Q And A Q1: What are the defects in Modern Periodic Table? - Class 11 PDF Download

 Page 1


     Q And A 
 
Q1: What are the defects in Modern Periodic Table? 
 
Answer: Following are the defects: 
? Position of hydrogen still dicey. It is not fixed till now.  
? Position of lanthanides and actinides has not been given inside the main body of periodic 
table.  
? It does not reflect the exact distribution of electrons of some of transition and inner transition 
elements.  
Q2: What are the salient features of Modern Periodic Table? 
 
Answer:  
? In a period of the periodic table, the number of valence electrons increases as atomic number 
increases. As a result, elements change from metal to metalloid to nonmetal to noble gas.  
? Atomic size is a periodic property. As atomic number increases in a period, atomic radius 
decreases. As atomic number increases in a group, atomic radius increases.  
? Positive ions have smaller atomic radii than the neutral atoms from which they derive. 
Negative ions have larger atomic radii than their neutral atoms.  
? Positive ions in the same group increase in size down the group.  
? In a group, each element has the same number of valence electrons. As a result, the 
elements 
in a group show similar chemical behavior.  
? Metallic character decreases from left to right in a period because of the increase in the 
effective nuclear charge.  
? Non-metallic character increase from left to right in a period because of ncrease in 
effective nuclear charge.Non-metallic character decreases down the group because of 
increase in the size of the 
atom.  
Q3: Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic 
configuration for these elements. To which period of the modern periodic table do these two 
elements belong? What type of bond will be formed between them and why?  
 
Answer: Electronic configuration of X (Z=12): 2, 8, 2 
Electronic configuration of Y (Z=16): 2, 8, 6 
Both these elements belong to third period. An ionic bond is formed between X and Y due to transfer 
of two electrons from X to Y. 
 
Q4: The present classification of elements is based on which fundamental property of 
elements? 
 
Answer: Atomic Number is the basis. 
 
Q5: Li, Na and K are the elements of a Dobereiners Triad. If atomic mass of Li is 7 and that of K 
is 39. What would be the atomic mass of Na? 
 
Answer: According of Dobereiner’s law of triads, the atomic mass of the middle element, in this case 
Na, should be the arithmetic mean of Li and K. 
Thus,  
arithmetic mean of Li and K = (7 + 39)/2 = 23 (Answer) 
 
 
Q6: Define Dobereiner’s law of triads. 
 
Answer: It states, "when elements are placed in order of the ascending order of atomic masses, 
Page 2


     Q And A 
 
Q1: What are the defects in Modern Periodic Table? 
 
Answer: Following are the defects: 
? Position of hydrogen still dicey. It is not fixed till now.  
? Position of lanthanides and actinides has not been given inside the main body of periodic 
table.  
? It does not reflect the exact distribution of electrons of some of transition and inner transition 
elements.  
Q2: What are the salient features of Modern Periodic Table? 
 
Answer:  
? In a period of the periodic table, the number of valence electrons increases as atomic number 
increases. As a result, elements change from metal to metalloid to nonmetal to noble gas.  
? Atomic size is a periodic property. As atomic number increases in a period, atomic radius 
decreases. As atomic number increases in a group, atomic radius increases.  
? Positive ions have smaller atomic radii than the neutral atoms from which they derive. 
Negative ions have larger atomic radii than their neutral atoms.  
? Positive ions in the same group increase in size down the group.  
? In a group, each element has the same number of valence electrons. As a result, the 
elements 
in a group show similar chemical behavior.  
? Metallic character decreases from left to right in a period because of the increase in the 
effective nuclear charge.  
? Non-metallic character increase from left to right in a period because of ncrease in 
effective nuclear charge.Non-metallic character decreases down the group because of 
increase in the size of the 
atom.  
Q3: Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic 
configuration for these elements. To which period of the modern periodic table do these two 
elements belong? What type of bond will be formed between them and why?  
 
Answer: Electronic configuration of X (Z=12): 2, 8, 2 
Electronic configuration of Y (Z=16): 2, 8, 6 
Both these elements belong to third period. An ionic bond is formed between X and Y due to transfer 
of two electrons from X to Y. 
 
