Why does the hydroxide ion have a negative charge?

I've been studying the roles of hydroxide & hydronium in acids and bases, and it was mentioned that a hydroxide ion (OH-) has a negative charge. Can someone give me a layman's explanation of what causes this charge?

Since hydrogen and oxygen are sharing a covalent bond, is the charge negative because an electron is donated from oxygen, thus giving hydrogen 2 electrons?

• In a sense, you are overthinking it. OH- has a negative charge, simply because it has a negative charge (it has one more electron than it has protons). There could also be an OH molecule with a negative 2 charge, but thats incredibly unlikely. Bonds come into play after you know the number of electrons a molecule has. Dec 4 '18 at 18:59

A water molecule is charge neutral because there is the same number of positive charges as there are negative charges.

In this diagram, called a Lewis structure, the dots represent electrons while the lines or dashes represent a covalent bond of two electrons.

When water ionizes one of the hydrogen atoms absconds with itself and leaves it's electron behind, giving us the hydroxide ion. The extra electron gives hydroxide a net charge of -1.

The brackets indicate that this is an ion, charge is denoted at top right.

To go deeper down the rabbit hole on this one I recommend reading up on the Octet rule and Electronegativity.

Hydrogen exists as $\ce{H2}$, while oxygen exists as $\ce{O2}$. They are both diatomic elements, meaning that their stable form is a pair. There are several others...

One Hydrogen ($\ce H$) has a positive charge +1. This has nothing to do with it having more protons than electrons, as it has one of each. Think of it as 'willing' to donate one electron, or it has +1 electrons... see, it is on the plus side.

One Oxygen ($\ce O$) has a negative charge of -2. Think of it as 'wanting' to borrow two electrons, or it has -2 electrons... see, it is on the plus side.

Now, put one $\ce H^+$ with one $\ce {O}$ and the $\ce H$ will donate, the $\ce O$ will borrow and the $\ce H$ will be 0, and the O will be -1. Now the combination has a -1 charge.

Now the Hydrogen is sharing one electron with the oxygen, filling its outermost shell with 2, but the Oxygen is needing 8 in its outermost shell, and is only at 7, so the compound is at -1, or 'wanting' one more electron to make it stable.

If you add one more $\ce H^+$ the $\ce{O}$ will be satisfied, and you will get $\ce {H2O}$, a very stable compound we know as water.

• Thanks for the information, that helps quite a bit! Since Hydrogen's charge has nothing to do with with having more protons than electrons, what does it have to do with? Is it a function of some atomic attribute, or is there table to reference? I'm curious about the states of other atomic ions as well. Thanks again! Dec 5 '12 at 23:37
• The charge on hydrogen ion does have to do with it having more protons (one) than electrons (zero), but it is more common and more convenient to think of hydrogen ion having fewer electrons (zero) than hydrogen atoms (one). Dec 6 '12 at 0:47
• Both O and H are neutral species. Do not confuse charge with oxidation number. Dec 6 '12 at 7:52
• Warning for reader: This is a too simplistic view on the matter to be useful. It doesn't take nitpick to know that thinking of $\ce{OH-}$ as $\ce{O^2-}$ and $\ce{H+}$ is flat out wrong. May 1 '16 at 11:08
• @H.Khan Because water isn't ionic compound! And in acid/base reactions there's no "free protons" to be found. Dec 4 '18 at 20:44

I may be wrong, but one way I understood the overall formal charge denoted outside the bracketed OH- molecule, was by giving OH- a backstory.

Without a backstory of how it came to form, I don't think the negative formal charge makes intuituve sense.

If we say OH- formed from the dissociation of H2O into H+ and OH-, then there is some sense.

What I mean is O and H and H came together to form water, amd did so by sharing electrons. At this point, before they arrived together and during, even the hydrogens had 1 electron each to give to help oxygen. The oxygen had 2 electrons to help out the hydrogens, one for each.

