Concise Chemistry Class 10 :  Periodic Table Exercise Intext 1 
     
     
    1. (i)  State modern periodic law. 
    Modern periodic law :  "The properties of elements are the periodic functions of their atomic number." 
     
    (ii) Name the scientist who stated the law. 
    Answer :  Henry Moseley stated the modern periodic law.  
     
    (iii) How many groups and periods does modern periodic table have? 
    Answer :   Modern Periodic table has 7 periods and 18 groups. 
     
    2.  What are horizontal rows and vertical columns in a periodic table known as? 
    Answer :   The horizontal rows are known as periods and vertical columns in the periodic table are known as groups. 
     
    3.  Periodicity is observed due to the similar............. 
    (number of valence electrons/ atomic number/ electronic configuration) 
    Answer :   Periodicity is observed due to the similar electronic configuration. 
     
    4.  How does electronic configuration in atoms change 
    (i) In a period from left to right? 
    (ii) In a group top to bottom? 
    Answer :   (i) In a period from left to right, the number of shells remain the same but number of valence electrons 
    increases by one. 
                                K    L 
    e.g.   Li (3)    :    2    1 
             Be (4)   :    2    2 
    (ii) In a group top to bottom, the number of shells increases successively i.e. one by one but the number of valence 
    electrons remains the same. 
                                K    L     M 
    e.g.   Li  (3)    :    2    1 
             Na (4)   :     2    8     1   
     
    5.  Name 2 elements in each case : 
    (i) Alkali metals 
    Answer :  Lithium (Li) and Sodium (Na) 
    (ii) Alkaline earth metals 
    Answer :  Magnesium (Mg) and Calcium (Ca) 
    (iii) Halogens 
    Answer :  Fluorine (F) and Chlorine (Cl) 
    (iv) Inert gas 
    Answer :  Helium (He) and Neon (Ne)  
     
    6.  Elements of group 1 and elements of group 17 both have valency 1. Explain. 
    Answer :  Valency is the combining capacity of the atom of an element. It is equal to the number of electrons an 
    atom can donate or accept or share. Valency depends on the number of electrons in the outermost shell (i.e. valence 
    shell). Group -1 elements have 1 electron in their outermost orbital, while Group-17 elements have 7 electrons in 
    their outermost orbital. 
    Group -1 elements can donate one electron while combining with other elements to obtain a stable electronic 
    configuration and hence they have valency = 1. 
     
       
      On the other hand, Group -17 elements can accept 1 electron from the combining atom to obtain a stable electronic 
      configuration and hence they have valency =( 8 – 7) = 1 
      7.  Correct the statements. 
      (i) Elements in the same period have equal valency. 
      Answer :  Elements in the same group have equal valency. 
      (ii) Valency depends upon the number of shells in an atom. 
      Answer :  Valency depends upon the number of valence electrons in an atom. 
      (iii) Copper and zinc are representative elements. 
      Answer :  Copper and zinc are transition elements. 
      (iv) Transition elements are placed at the extreme right of the periodic table. 
      Answer :  Noble gases are placed at the extreme right of the periodic table. 
       
      8.  What do you understand by? 
      (i) Periodicity   
      Periodicity : The properties that reappear at regular intervals, or in which there is a gradual variation at regular 
      intervals, are called periodic properties and the phenomenon is known as the periodicity of elements. 
       
      (ii) Typical elements 
      Typical elements  :  The third-period elements, Na, Mg, Al, Si, P, S and Cl summarize the properties of their respective 
      groups and are called typical elements. 
       
      (iii) Orbits 
      Orbits  :  Electrons revolve around the nucleus in certain definite circular paths called orbits or shells. 
       
      9.  Name 2 elements you would expect to show chemical reactions similar to calcium. What is the basis of your 
      choice? 
      Answer :  Beryllium and magnesium will show similar chemical reactions as calcium. Since these elements belong to 
      same group 2 and also have two electrons in their outermost shell like calcium. 
       
      10.  Name the (i) metals, (ii) metalloids and (iii) non-metals in the first twenty elements. 
      Answer : (i)  Metals : Lithium, Beryllium, Sodium, Magnesium, Aluminium, Potassium, Calcium 
      (ii) Metalloids : Boron, Silicon 
      (iii) Non-metals : Hydrogen, Helium, Carbon, Nitrogen, Oxygen, Fluorine, Neon, Phosphorous, Sulphur, Chlorine, 
      Argon  
       
      11.  Fluorine, Chlorine and Bromine are put in one group on the basis of their similar properties. 
       (i) What are those similar properties?    
      Similar Properties :  a) They are non-metals with seven valence electrons.  
      b) They have the highest electronegativity, ionisation potentials and electron affinity in the respective periods.  
      c) They exist as diatomic molecules (F , Cl , Br ). 
                              2 2  2
      d) They form ionic compounds with alkali metals( e.g. NaCl, KF, KBr). 
      (ii) What is the common name of this group or family?  
      Answer : The common name of this group or family is halogens i.e. Salt-forming.  
       
