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Why is K+ larger than Ca^2+?

The two ions K+ and Ca2+ each have 18 electrons around the nucleus. Which would have the smaller radius? Why??

  • Answer:

    It has to do with a trend on the periodic table called electronegativity. Basically electronegativity is a measure of how powerfully an element is able to attract its electrons inward towards the nucleus. If you look at the periodic table, elements electronegativity will GROW as you go across the table in a straight line, or up in a straight line (can work diagonally as well, but only for specific areas of the table.) K+ and Ca2+ both have the same number of electrons, but the calcium has a higher electronegativity. Calcium's electronegativity is 1.00 while potassium's is only 0.82. Thus calcium it is able to hold its electrons much closer into the nucleus which in turn will lower the size of it's radius. Wikipedia has a pretty decent picture of the period table show the trends at the following link: http://en.wikipedia.org/wiki/Electronegativity#Electronegativities_of_the_elements

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Ca 2+ because the stronger charge will hold the electrons closer.

Karen g

Careful on the wording, this actually has nothing to do with electronegativity, but it has to do with the nuclear charge. K+ and Ca2+ both have 18 electrons, but Ca2+ has a greater nuclear charge to pull those electrons in, so the electrons in Ca2+ will be closer to the nucleus, resulting in a smaller radius. Cl- would also contain 18 electrons, and would have a larger radius than both K+ and Ca2+ even though it has a higher electronegativity because its nuclear charge is less than that of K and Ca.

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