cation size

[medicinegoal]

Hi,

In the BR General Chemistry book in question 28 of the Atomic Theory chapter, while comparing the size of Mg 2+ and Na+, it says that Mg 2+ <Na+ but don't they both have the same electronic configuration as Ne (1s^2 2s^2 2p^6)? So then how can we say that Na+ is bigger than Mg2+ ?

Thanks so much!

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[Bacchus]

Think of effective nuclear charge for the explanation. The nuclear charge (Z) of Na is 11 whereas it is 12 for Mg. Removing one electron for sodium and two for magnesium gives them the same amount of electrons, complete 1s, 2s, and 2p orbitals. However, the 2p electrons feel a greater effective nuclear charge in magenesium becuase +12 - 4 = 8 whereas for sodium +11 - 4 = 7. Since there is a greater effective nuclear charge on magenesium, the outer electrons will be drawn in closer decreasing the cationic radius. I attached a JPEG to explain the calculation of effective nuclear charge.

 

[medicinegoal]

thanks a lot; that clears it up--i like the picture!