Chem – College: Ionic Radius (Ionic Size)

 How do you compare the ionic radius (ionic size) of two or more atoms?

Ionic radius is really the same as atomic radius with some modifications.  Here the ionic radius table  is not very important for solving possible problems you will encounter on a test, but it does give you a good idea of how the atomic radius changes as you take a non-charged element and turn it into an ion.  The ionic radius describes how the radius or size of an atom changes when electrons are added or taken away to form an ion. The general rule for ionic radius is that as you add electrons the ionic radius gets bigger and as you take way electrons the ionic radius gets smaller. Therefore:


O2- is larger that O


Ca2+ is smaller than Ca


However, it can be more complicated than that. If there is no difference in the amount of electrons (if they are isoelectric) then you have to look to see if you have a difference in the amount of protons. The more protons an atom or ion has the smaller it is. The less protons an atom or ion has the larger it is. Therefore:


Na+ is smaller than N3-

Both have the same number of electrons = 10

But they have different amounts of protons

Na = 11            N = 7


What you want to always keep in mind for ionic radius is that you should count up the electrons and protons in every case and then compare it to each other.  Remember, electrons are the main reason that atoms are bigger so they are more important but if the electron numbers are the same then look at the proton numbers.


Examples: Put the Ions below in order from LARGEST to SMALLEST.


Ions Largest to Smallest
S2-, Cl, Si4-, P3- Si4->P3->S2->Cl
Be2+, B3+, Li+, C4+ Li+>Be2+>B3+>C4+
F, Na+, Se2-, Sr2+ Se2->Sr2+>F>Na+


VIDEO Ionic Radius Demonstrated Example 1: Put the ions below in order form LARGEST to SMALLEST.


Rb+, As3-, K+, S2-


How many electrons do each of them have?

Rb = 36

As = 36

K = 18

S = 18

So we see that we have two groups. The group with more electrons will be larger. So far we know that Rb and As will be larger than K and S. To find out more we look at the protons.


How many protons do each have?

Rb = 37

As = 33

K = 19

S = 16


Since electrons contribute more to the size, we look at them first. Rb and As have 36 electrons. Which one has less protons?

As….Therefore As is the largest and Rb is the next largest.


Between K and S which both have 18 electrons which one has more protons?

K….Therefore K is the smallest and S is the next smallest.




PRACTICE PROBLEMS: Put the ions below in order from SMALLEST to LARGEST using just a regular periodic table.


Ions Smallest to Largest
N3-, Mg2+, F, Na+ Mg2+<Na+<F<N3-
P3-, Cl, Cs+, I Cl<P3-<Cs+<I
Te2-, Br, Ba2+, Sr2+ Sr2+<Br<Ba2+<Te2-


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