(solved)Question 3.12 of NCERT Class XI Chemistry Chapter 3

Consider the following species : N3- , O2-, F- , Na+ , Mg2+ and Al3+ (a) What is common in them? (b) Arrange them in the order of increasing ionic radii.

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Parveen,

Question 3.12
NCERT Class XI Chemistry

Consider the following species : N3- , O2-, F- , Na+ , Mg2+ and Al3+ (a) What is common in them? (b) Arrange them in the order of increasing ionic radii.

Solution in Detail
(video solution below this)

$\displaystyle \underline{\underline{\text{Part (a)}}}$

N3- has N(Z=7) + 3 = 10 electrons

O2- has O(Z=8) + 2 = 10 electrons

F- has F(Z=9) + 1 = 10 electrons

Na+ has Na(Z=11) - 1 = 10 electrons

Mg2+ has N(Z=12) - 2 = 10 electrons

Al3+ has N(Z=13) - 3 = 10 electrons

All are iso-electronic with 10 electrons $\displaystyle \underline{Ans}$

$\displaystyle \underline{\underline{\text{Part (b)}}}$

Point 1. All have the same electrons but nuclear charge is increasing from 7 (N3-) to 13 (Al3+)

Point 2: So there is a greater attraction between the nucleus and its electrons that decreases the radius

Hence the cation Al3+ is the smallest and the anion N3- the largest

Al3+ < Mg2+ < Na+ < F- < O2- < N3- $\displaystyle \underline{Ans}$

Note: Noble gas Ne is also isolectronic but it doesn't form covalent bonds like others, but its vanderwal radius rather than ionic/atomic radius is more meaningful.

Video Explanation

Please watch this youtube video for a quick explanation of the solution:

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