Periodic Classification Of Elements Page 81

  1. Did Dobereiner’s  triads also exist in the columns  of  Newland’s Octaves? Compare and find out.

Ans:- yes, Dobereiner’s triads also exist in the column of Newland’s law of octaves because both were based on increasing order of their atomic masses and the element belonging to Dobereiner’s triads were present in the same manner in Newlands column.

For example if we see the second column of newland’s  law of octaves  

Li, Na, K, Cu

2. What are the limitation of Dobereiner’s classification?

Ans:- The limitations are   (a) he was only to give only three traids (b) he was able to classify only 9 elements but more than 9 elements were already discovered.

3. What are the limitation of Newland’s law of octaves?

Ans:-  (i) his major mistake was that he told no elements were discovered after 56.

(II) To full fill his own condition he put even two elements in a single slot.  Ex – (Co and Ni), (Ce and La)

(iii) when noble elements were discovered the properties of 9th element become similar to the first one from where we start.

(iv) He gave the wrong order of atomic masses (a) Co and Ni (b) Ce and La , but  his periodic chart was based on increasing order of their atomic masses.

(v) it was applicable up to calcium only.

(vi) The properties of Cobalt and Nickel are very similar  to iron, but according to his rule cobalt and nickel were very far from iron.

Page 85

  1. Use Mendeleev’s  periodic  table  to predict the formulae for the oxides of following elments?

K,   C,  Al,   Si,  Ba

Ans:-  K2O,  CO or CO2 ,  Al2O3,  SiO2 ,   BaO

2. Beside Gallium,  which other elements have since been discovered that were left by Mendeleev in his periodic  table?(Any two)

Ans:-  Mendeleev  predicted                     discovered element
Eka boron                                          Scandium
Eka alluminium                               Gallium
Eka Silicon                                            Germanium
Eka Magnese                                   Rhenium

3. What were the criteria used by Mendeleev in creating his periodic table?

Ans:- when he was arranging elements he put following  criteria in his mind

  • He arranged all the know elements in the increasing order of their atomic masses. i.e the properties of elements are periodic of atomic masses.
  • In a single vertical column, all the elements have similar properties.
  • He used the formulae of elements with hydrogen and oxygen. ( the formulae of hydrides and oxides)

4. Why do you think the noble gases are placed in a separate groups?

Ans:- After a long period of time of Mendeleev’s periodic table, the noble or inert gases were discovered and they were not disturbing the  Mendeleev periodic rules if these noble gases elements were separated in a group.

Page 90

  1. How could modern periodic Table remove various anomalies of Mendeleev Periodic Table?

Ans :- There are different rectification in Mendeleev periodic table after Modern Periodic table  

  • Mendeleev gave the wrong order of Cobalt-Nickel according to his law because his law was based on the increasing order of atomic masses. This was rectified after the modern periodic table because the modern periodic table is based on an increasing atomic number.
  • He was unable to give the position of isotopes because his periodic chart was based on increasing atomic mass but after the modern periodic table, this was also rectified because the modern periodic table is based on increasing atomic numbers.
  • He gave subgroup A and subgroup B in a single group but when a modern periodic table came these two subgroups divided into separate groups.

2. Name two elements you would expect to same kind of chemical reactivity as magnesium. What is the basic for your choice?

Ans:- Ca, Sr,  Ba,  Ra are the elements having similar chemical properties because they belong to the same group and they have a similar electronic configuration.

3. Name

(a) three elements that have only single electron in their outermost shells.

(b) two elements that have two electrons in their outermost shells.

(c) there elements with filled outermost shells.

Ans:- (a) group 1 elements:- Li, Na, k, Rb, Cs, Fr

           (b) group 2 elements:-  Be, Mg, Ca, Sr, Ba, Ra

          (c) group 13 elements:- B, Al, Ga, In, Tl

4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What if anything, do their atoms have in common?

Ans:- (a) They have similar electronic configuration and their valance electron is 1.

          (b) they have their outermost shell are completely filled.

5. In the modern periodic table, which are the metals among the first ten elements?

Ans:- The first ten elements are:- Hydrogen(H), Helium(He), Lithium(Li), Beryllium(Be),  Boron(B), Carbon(C), Nitrogen(N), Oxygen(O), Fluorine(F), Neon(Ne)

Among above  Li, Be are metals. 

6. By considering their position in the periodic table,  which one of the following elements would you expect to have maximum metallic Characteristics?  Ga,  Ge,  As,  Se,  Be  

Ans:- Be and Ga should be more metallic in nature. Because as we move left to right in a period the metallic character decreases and moving down in a group the metallic character increases because the size of atom increases that makes easy to lose electrons of the outermost electrons, So gallium(Ga) will have the most metallic character due to more atomic size. 