Q4: The present classification of elements is based on which fundamental property of 
elements? 
 
Answer: Atomic Number is the basis. 
 
Q5: Li, Na and K are the elements of a Dobereiners Triad. If atomic mass of Li is 7 and that of K 
is 39. What would be the atomic mass of Na? 
 
Answer: According of Dobereiner’s law of triads, the atomic mass of the middle element, in this case 
Na, should be the arithmetic mean of Li and K. 
Thus,  
arithmetic mean of Li and K = (7 + 39)/2 = 23 (Answer) 
 
 
Q6: Define Dobereiner’s law of triads. 
 
Answer: It states, "when elements are placed in order of the ascending order of atomic masses, 
groups of three elements having similar properties are obtained. The atomic mass of the middle 
element of the triad is equal to the mean of the atomic masses of the other two elements of the triad." 
 
 
Q7: Why did Dobereiner's system of classification fail? 
 
Answer: 
? Major drawback of Deberneir’s classification was that it was valid only for a few groups of 
elements known during that time. He was able to identify three triads only.  
? Also with more accurate measurements of atomic masses showed that the mid element of the 
triad did not really have the mean value of the sum of the other two elements of the triad.  
? For elements of very low mass or very high mass, the law did not hold good. For example 
Flourine(F), Chlorine (Cl), Bromine(Br). Atomic mass of Cl is not an arithmetic mean of atomic 
masses of F and Br  
Q8: Explain about position of metalloids in modern periodic table. 
Answer: In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The 
borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are 
intermediate in properties and are called metalloids or semi-metals. 
 
Q9: Why does Silicon classified as metalloid? 
 
Answer: Silicon is classified as a semi-metal or metalloid because it exhibits some properties of both 
metals and non-metals. 
 
Q10: State Newland's law of octaves. 
 
Answer: Elements are arranged in increasing order of their atomic masses such that the properties of 
the eighth element is repetition of the properties of first element ( similar to eighth note in an octave of 
music). 
 
 
Q11: X and Y are the two elements having similar properties which obey Newlands law of 
octaves. How many elements are there in between X and Y? 
 
Answer: The law states there are eight elements in an octave (row). No. of elements between X and 
Y is six. 
 
Q12: What are the drawbacks of Newland's law of octaves? 
 
Answer: Following are the major drawbacks: 
1. Worked well with lighter elements (upto Calcium). After that elements in eighth column did not 
possess properties similar to element in first column.  
2. Newland assumed only 56 elements existed so far. Later new elements were discovered 
which did not fit into octaves table.  
3. Newland adjusted few elements in a same slot though their properties were quite different. 
E.g. Cobalt and nickel are in the same slot and these are placed in the same column as 
fluorine, chlorine and bromine which have very different properties than these elements. Iron, 
which resembles cobalt and nickel in properties, has been placed far away from these 
elements.  
Q13: What periodic trends do we observe in terms of atomic radii or atomic sizes in Modern 
periodic table? 
Answer: Following two trends are observed. 
Page 3


     Q And A 
 
Q1: What are the defects in Modern Periodic Table? 
 
Answer: Following are the defects: 
? Position of hydrogen still dicey. It is not fixed till now.  
? Position of lanthanides and actinides has not been given inside the main body of periodic 
table.  
? It does not reflect the exact distribution of electrons of some of transition and inner transition 
elements.  
Q2: What are the salient features of Modern Periodic Table? 
 
