Why Does OH- Have a Negative Charge?

[andyrk]

Why does the hydroxide ion have a negative charge? , i.e OH^-?

 

[DrDu]

Because if not, it would be a hydroxide radical and not an ion?

 

[ArielRodriguez]

Think octet rule and lewis dot structures... you need one more electron to make the oxygen stable, so it has a 1- charge.

 

[adjacent]

Why does the hydroxide ion have a negative charge? , i.e OH^-?

The simplest way for explaining this is this:
O^2-+H⁺=(OH)^- (It's actually (OH)^-)

This is not stable(Stable form is H₂O)

Here H⁺ Cancels out one electron of O^2-

So one electron is left.Therefore it is left with a negative charge
-2+1=-1

 

[Enigman]

Why does the hydroxide ion have a negative charge? , i.e OH^-?

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...}

 

[Yanick]

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.

See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2644638/

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.

 

[Enigman]

Radicals need not always be neutral.

See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2644638/

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.