Question

# Which of the following species will have the largest and the smallest size?$$Mg,\quad { Mg }^{ +2 },\quad Al\quad { Al }^{ +3 }$$

Solution

## (i) $$Mg$$ and $$Al$$ belong to the third period. Across a period , atomic radii decrease due to increased nuclear charge. Therefore, atomic size of $$Al$$ is smaller than that of $$Mg$$.(ii) Further cations are smaller than their parent atom. Therefore , $$Mg^{2+}$$ is smaller than $$Mg$$ and $$Al^{3+}$$ is smaller than $$Al$$.(iii) $$Mg^{2+}$$ and $$Al^{3+}$$ are isoelectronic ions. Among isoelectronic ions, higher the +ve charge, smaller the size. Therefore , atomic radius of $$Al^{3+}$$ is smaller than that of $$Mg^{2+}$$.From the above discussion, it follows that $$Mg$$ has the largest while $$Al^{3+}$$ has the smallest size.Physics

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