Answer:  
? In a period of the periodic table, the number of valence electrons increases as atomic number 
increases. As a result, elements change from metal to metalloid to nonmetal to noble gas.  
? Atomic size is a periodic property. As atomic number increases in a period, atomic radius 
decreases. As atomic number increases in a group, atomic radius increases.  
? Positive ions have smaller atomic radii than the neutral atoms from which they derive. 
Negative ions have larger atomic radii than their neutral atoms.  
? Positive ions in the same group increase in size down the group.  
? In a group, each element has the same number of valence electrons. As a result, the 
elements 
in a group show similar chemical behavior.  
? Metallic character decreases from left to right in a period because of the increase in the 
effective nuclear charge.  
? Non-metallic character increase from left to right in a period because of ncrease in 
effective nuclear charge.Non-metallic character decreases down the group because of 
increase in the size of the 
atom.  
Q3: Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic 
configuration for these elements. To which period of the modern periodic table do these two 
elements belong? What type of bond will be formed between them and why?  
 
Answer: Electronic configuration of X (Z=12): 2, 8, 2 
Electronic configuration of Y (Z=16): 2, 8, 6 
Both these elements belong to third period. An ionic bond is formed between X and Y due to transfer 
of two electrons from X to Y. 
 
Q4: The present classification of elements is based on which fundamental property of 
elements? 
 
Answer: Atomic Number is the basis. 
 
Q5: Li, Na and K are the elements of a Dobereiners Triad. If atomic mass of Li is 7 and that of K 
is 39. What would be the atomic mass of Na? 
 
Answer: According of Dobereiner’s law of triads, the atomic mass of the middle element, in this case 
Na, should be the arithmetic mean of Li and K. 
Thus,  
arithmetic mean of Li and K = (7 + 39)/2 = 23 (Answer) 
 
 
Q6: Define Dobereiner’s law of triads. 
 
Answer: It states, "when elements are placed in order of the ascending order of atomic masses, 
groups of three elements having similar properties are obtained. The atomic mass of the middle 
element of the triad is equal to the mean of the atomic masses of the other two elements of the triad." 
 
 
Q7: Why did Dobereiner's system of classification fail? 
 
Answer: 
? Major drawback of Deberneir’s classification was that it was valid only for a few groups of 
elements known during that time. He was able to identify three triads only.  
? Also with more accurate measurements of atomic masses showed that the mid element of the 
triad did not really have the mean value of the sum of the other two elements of the triad.  
? For elements of very low mass or very high mass, the law did not hold good. For example 
Flourine(F), Chlorine (Cl), Bromine(Br). Atomic mass of Cl is not an arithmetic mean of atomic 
masses of F and Br  
Q8: Explain about position of metalloids in modern periodic table. 
Answer: In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The 
borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are 
intermediate in properties and are called metalloids or semi-metals. 
 
Q9: Why does Silicon classified as metalloid? 
 
Answer: Silicon is classified as a semi-metal or metalloid because it exhibits some properties of both 
metals and non-metals. 
 
Q10: State Newland's law of octaves. 
 
Answer: Elements are arranged in increasing order of their atomic masses such that the properties of 
the eighth element is repetition of the properties of first element ( similar to eighth note in an octave of 
music). 
 
 
Q11: X and Y are the two elements having similar properties which obey Newlands law of 
octaves. How many elements are there in between X and Y? 
 
Answer: The law states there are eight elements in an octave (row). No. of elements between X and 
Y is six. 
 
Q12: What are the drawbacks of Newland's law of octaves? 
 
Answer: Following are the major drawbacks: 
1. Worked well with lighter elements (upto Calcium). After that elements in eighth column did not 
possess properties similar to element in first column.  
2. Newland assumed only 56 elements existed so far. Later new elements were discovered 
which did not fit into octaves table.  
3. Newland adjusted few elements in a same slot though their properties were quite different. 
E.g. Cobalt and nickel are in the same slot and these are placed in the same column as 
fluorine, chlorine and bromine which have very different properties than these elements. Iron, 
which resembles cobalt and nickel in properties, has been placed far away from these 
elements.  
Q13: What periodic trends do we observe in terms of atomic radii or atomic sizes in Modern 
periodic table? 
Answer: Following two trends are observed. 
1. Within each column (group), atomic radius tends to increase from top to bottom. This trend 
results primarily from the increase in the number of the outer electrons. As we go down a 
column, the outer electrons have a greater probability of being farther from the nucleus, 
causing the atom to increase in size.  
2. Within each row (period), atomic radius tends to decrease from left to right. The major factor 
influencing this trend is the increase in the nuclear charge as we move across a row. The 
increasing effective nuclear charge steadily draws the valence electrons closer to the nucleus, 
causing the atomic radius to decrease.  
 
