NCERT Solutions For Class 10 Science Chapter 5 Periodic Classification Of Elements

Class 10 Science Chapter 5 Periodic Classification Of Elements

1. Excercise Questions

2. Intext Questions

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NCERT Solutions For Class 10 Science Chapter 5 Periodic Classification Of Elements

Class 10 Science Chapter 5 Periodic Classification Of Elements

EXERCISE QUESTIONS

 

Q. 1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of Periodic Table :

( a ) The elements become 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 . Correct option : ( c )

Explanation : The atoms lose their electrons more easily . Because on moving from left to right across the periods of the periodic table , the non – metallic character increases . Hence , the tendency to lose electrons decreases .

Q. 2. Element X forms a chloride with the formula XCI₂ , which is a 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 . Correct option : ( b )

Explanation : Magnesium has the valency 2 and valency in a group remains the same . When it combines with chlorine having valency 1 it forms MgCl₂ . Na has valency 1 ( NaCl ) , Al has valency 3 ( AICl₃ ) ; Si has valency 4 ( SiCl₄ ) .

Q. 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 has two shells , both of which are completely filled with electrons ( 2 electrons in K shell and 8 electrons in L shell ) .

( b ) Magnesium has the electronic configuration 2 , 8 , 2 .

( c ) Silicon has a total of three shells , with four electrons in its valence shell ( 2 electrons in K shell , 8 electrons in L shell and 4 electrons in M shell ) .

( d ) Boron has a total of two shells , with three electrons in its valence shell ( 2 electrons in K shell and 3 electrons in L shell ) .

( e ) Carbon has twice as many electrons in its second shell as in its first shell ( 2 electrons in K shell and 4 electrons in L shell ) .

Q. 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 ) All the elements in the same column as boron have the same number of valence electrons ( 3 ) . Hence , they all have valency equal to 3 .

( b ) All the elements in the same column as fluorine have the same number of valence electrons ( 7 ) . Hence , they all have valency equal to 1 .

Q. 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 ) The atomic number of this element is 17 .

( b ) It would be chemically similar to F ( 9 ) with configuration as 2 , 7 .

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

( 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 a non – metal .

( b ) C is less reactive than A , as reactivity decreases down the group in halogens .

( c ) C will be smaller in size than B as moving across a period , the nuclear charge increases and therefore , electrons come closer to the nucleus .

( d ) A will form an anion as it accepts an electron to complete its octet .

Q. 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 .

Nitrogen is more electronegative than phosphorus . On moving down the group , the number of shells increases . Therefore , the valence electrons are from the nucleus and the effective nuclear charge decreases . This causes the decrease in the tendency to attract electron and hence electronegativity decreases .

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

Ans . 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 .

Q. 9. 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 ?

Ans . 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 ) .

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

Ans .

INTEXT QUESTIONS

Page 81

Q. 1. Did Dobereiner’s triads also exist in the columns of Newlands ‘ Octaves ? Compare and find out . [ NCERT Q. 1 , Page 81 ]

Ans . Only one triad of Dobereiner’s triads exists in the columns of Newlands ‘ octaves . The triad formed by the elements Li , Na , and K of Dobereiner’s triads also occurred in the columns of Newlands ‘ octaves .

Q. 2. What were the limitations of Dobereiner’s classification ? [ NCERT Q. 2 , Page 81 ]

Ans . Limitation of Dobereiner’s classification :

( i ) All known elements could not be classified into groups of triads on the basis of their properties .

( ii ) He could find only three triads from all the elements known .

Q. 3. What were the limitations of Newlands ‘ Law of Octaves ? [ NCERT Q. 3 , Page 81 ]

Ans . Limitations of Newlands ‘ law of octaves :

( i ) It was not applicable throughout the arrangements .

( ii ) It was applicable up to calcium only . The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them .

( iii ) Those elements that were discovered after New lands ‘ octaves did not follow the law of octaves .

( iv ) The position of cobalt and nickel in the group of the elements ( F , Cl ) which have different proper ties could not be explained .

