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determining the charge of OH ion

Sorry, this is a dumb question.
Ok, so i know an OH ion has a negative charge and some other substances have different charges. But is there a way to determine its charge? i.e by counting the electrons, OR am i just supposed to know and memorize the charge of different chemicals.

what I mean is, imagine that you know you are given shown a dot and cross diagram of a molecule with its empirical formula, imagine you dont know whether the molecule is charged or what charge it is.How can you find out what charge this molecule is?

Thanks

Reply 1

xpineda

you find the charge, by looking at the amount of atom(s) that surrounds it and their charge. Don't forget that every atom in a molecule is trying to attain a pseudo-noble gas configuration, this might be a big factor on their charge!

Reply 2

appletree_1

okay, so let's say youre given an OH ion. it has a negative charge so you know that it has one more electron than it needs.
but how do you deduce that it has a negative charge in the first place. You know that the Oxygen atom has 6 outer electrons and the hydrogen atom has 1 extra electron, but how do you find out that it has a charge of 1-?

Reply 3

w-inds

You look at the outermost electron shell of both combined. In OH-, there are 2 electrons coming from the O ion, and the H+ ion needs one. H+ takes one electron from the O ion to combine to make OH- because there is only one electron in the outer shell.

Reply 4

EierVonSatan

To really answer the question you're suggesting (being given a blank dot-and-cross diagram with O and H) you need to be told either explicitly or implicitly how many electrons there are. The charge is simply then: number of protons - number of electrons.

Reply 5

-Kav-

As daft as it sounds, you only know that an OH^- ion has a negative charge; you can't work it out simply by counting atomic electrons because that'll always give you a neutral charge. The formation of ions depends on electrons coming or going, and you can't know for sure how that's going to go. It's possible, for example, to form an OH⁺ ion under the right conditions. All that said, there's a decent rule of thumb you can use for simple ions liked hydroxyl, ammonium and amine. A lot of elements only have one common oxidation state, and you can count the charge based on that. For example, adding two electrons to O gives it a 'complete shell,' and makes it O^2-. Removing one from H gives it an 'empty shell' and makes it H⁺. Adding them together gives you OH^-. It's handy for those examples, but it's not Gospel, and should be treated with caution, because variable oxidation states are always possible. Do it for a nitrite, for example, and it's all going to go very wrong.

In practice I always found it easiest to remember a few formulae containing one 'compound' ion and one simple one that generally has a constant charge. Lab acids are perfect; you know they all contain H⁺ so H₂SO₄, HNO₃, HNO₂ and H₃PO₄ must contain SO₄^2-, NO₃^-, NO₂^- and PO₄^3-. For OH^-, NaOH works, since Na is almost always Na⁺.

I hope that helps, and I really hope all those tags work. :p:

EDIT: Damn.

Reply 6

EierVonSatan

you want [noparse][/noparse] tags :biggrin:

Reply 7

-Kav-

EierVonSatan

you want [noparse][/noparse] tags :biggrin:

Cheers. You learn something new every day, eh? :o:

Reply 8

EierVonSatan

-Kav-

Cheers. You learn something new every day, eh? :o:

Not unless you're dumb like me...

Reply 9

-Kav-

Hah, there's a difference between incapability and unavailability of opportunity. I'd like to think I'll not be picking up much from this forum if and when I'm slogging through my PhD.