Periodic Classification of Elements : NCERT Intext Questions


Q.1       Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Sol.         

Yes, Dobereiner’s traiads exists in the columns of Newland’s Octaves.
Example - In the Dobereiner’s triads Lithium, Sodium and Potassium make a triad, and these elements also present in the same column of Newland’s Octaves as these have same properties.


Q.2       What were the limitations of Döbereiner’s classification?
Sol.  

Limitations of Dobereiner’s classification:
(i) Dobereiner could find only three triads; .i.e total of 9 elements only.
(ii) 
However the total number of elements were more than that of those encompassed in Dobereiner’s Triad.
(iii) 
Thus, Dobereiner’s could not be classify most of the elements known at that time.


Q.3       What were the limitations of Newlands’ Law of Octaves?
Sol.

Limitations of Newland’s Law of Octaves:
(i) Newland’s Law of Octaves was applicable up to calcium only.
(ii) Elements after Calcium did not obey the Newland’s Law of Octaves.
(iii) Newland adjusted two elements under the same slot inspite of their different properties, such as cobalt and chlorine were put in the same note while there properties were different.
(iv) It appears that Newland assumed that only 56 elements existed in nature; after discovery of more elements his Law could no longer hold true.


Q.4       Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.
Sol. 

(i) The general formula for oxides of 1st, 2nd, third and 4th group are R2O, RO, R2O3 and RO2 respectively in the Mendeleev’s Periodic Table, where R denotes the element.
(ii) 
Since, Potassium (K) belongs to 1st group, thus, formula of its oxide would be K2O.
(iii) 
Barium (Ba) belongs to 2nd group, thus formula of its oxide would be BaO.
(iv) 
Aluminium (Al) belongs to the 3rd group, thus, formula of its oxide would be Al2O3.
(v) 
Carbon (C) and Silicon (Si) belong to the 4th group, thus, formula of its oxide would be CO2 and SiO2 respectively.
(vi) 
Thus, formula of oxides of the given elements would be K2O, CO2, Al2O3, SiO2 and BaO; respectively.


Q.5       Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)
Sol.         Scandium and Germanium


Q.6       What were the criteria used by Mendeléev in creating his Periodic Table?

Sol.

Mendeleev used atomic mass of the elements as the unique criteria of the elements. He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.


Q.7       Why do you think the noble gases are placed in a separate group?
Sol.

Since, noble gases are inert and all of them show similar properties, thus they have unique properties compare to other elements, thus they are placed in a separate group in Periodic Table.


Q.8       How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?

Sol.

Various anomalies of Mendeleev’s Periodic Table removed as follows in the Modern Periodic Table:
(i) 
Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.
(ii) 
In Modern Periodic Table there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.
(iii) 
Elements having same valence electron are kept in same group.
(iv) 
Elements having same number of shells were put under the same period.
(v) 
Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.


Q.9       Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Sol.

Beryllium and Calcium. These elements show chemical reactions similar to magnesium because magnesium, calcium and beryllium belong to same group, i.e. Group 2nd in the Modern Periodic Table.


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.
Sol.         (a) Hydrogen, Lithium and Sodium
               (b) Answer: Calcium and Magnesium
               (c) Neon, Argon and Krypton


Q.11     Explain the following:

            (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?
Sol.

(a) Lithium, Sodium and Potassium have following similarities:
(i) 
They all have valence electron equal to 1.
(ii) 
They all belong to same group, i.e. Group 1st in Periodic Table.
(iii) 
They all form ionic bond with non-metals.
(iv) 
They all are highly electropositive.
(v) 
They all are highly reactive.

(b) Following properties are common in the atoms of Helium and Neon:
(i) 
Both have completely filled outermost orbit.
(ii) 
Both have stable electronic configuration.
(iii) 
Both belong to same group, i.e. zero group in Modern Periodic Table.


Q.12     In the Modern Periodic Table, which are the metals among the first ten elements?

Sol.         Lithium and Beryllium


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

Sol.

Metallic character of elements depends upon the position in periodic table. Group numbers of given elements are as follows:
(i) 
Beryllium (Be) – Group 2nd
(ii) 
Ga (Gallium), Ge(Germanium), As (Arsenic) and Se (Sellenium) belongs to 13th, 14th, 15th and 16th groups respectively.
(iii) 
Since, metallic character decreases by moving left to right in a period, thus, Beryllium is the most metallic as it is at the left of 2nd period and Selenium is a non-metal as it is right most in the third period.
(iv) 
Thus, Beryllium is most metallic and selenium has least metallic character as it is a non-metal.



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