 
Q14: Name the scientist who proposed modern periodic law? On which fundamental property 
of elements it is based? 
 
Answer: Henry Moseley. He proposed atomic number should be chosen as fundamental property 
(instead of atomic mass) to classify elements. 
 
Q15: On what basis did Mendeleev arrange the elements? 
 
Answer: On the basis of atomic masses. 
 
Q16: An element R of atomic number 12 combines separately with NO
3
-
 and (SO
4
)
2-
, (PO
4
)
3-
 
radicals. 
a) Write the electronic configuration of element R 
b) Write the formulae of the three compounds so formed.  
c) To which group of the periodic table does the element ‘R’ belong?  
d) Does it form covalent or ionic compound? Why? 
 
Answer: 
a) Electronic configuration of R is 2,8,2.  
b) Compounds formed are: R(NO
3
)
2
, R(SO
4
) and R(PO
4
)
2
 
c) R has two valence electrons and hence it belongs to second group. 
d) It forms ionic compound by loosing two electrons to attain stable electronic configuration. 
 
Q17: Why do you think the noble gases are placed in a separate group? 
 
Answer: Due to following reasons: 
Page 4


     Q And A 
 
Q1: What are the defects in Modern Periodic Table? 
 
Answer: Following are the defects: 
? Position of hydrogen still dicey. It is not fixed till now.  
? Position of lanthanides and actinides has not been given inside the main body of periodic 
table.  
? It does not reflect the exact distribution of electrons of some of transition and inner transition 
elements.  
Q2: What are the salient features of Modern Periodic Table? 
 
Answer:  
? In a period of the periodic table, the number of valence electrons increases as atomic number 
increases. As a result, elements change from metal to metalloid to nonmetal to noble gas.  
? Atomic size is a periodic property. As atomic number increases in a period, atomic radius 
decreases. As atomic number increases in a group, atomic radius increases.  
? Positive ions have smaller atomic radii than the neutral atoms from which they derive. 
Negative ions have larger atomic radii than their neutral atoms.  
? Positive ions in the same group increase in size down the group.  
? In a group, each element has the same number of valence electrons. As a result, the 
elements 
in a group show similar chemical behavior.  
? Metallic character decreases from left to right in a period because of the increase in the 
effective nuclear charge.  
? Non-metallic character increase from left to right in a period because of ncrease in 
effective nuclear charge.Non-metallic character decreases down the group because of 
increase in the size of the 
atom.  
Q3: Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic 
configuration for these elements. To which period of the modern periodic table do these two 
elements belong? What type of bond will be formed between them and why?  
 
Answer: Electronic configuration of X (Z=12): 2, 8, 2 
Electronic configuration of Y (Z=16): 2, 8, 6 
Both these elements belong to third period. An ionic bond is formed between X and Y due to transfer 
of two electrons from X to Y. 
 
Q4: The present classification of elements is based on which fundamental property of 
elements? 
 
Answer: Atomic Number is the basis. 
 
Q5: Li, Na and K are the elements of a Dobereiners Triad. If atomic mass of Li is 7 and that of K 
is 39. What would be the atomic mass of Na? 
 
Answer: According of Dobereiner’s law of triads, the atomic mass of the middle element, in this case 
Na, should be the arithmetic mean of Li and K. 
Thus,  
arithmetic mean of Li and K = (7 + 39)/2 = 23 (Answer) 
 
 
Q6: Define Dobereiner’s law of triads. 
 
Answer: It states, "when elements are placed in order of the ascending order of atomic masses, 
groups of three elements having similar properties are obtained. The atomic mass of the middle 
element of the triad is equal to the mean of the atomic masses of the other two elements of the triad." 
 
 
Q7: Why did Dobereiner's system of classification fail? 
 