But when the water molecule split apart, there was a tug of war over their shared electrons.

Oxygen turned out to be stronger and yanked away the only electron one of the hydrogens had. This hydrogen was sent loose, without an electron to help it make another bond or be useful by sharing its bond making electron with another element.

The other hydrogen stayed attached to the oxygen.

So the oxygen had an extra electron, torn from the hydrogen now detached. This is written as O-. But because there is an H still attached we write OH-. (Note, the H in OH is not carrying the stolen electron but the negative charge symbol is written by it, though we could write HO- to be more clear. (On another level however, I think this stolen electron would be whizzing around near the H in OH too, hence some negative charge of this electron woukd be exerting itself over the H in the OH from time to time.

So if ai am correct, without this backstory or creation story for OH- or HO-, it is not intuitive ...

... because if I think simply yet logically, if I take a neutral atomic O, and a neutral atomic H and make them bond, I don't think we actually have an overall formal charge. Why ? Because O has 6 valence electrons. H has 1 valence electron. When they bond, they each give eachother a valence electron. O ends up with 7. H ends up with 2. In this OH or HO bond, neither atom has lost an electron. No tug of war has been won or lost. (Some might argue that oxygen might be pulling hydrogen's electron a bit closer to itself for a few physical reasons, and hence 'winning' - but the match has not ended, so no one can be declared a winner or loser, a taker of electrons, hence no charge, (as a prize or show of strength), can yet be denoted.

Oxygen would have seven electrons in its orbital flux aura, one away from having an octet and a complete shell.

In this way I think that in theory we coukd see O(2-). I think this woukd refer to a single monatomic oxygen atom that has taken two extra electrons, and hence, will be somewhat stabke with a full outer shell.

It seems therefore, that when we see an atom or compound polyatimic molecule with a charge, we should be reminded to think of battle scars and spoils of war - that this atom or polyatom has a history of wars over electrons with other atoms and polyatoms, and its charge is a symbol of its current state of winnings or losings.

Some don't seem to even need to go to war. Noble gases have full outer shells before they have even gone to battle over electrons. They are sort of born into a position of abundance and luxury, though it may be lonely nonetheless. It is also pretty hard for other atoms to even get into a tug of war with a noble gas, let alone win.

Some atoms like Al(3+), meaning 3 electrons lost, not 3 atoms of aluminium as in (Al3) It is found in soil quite a lot. I think this refers to the notion that this aluminium has lost 3 electrons. If so, it has lost 3 valence electrons to something or many things, at some point in the past. It is quite stable having lost its valence electrons because only had 3, which means its valence electrons are now gone. Can it be made to react again then ? Yes, I think so, but the chemistry eludes me.

I think Al (3+) is considered pseudo noble because it has lost its only 3 valence electrons at some prior time and has lost an oribtal, but demoted itself to a stable orbital nonetheless. The pseudo aspect might be that this stable state is not as strong as a real noble. One reason might be that Al (3+) now has an abundance of protons compared to electrons. This means it will have a slight tendency to pull in electrons and create a new shell that is not full, even though it has a full outer shell now.

I think the same is true of certain pseudo noble anions, in an opposite sense. They may have too much electron charge even if they have a full outer shell and might only weakly bind their newly acquired electron/s. Some pseudo noble anions might really be pretty stable though. Same with some cations. In the soil I believe Al (3+) and Cl(-) are pretty stable pseudo noble ions.

I think the way Al (3+) starts to destabilize is when electrons start getting close in abundance around it and cause electron gains to take place upon the aluminium metal, rather than abundances cations coming and stealing even more electrons from it. I think it is pretty hard for cations or any atoms to steal electrons from non bonding non valence orbitals like inner ones. I think x rays might be able to temporarily knock out a inner core electron, but possibly not without causing a lot of energy to be released as well.

I could be completely wrong in my understanding of the basics though.

I hope someone will correct me if I am. I would like to know.