Anand Classes provides the NCERT Intext 3.5 solution for Class 11 Chemistry: Which of the following species will have the largest and the smallest size: Mg, Mg²⁺, Al, Al³⁺? The solution is explained using periodic trends, effect of cation formation, and the concept of isoelectronic species. It is concluded that the largest species is Mg, while the smallest species is Al³⁺. This step-by-step explanation is very useful for JEE, NEET, and CBSE Class 11 Chemistry students. Click the print button to download study material and notes.
❓ Question
Which of the following species will have the largest and the smallest size? Mg, Mg²⁺, Al, Al³⁺.
✅ Step-by-Step Explanation
Periodic Trend in Atomic Size
Atomic radius decreases across a period from left to right because the nuclear charge increases while electrons are added to the same shell.
Thus, for neutral atoms: Mg (Z = 12) > Al (Z = 13)
Effect of Cation Formation
When a cation is formed, electrons are lost → effective nuclear charge per electron increases → atomic size decreases.
So, Mg²⁺ smaller than Mg,Al³⁺ smaller than Al
Isoelectronic Species Concept
Mg²⁺ (10 e⁻) and Al³⁺ (10 e⁻) are isoelectronic.
Among isoelectronic species, the one with the greater nuclear charge (Z) will pull electrons more strongly → smaller size.
Here, Z(Al³⁺)=13 > Z(Mg²⁺)=12. Hence, Al³⁺ is smaller than Mg²⁺.
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