## Chem – College: Ionic Radius (Ionic Size)

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

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+