oxide and water combines to form a single product, calcium hydroxide.

(iii) OBSERVATION

  1. Calcium oxide reacts vigorously with water to produce slaked lime releasing a large amount of heat.
  2. A hissing sound is produced. Precipitate is formed.

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