S8E3 - Ion Electron Configurations and Ionic Radius Trends

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

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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).

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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⁺



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(b)   Ba²⁺ ,  Cs⁺ ,  I^- ,  Te^2-

➞  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.

Result:

Ba²⁺   <   Cs⁺   <   I^-   <   Te^2-

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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⁺.
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answer:

➞  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^-

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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^-

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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.

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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!