Page number 91 and 92 Exercises

  1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of the periodic table. (a) the element becomes less metallic in nature.   (b) the number of valence electrons increases  (c) the atoms lose their electrons more easily.  (d) the oxides become more acidic

Ans:- (C) the atoms lose their electrons easily

2. Element X forms a chloride with the formula XCl2, which is solid with a high melting point. X would most likely be in the same group of the periodic table as (a) Na  (b) Mg  (c) Al  (d) Si

Ans:- (b) Mg

3. Which element has

(a) Two shells, both of which are completely filled with electrons.

(b) the electronic configuration 2, 8, 2?

(c) a total of three shells, with four electrons in its valence shell?

(d) A total of two shells, with three electrons in its valence shell?

(e) twice as many electrons in its second shell as in its first shell?

   Ans:- (a) neon [Ne] (b) magnesium [Mg]  (c) silicon [Si] (d) boron  [B] (e) carbon [C]

4. (a) What property do all elements in the same column of the periodic table as boron have in common?

(b) What property do all elements in the same column of the periodic table as fluorine have in common?

  Ans:- (a) (i) they have same valence electrons  (ii) they have same valency

(b), (i) they have same valence electrons  (ii) they have same valency (iii) all are non-metals

5. An atom has electronic configuration 2, 8, 7  (a) what is the atomic number of this element?   (b) to which of the following elements would it be chemically similar? (atomic numbers are given in parentheses.)   N(7)   F(9)    P(15)    Ar(18)

Ans:-  (a) 17  (b) The element will be chemically similar to F(9) because of similar electronic configuration.

6. The position of three elements A, B and C in the Periodic Table are shown below –                                                               


Group 16
Group 17
……….……….
……..A
……………..
BC
  • (a )State whether A is a metal or non-metal.

(b)State whether C is more reactive or less reactive than A.

(c) Will C be larger or smaller in size than B?

(d) Which type of ion, cation or anion, will be formed by element A?

Ans:- (a) A is non-metal.

(b)C is less reactive than A because in non-metallic group on moving down the reactivity decreases

(c) the atomic size of C is smaller than B because on moving left to right in a period the atomic size decreases.

(d) A will produce anion(A)  because A is non-metal.

7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

Ans:-  the electronic configuration of nitrogen:- 2, 5

           The  electronic configuration of phosphorus:- 2, 8, 5

Nitrogen is more electronegative than phosphorus because nitrogen has less atomic size than phosphorus and we also know that on moving down in a group the electronegativity decreases.

8.How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Ans:- if we know the atomic number of the element and we write the electronic configuration then we will come to know the following things

(i) By knowing the numbers of shells or orbits we will come to know the number of period

Eg. the electronic configuration of K= 2, 8, 8, 1 it has 4 orbits then number of period of the K= 4

(ii) By writing electronic configuration we will come to know which block that element belongs.

Eg. K has last electron enters the S subshell so it belongs to S block

(iii) By writing electronic configuration we come to know which group that element belongs. Eg. K belongs to S block so K belongs to group number 1

Note:- Rule for obtaining number of group :- (a) First of all we decide the  the block of element

(b)After deciding the block of element we use the following rules   (i) for S block , the number of group = the valance electrons

(ii) for p block, the number of group= 10 + valence electrons  

(iii)For d block, the number of group= (valence electrons + second last orbit electron) – 8

(iv) f-block elements always lie in group number 3

9.   In Modern Periodic Table, calcium (atomic number 20) is surrounded by elements numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

Ans:-

 Mg(12) 
K(19)Ca(20)Sc(21)
 Sr(38) 

The atomic numbers 12 and 38 have similar properties with calcium because they belong to the same group and have a similar electronic configuration.

10. Compare and contrast the arrangement of elements on Mendeleev’s Periodic Table and the Modern Periodic Table.

Ans:- the differences and similarities in Mendeleev’s periodic table and Modern periodic table.

Differences 

Mendeleev’s periodic TabelModern periodic table
the number of group 8the number of group 18
the elements were arranged in increasing order of atomic masses.2. the elements were arranged in increasing order of atomic numbers.
Noble gases were not included3.Nobles gases were included
A group was divided into two sub groups    4.All groups are independent.
This periodic table doesn’t give the position of isotopes5. This periodic table gives the position of isotopes
The number of period is 66. The number of period is 7

Similarities

  • In both the periodic table elements were classified in group and periods.
  • Elements with similar properties are placed in the same group.
  • Metals and Non-metals are placed separately.
  • Both the classification makes the study easy.