[tenbee]

  Okay I'm a bit embarrassed to ask - Why does oxonium (oxygen with 3 bonds and a lone pair) have a positive charge?

Ah another question I never asked in undergrad... I simply memorized O + 3 bonds + a lone pair is positive, while O + 1 bond + 3 lone pairs is negative. What fundamental concept have a I forgotten?

Thanks!

[Dan]

Draw Lewis structures for the ions and apply the octet rule. Count the protons and electrons, the difference is the charge.

[orgopete]

This is not the standard answer about the charges. Actual charges do not change, protons are positive and electrons are negative. If you add a proton to water to form H₃O⁺, the plus sign indicates there are more protons than electrons. Putting it on the oxygen is a convention of applying formal charges. They can be useful for electronic bookkeeping.

Formal charges can be useful to understand how electrons may be affected. Protonating water results in the remaining lone pair (-2) being pulled closer to the nucleus and thus less basic. Similarly, removing a proton from water results in the electrons (still -2) extending further from the nucleus and thus more basic. What I have described about formal charges is the same principle as the elements of the periodic table. If you look at the bond lengths of methane, ammonia, water, and HF, the bond lengths become shorter as the nuclear charge increases. A non-bonded pair of electrons on carbon are more basic than nitrogen simply because they extend further from the nucleus. The charge of a pair of non-bonded electrons (-2) on a carbon (as its anion) is the same as the charge of a pair of non-bonded electrons (-2) of water. The difference is the effective field they exert in being able to attract opposite charges.