Does basicity always vary inversely with ionic atomic radius???

1 Answer
Jun 22, 2016

Not necessarily... It's only a guideline, and one that easily gets confusing. So it's not a good guideline to try to make sense of.


We need to determine what "basicity" means. You did not specify either Lewis or Bronsted basicity, which both make a difference.

Take #"I"^(-)# and #"Cl"^(-)#, for example...


BRONSTED BASICITY

Bronsted basicity generally depends on the ability to polarize a proton's bond with another atom, weakening the bond and promoting the Bronsted base's acceptance of that proton.

When you place #"I"^(-)# and #"Cl"^(-)# in water... #"Cl"^(-)# is the stronger Bronsted base, because:

  1. It is smaller, making its electron density, well, denser, making it less polarizable by another species. Thus, it is a harder base.
  2. It is more electronegative, making it more polarizing towards the proton on water than #"I"^(-)# is on water.

That allows a stronger hydrogen-bonding interaction than #"I"^(-)# would have with water, which in this case makes it a stronger Bronsted base (a stronger proton acceptor).

(Indeed, we say that #"HI"# is the stronger acid, and so its conjugate base is generally stated to be a weak base.)


LEWIS BASICITY

Lewis basicity may depend on the following factors:

  • electronegativity
  • solvent effects (hydrogen-bonding, for example)
  • strength of the conjugate acid's #"H"-"A"# bond(s), and the trade-off you get upon trying to make that bond.
  • and probably other stuff.

In terms of Lewis basicity, since #"I"^(-)# is less electronegative, its valence electrons are held less tightly to the nucleus (it's a softer base).

That increases its effectiveness as a Lewis base.

http://crab.rutgers.edu/~alroche/Ch06.pdf, pg. 16

However, #"I"^(-)# is larger than #"Cl"^(-)#, so the bonds it makes are naturally weaker, which is not a great tradeoff for donating its electrons, lowering its Lewis basicity.

http://www.t.ibchem.com/

It's unclear how this all balances out without more context. So, keep in mind... context is extremely important, and defining your words is as well.