      12.  What is the main characteristic of the last element in each period of the Periodic Table? What is the general 
      name of such elements? 
      Answer : #  The main characteristic of the last element in each period of the periodic table is they are inert or 
      chemically unreactive. 
      ##  The general name of such elements is 'Noble gases'. 
       
       
          
         13.  According to atomic structure, what determines which element will be the first and which will be the last in a 
         period? 
         Answer :  According to atomic structure, the number of valence electrons determines the first and the last element 
         in a period.  For first element in a period, the number of valence electron is 1  and the last element in a period, the 
         number of valence electron is 8. (except for He, it is 2) 
          
         14.  How does the number of : 
         i. Valence electrons and 
         ii. Valency vary on moving from left to right in the third period of the periodic table? 
         Answer :  i. The valence electrons increase from 1 to 8 in the 3rd period of the periodic table. 
          ii. On moving from left to right, the valency increases from 1 to 4 [Na(1), Mg(2), Al(3), Si(4) ]and then decreases from 
         4 to 0 [P(3), S(2), Cl(1), Ar(0)].  
          
         15.  Name the type of elements, which have their 
         (i) Outermost shell complete 
         Ans :  Noble gases 
         (ii) Outermost shell incomplete 
         Ans :  Representative elements 
         (iii) Two outermost shells incomplete 
         Ans :  Transition elements 
         (iv) One electron short of octet 
         Ans :  Halogens 
         (v) Two electrons in the outermost orbit. 
         Ans :  Alkaline Earth metals  
           
         16.  An element has 2 electrons in its N shell 
         (i) What is its atomic number? 
         Ans :    K    L    M    N 
                      2    8     8    2  
         Atomic number = 20 
         (ii) State its position in periodic table 
         Ans :  Period = 4 and Group = 2 
         (iii) Is it metal or non-metal? 
         Ans : It is a metal. 
         (iv) State the name assigned to this group. 
         Ans :  Alkaline Earth metals  
         (v) What is the valency of this element? 
         Ans :  2 
          
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         17.  Answer the following in respect of element 16S   
         (i) Give its electronic configuration. 
         Ans :                                                                   K     L    M     
                    Electronic configuration of S(16) :     2     8    6      
          
         (ii) To which group and period does it belong? 
         Ans :  Group = 16 and Period = 3 
         (iii) What is its valency? 
         Ans :   Valency = (8-6) = 2 
         (iv) Is it metal or non-metal? 
         Ans : It is a non-metal. 
         (v) Is it a reducing agent or oxidizing agent? 
          
       
      Ans : It is an oxidizing agent. 
      (vi) Give its formula with hydrogen. 
      Ans :  Formula with hydrogen = H S 
                           2
       
      18.  Name  
      a)  An alkali metal in period 3 and halogen in period 2 
      Ans :   Alkali metal in period 3 is Sodium (Na) halogen in period 2 is Fluorine (F).       
      b) The noble gas with 3 shells 
      Ans :   The noble gas with 3 shells  is Argon (Ar).       
      c) The non-metals present in period 2 and metals in period 3. 
      Ans :   The non-metals present in period 2 are Carbon (C), Nitrogen(N), Oxygen(O), Fluorine(F) & Neon(Ne) and 
      metals in period 3 are Sodium(Na), Magnesium(Mg) & Aluminium(Al). 
      d) The element of period 3 with valency 4. 
      Ans :   The element of period 3 with valency 4 is Silicon (Si) 
      e) The element in period 3 which does not form an oxide. 
      Ans :   Argon 
      f) The element of lower nuclear charge out of Be and Mg. 
       Ans :   The atomic number of Be is 4 (p = 4 ), whereas the atomic number of Mg is 12 (p = 12). Therefore, Be will 
      have lower nuclear charge as compared to Mg.  
       
      19.  The electronic configuration of an element T is 2, 8, 8, 1.  
      (i) What is the group number of T?  
       Ans :  As T has 1 valence electron,  Group = 1 
      (ii) What is the period number of T?  
      Ans :  As T has 4 shells,  Period = 4 
      (iii) How many valence electrons are there in an atom of T? 
      Ans :  Valence electrons = 1  
       (iv) What is the valency of T? 
      Ans :  Valency = 1  
      (v) Is it a metal or a non-metal?  
      Ans : It is a metal. 
       
      20.  Match the atomic number 19, 15, 8, 4 and 2 with each of the following: 
      (i) A metal of valency one 
      Ans :  A metal of valency one = 19 (K) 
      (ii) A solid non-metal of period 3 
      Ans :  A solid non-metal of period 3 = 15 (P) 
      (iii) A rare gas 
      Ans :  A rare gas = 2 (He) 
      (iv) A gaseous element with valency 2 
      Ans :  A gaseous element with valency 2 = 8 (O) 
      (v) An element of group 2 
      Ans :   An element of group 2 = 4 (Be)