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MCAT Question A Day - 10/16/14 - Answer!

Which of the following most likely represents the correct order of ion size from greatest to smallest?

A.  O 2- , F, Na+, Mg2+
B.  Mg2+, Na+, F, O2-
C.  Na+, Mg2+, O2- , P
D.  Mg2+, Na+, O2-, P-

A is correct. This is an isoelectronic series, which means that the number of electrons on each ion is the same. In an isoelectronic series of ions, the nuclear charge increases with increasing atomic number and draws the electrons inward with greater force. The ion with fewest protons produces the weakest attractive force on thelectrons and thus has the largest size.

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