Hard And Soft Elements: Size Does Matter

Here's a great Periodic Table showing the relative sizes of common ions. Cations are shown in red, anions are in blue.

Click To Enlarge

Cool things to note:

• Ions in the same column get bigger as one moves down a column.
• Look how ginormous cesium (bottom left) and iodide (bottom right) are.
• Look how small some ions are (Be²⁺ in particular).
• Look how invisibly small the proton is because H⁺ has no electrons. Hydride, H^-, having two electrons, is comparatively huge. It's almost like the planets Mercury and Jupiter. I wrote about Dr. Proton and Mr. Hydride back here.

A chemist named Ralph Pearson invented the concept of Hard Soft Acid Base (HSAB) Theory in the 1960s.  According to Pearson, "hard" (small) acids like Li⁺, Be²⁺, etc., naturally prefer binding with "hard" (small) bases like [OH]^- and O^2-.  Likewise, "soft" (larger) acids like silver, Ag⁺ and mercury, Hg²⁺ (when they aren't found in their elemental state) will invariably be found with a "soft" base, i.e., sulfide, S^2-.

So it goes.

Funny story about Pearson.  I saw him speak once at a special symposium dedicated to Henry. Pearson caught everyone's attention when he showed up late in the middle of a talk, entering at the rear, striding to the front of the room escorted arm-in-arm by two beautiful 20-something women (they turned out to be his grand nieces or something but everybody else was thinking "hired").  The women were dressed for cocktails too, not for a roomful of chemistry geeks. Pearson made his entry, said his hellos, and announced that he was just testing his principle of maximum hardness.