Answer: 
? Major drawback of Deberneir’s classification was that it was valid only for a few groups of 
elements known during that time. He was able to identify three triads only.  
? Also with more accurate measurements of atomic masses showed that the mid element of the 
triad did not really have the mean value of the sum of the other two elements of the triad.  
? For elements of very low mass or very high mass, the law did not hold good. For example 
Flourine(F), Chlorine (Cl), Bromine(Br). Atomic mass of Cl is not an arithmetic mean of atomic 
masses of F and Br  
Q8: Explain about position of metalloids in modern periodic table. 
Answer: In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The 
borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are 
intermediate in properties and are called metalloids or semi-metals. 
 
Q9: Why does Silicon classified as metalloid? 
 
Answer: Silicon is classified as a semi-metal or metalloid because it exhibits some properties of both 
metals and non-metals. 
 
Q10: State Newland's law of octaves. 
 
Answer: Elements are arranged in increasing order of their atomic masses such that the properties of 
the eighth element is repetition of the properties of first element ( similar to eighth note in an octave of 
music). 
 
 
Q11: X and Y are the two elements having similar properties which obey Newlands law of 
octaves. How many elements are there in between X and Y? 
 
Answer: The law states there are eight elements in an octave (row). No. of elements between X and 
Y is six. 
 
Q12: What are the drawbacks of Newland's law of octaves? 
 
Answer: Following are the major drawbacks: 
1. Worked well with lighter elements (upto Calcium). After that elements in eighth column did not 
possess properties similar to element in first column.  
2. Newland assumed only 56 elements existed so far. Later new elements were discovered 
which did not fit into octaves table.  
3. Newland adjusted few elements in a same slot though their properties were quite different. 
E.g. Cobalt and nickel are in the same slot and these are placed in the same column as 
fluorine, chlorine and bromine which have very different properties than these elements. Iron, 
which resembles cobalt and nickel in properties, has been placed far away from these 
elements.  
Q13: What periodic trends do we observe in terms of atomic radii or atomic sizes in Modern 
periodic table? 
Answer: Following two trends are observed. 
1. Within each column (group), atomic radius tends to increase from top to bottom. This trend 
results primarily from the increase in the number of the outer electrons. As we go down a 
column, the outer electrons have a greater probability of being farther from the nucleus, 
causing the atom to increase in size.  
2. Within each row (period), atomic radius tends to decrease from left to right. The major factor 
influencing this trend is the increase in the nuclear charge as we move across a row. The 
increasing effective nuclear charge steadily draws the valence electrons closer to the nucleus, 
causing the atomic radius to decrease.  
 
 
Q14: Name the scientist who proposed modern periodic law? On which fundamental property 
of elements it is based? 
 
Answer: Henry Moseley. He proposed atomic number should be chosen as fundamental property 
(instead of atomic mass) to classify elements. 
 
Q15: On what basis did Mendeleev arrange the elements? 
 
Answer: On the basis of atomic masses. 
 
Q16: An element R of atomic number 12 combines separately with NO
3
-
 and (SO
4
)
2-
, (PO
4
)
3-
 
radicals. 
a) Write the electronic configuration of element R 
b) Write the formulae of the three compounds so formed.  
c) To which group of the periodic table does the element ‘R’ belong?  
d) Does it form covalent or ionic compound? Why? 
 
Answer: 
a) Electronic configuration of R is 2,8,2.  
b) Compounds formed are: R(NO
3
)
2
, R(SO
4
) and R(PO
4
)
2
 
c) R has two valence electrons and hence it belongs to second group. 
d) It forms ionic compound by loosing two electrons to attain stable electronic configuration. 
 
Q17: Why do you think the noble gases are placed in a separate group? 
 
Answer: Due to following reasons: 
1. Similar inert behaviour  
2. Similar electronic configuration  
3. Available in low concentration.  
Q18: Besides gallium, which other elements have since been discovered that were left by 
Mendeléev in his Periodic Table? (any two)  
 
Answer: Scandium (Sc) and Germanium(Ge)