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Ion Electron Configurations and Ionic Radius Trends

SECTION 8 - Chemical Bonding

In this post we'll discuss the relative sizes of ions, and the three ways to write an ion's electron configuration:

- complete electron configuration
- noble-gas notation (shorthand e- configuration)
- orbital notation


Relative Sizes of Ions

➞  generally, a cation (X+) is smaller than the parent atom (X).

➞  generally, an anion (X-) is larger than the parent atom (X).

Relative Sizes of Cations and Anions


ex:  Order the ions from smallest to largest ionic radius.

(a)   Li+ ,  Na+ ,  K+ ,  Rb+ ,  Cs+

➞  well... all of these are group 1 cations and size increases down a group, remember?  So we have:

Li+   <   Na+   <   K+   <   Rb+   <   Cs+

Order Ions from Smallest to Largest Ion


(b)   Ba2+ ,  Cs+ ,  I- ,  Te2-

➞  in this grouping, it's not so easy as in part (a).

All four ions have 54 e- , so their electron clouds would be the same size.

However, each ion has a different amount of protons (+) in it's nucleus (differing atomic numbers).

The ion with the most protons is best at "pulling" the 54 e- in, toward the nucleus, thus decreasing the size of the electron cloud.

This "pulling" is a direct result of the (+)⇿(-) electrostatic interaction.


Ba2+   <   Cs+   <   I-   <   Te2-

Relative Sizes of Ions


Ion Electron Configurations

Electron configurations for cations and anions are very similar to the way I taught you previously...

Only now, we have to first add or subtract the proper number of specific electrons.  Here's how:

ex:  Write the electron configuration, noble-gas notation, and orbital notation for O-, Na+, and Fe2+.

➞  to do this, we really only need to know the number of electrons for each ion.

O-   ,   O = atomic number 8, so 8 e-, plus 1 more e- from the negative charge = 9 e-

Oxide Ion Electron Configuration

Now, we'll do the same thing for Na+

Na+   ,   Na = atomic number 11, so 11 e-, then 1 less e- from the +1 charge = 10 e-

Sodium Cation Ion Electron Configuration

Finally, the last one is the Fe2+ ion...

Fe2+   ,   Fe = atomic number 26, so 26 e-, then 2 less e's from the +2 charge = 24 e-

Beware!  24 electrons is the same number of electrons as the chromium atom, which we know is an exception to the diagonal rule.

So we'll end up with ...4s13d5, as shown below.

Ferrous Ion Electron Configuration


Whew!  That's a lot of configurations and notations! 

Okay, in our next post covering SECTION 8 - Chemical Bonding,

We'll cover lattice energy and bond dissociation energy (bond enthalpies).

See you on the other side!