# 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. – Jordan Epstein 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! – dhulihan 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). – Ben Norris Dec 6 '12 at 0:47
• Both O and H are neutral species. Do not confuse charge with oxidation number. – Richard Terrett 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. – M.A.R. 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. – Mithoron Dec 4 '18 at 20:44