( v ) Placement of iron far away from cobalt and nickel , which have similar properties as iron , could also not be explained .

Page 85

Q.4 . Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements :

K , C , Al , Si , Ba . [ NCERT Q. 1 , Page 85 ]

Ans . K is in group 1. Therefore , the oxide will be K₂O .

C is in group 4. Therefore , the oxide will be CO₂ .

Al is in group 3. Therefore , the oxide will be Al₂O₃ .

Si is in group 4. Therefore , the oxide will be SiO₂ .

Ba is in group 2. Therefore , the oxide will be BaO .

Q. 5. Besides gallium , which other elements have since been discovered that were left by Mendeleev in his Periodic Table ? ( Any two ) [ NCERT Q. 2 , Page 85 ]

Ans . Scandium and Germanium .

Q. 6. What were the criteria used by Mendeleev in creating his periodic table ? [ NCERT Q. 3 , Page 85 ]

Ans . Mendeleev’s periodic table was based on the observation that the properties of elements are the periodic function of their atomic masses . This means that if elements are arranged in the increasing order of their atomic masses , then their properties get repeated after regular intervals .

Q. 7. Why do you think the noble gases are placed in a separate group ? [ NCERT Q. 4 , Page 85 ]

Ans . Noble gases are inert elements . Their properties are different from all the other elements . Therefore , the noble gases are placed in a separate group .

Page 90

Q. 8. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table ? [ NCERT Q. 1 , Page 90 ]

Ans . ( i ) Mendeleev was unable to give fixed position to hydrogen and isotopes in the periodic table .

( ii ) In Mendeleev’s periodic table , the increasing manner of atomic mass of the elements is not always regular from one to its next .

( iii ) It was believed that a more fundamental property than atomic mass could explain periodic properties in a better manner .

( iv ) It was Henry Moseley who demonstrated that atomic number of an element could explain peri odic properties in a better way than atomic mass of an element and arranged the elements in increas ing order of their atomic numbers .

( v ) Then it was found that the various anomalies of Mendeleev’s periodic table were removed by the Modern Periodic Table .

Q. 9. Name two elements you would expect to show chemical reactions similar to magnesium . What is the basis for your choice ? [ NCERT Q. 2 , Page 90 ]

Ans . Calcium ( Ca ) and strontium ( Sr ) are expected to show chemical reactions similar to magnesium ( Mg ) . This is because the number of valence electrons ( 2 ) is same in all these three elements . And since chemical properties are due to valence electrons , they show same chemical reactions .

Q. 10. Name

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

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

( c ) three elements with filled outermost shells . [ NCERT Q. 3 , Page 90 ]

Ans . ( a ) Lithium ( Li ) , sodium ( Na ) , and potassium ( K ) have a single electron in their outermost shells .

( b ) Magnesium ( Mg ) and calcium ( Ca ) have two electrons in their outermost shells .

( c ) Neon ( Ne ) , argon ( Ar ) , and xenon ( Xe ) have filled outermost shells .

Q. 11. ( 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 ? [ NCERT Q. 4 , Page 90 ]

Ans . ( a ) Yes . The atoms of all the three elements lithium , sodium , and potassium have one electron in their outermost shells .

( b ) Both helium ( He ) and neon ( Ne ) have filled outermost shells . Helium has a duplet in its K shell , while neon has an octet in its L shell .

Q. 12. In the Modern Periodic Table , which are the metals among the first ten elements ? [ NCERT Q. 5 , Page 90 ]

Ans . Among the first ten elements , lithium ( Li ) and beryllium ( Be ) are metals .

Q. 13. By considering their position in the Periodic Table , which one of the following elements would you expect to have maximum metallic characteristic ?

Ga , Ge , As , Se , Be ? [ NCERT Q. 6 , Page 90 ]

Ans . Since ‘ Be ‘ lies to the extreme left hand side of the periodic table , Be is the most metallic , among the given elements .

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