Subjective Type Questions: Classification of Elements & Periodicity in Properties | JEE Advanced | Question Bank for JEE Main & Advanced (350+ Tests) PDF Download

Q.1. Arrange the following in :

(i) Decreasing ionic size :  Mg²⁺, O^2–, Na⁺, F^– (1985 - 1 Mark) 

Ans. Sol. O2– > F– > Na+ > Mg2+
NOTE : All the above ions are isoelectronic having 10 electron each.
In such case the greater the nuclear charge, the greater is the attraction for electrons and smaller is the ionic radius. Hence O^2– has the highest and Mg²⁺ has the least ionic size.

(ii) Increasing acidic property :  ZnO, Na₂O₂, P₂O₅, MgO (1985 - 1 Mark)

Ans. Sol. 

Among oxides the acidic strength increases with oxidation state. So Na₂O₂ is least acidic and P₂O₅ is most acidic. Further Na₂O₂ and MgO are basic, ZnO is amphoteric and P₂O₅ is acidic.

(iii) Increasing first ionization potential :  Mg, Al, Si, Na (1985 - 1 Mark) 

Ans. Sol. The first ionization potential of the 3rd period elemens follows the order : Na < Al < Mg < Si

NOTE : Ionisation energy increases across a period but not regularly. Mg (1s², 2s²p⁶, 3s²) is more stable because the electron is to be removed from 3s which is difficult as compared to Al (1s², 2s²p⁶, 3s²p¹) where electron is to be removed from 3p.

(iv) Increasing size :  Cl^–, S^2–, Ca²⁺, Ar (1986 - 1 Mark) 

Ans. Sol. Ca²⁺ < Ar < Cl^– < S^2–; All of these are isoelectronic. In such cases the greater the nuclear charge, the greater is the attraction for electrons and smaller is ionic size.

ionic radius  

(v) Increasing order of ionic size :  N^3–, Na⁺, F^–, O^2–, Mg²⁺ (1991 - 1 Mark) 

Ans. Sol. Increasing order of ionic size : Mg²⁺ < Na⁺ < F^– < O^2– < N^3–

NOTE : All the above ions are isoelectronic having 10 electrons each.
In such a case the greater the nuclear charge, the greater is the attraction for electrons and smaller is the ionic radius. Hence N^3– has the highest and Mg²⁺ has the least ionic size.

(vi) Increasing order of basic character : MgO, SrO, K₂O, NiO, Cs₂O (1991 - 1 Mark)

Ans. Sol. Increasing order of basic character : NiO < MgO < SrO < K₂O < Cs₂O The basic character of oxides increases when we move down the group. So, K₂O < Cr₂O and MgO < SrO.
Further higher the group number lesser is the basic character. Hence NiO is the least basic.

(vii) Arrange the following ions in order of their increasing radii :  Li⁺, Mg²⁺, K⁺, Al³⁺. 

Ans. Sol. Al³⁺  < Mg²⁺  < Li⁺ < K⁺ In these Al³⁺ & Mg²⁺ are isoelectronic species, so in these size decreases with increase in atomic number because increase in  atomic number decreases Z_eff. 

In Li⁺ & K⁺, K⁺ is bigger in size than Li⁺ because on moving from top to bottom in a group, the group size increases.

Q.2. The first ionization energy of carbon atom is greater than that of boron atom whereas, the reverse is true for the second ionization energy. (1989 - 2 Marks)

Ans. Sol. C (1s22s2p2) h as h alf filled orbitals in its excited state due to which it becomes more stable and hence IE1 for C is greater than B.
Further for second ionization energy (IE₂) in C⁺ (1s²2s²p¹) the electron is to be removed from 2p which is easy as compared to B⁺ (1s²2s²), where it has to be removed from 2s.