Why is OH ion negative?

I am slightly confused with the OH ion’s charge. I know that it’s covalently bonded and it’s polar. Oxygen is slightly negative and hydrogen is slightly positive. But I didn’t think covalent bonds could have a charge.
Could someone plz explain this to me?
Thanks in advance

Reply 1

username3700840

covalent bonds can have charges due to the electronegativity of the two atoms which are bonded.

electronegativity is the ability of a nucleus to attract electrons, in simpler words the more protons an atom has, the higher the electro negativity.

because oxygen has 8 protons and hydrogen only has 1, oxygen is more electronegative and so the electron in the covalent bond, moves slightly towards the oxygen atom, meaning that oxygen is slightly more negative that hydrogen

(edited 6 years ago)

Reply 2

haarithiop

Imagine you have an H2O molecule and think of its dot and cross diagram. Now imagine removing one of the hydrogens but leaving its electron on the oxygen (i.e. you are just removing a proton).You're left with an oxygen with 8 electrons in its outer shell, one is from the covalent bond with the unremoved hydrogen, 6 are its own original electrons and the final one is from the hydrogen that was removed. As you have removed a proton (and left an electron) the OH molecule now has a 1- charge.

(edited 6 years ago)

Is that a level?

Original post by Janej77

The hydroxide ion has a negative charge because it has 10 electrons but only 9 protons in total.

The first reply above has talked about bond polarity, but a polar covalent bond isn't "charged". The charge comes from the extra electron.

Reply 5

Imstudying...

Original post by K-Man_PhysCheM

Thanks

Reply 6

Imstudying...

Original post by haarithiop

Thanks

Reply 7

uchase

This now makes sense.

Reply 8

uchase

Original post by haarithiop

Wow. Thanks. This really helped me a lot.

Reply 9

haarithiop

Original post by uchase

Wow. Thanks. This really helped me a lot.

no problem

Reply 10

jack_harrison

The 'charge' that you see on some atoms like in the hydroxide ion is known as formal charge. To start calculating it, you pretend that the electrons in each covalent bond are equally shared, and then count the number of electrons an atom effectively 'owns' based on this assumption.

For the oxygen in the hydroxide ion, you assume that the electrons in the O-H bond are equally shared, so the oxygen 'owns' 1 electron from this bond. If you then add on the oxygen's 3 lone pairs, the oxygen owns 7 electrons total.

To find the formal charge, you subtract this value from the number of electrons that that atom would have if it were completely neutral.

Oxygen is a 6 valence electron species, so a neutral oxygen atom would have 6 electrons. Taking 7 from 6 gives the formal charge: -1. Intuitively you've just calculated that there is 1 more electron on the oxygen than there is supposed to be to make the oxygen neutral, so it has a negative 1 charge.

You'll notice this method uses the assumption that electrons are shared equally. Obviously in a hydroxide ion the bond is very polar so this assumption is not very good. So while this calculation may not be accurate to charges of specific atoms, it does reflect the charge of the whole molecular species, eg. the hydroxide ion, because it just calculates how many extra/missing electrons you have.