Why does the hydroxide ion have a negative charge? , i.e OH-?
Because if not, it would be a hydroxide radical and not an ion?
Think octet rule and lewis dot structures... you need one more electron to make the oxygen stable, so it has a 1- charge.
The simplest way for explaining this is this:
O2-+H+=(OH)- (It's actually (OH)-)
This is not stable(Stable form is H2O)
Here H+ Cancels out one electron of O2-
So one electron is left.Therefore it is left with a negative charge
Count the number of protons and number of electron in the ion.
With reference to what Drdu said: ions are charged, radicals are neutral.
chemistry sometimes overcomplicates things to simplify them...
Radicals need not always be neutral.
Also you can have stuff like the superoxide anion which is a anion radical. See: http://en.wikipedia.org/wiki/Superoxide
Plenty of examples around, these are just off the top of my head.
Sorry for not being clearer, I was talking about hydroxyl radicals which DrDu mentioned rather than radicals in general.
As for the definition of radical I believe the only prerequisite is having an unpaired electron/ incompletely filled valence shell and the species in question may be an atom, ion or a molecule.
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