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This is a quite elementary question, but I still can't wrap my head around it.

I know how to calculate the formal charges of an ion or a molecule, but I don't understand what happens when oxygen (when sharing a single covalent bond with sulfur in a sulfate ion) has a lone electron that doesn't bond nor pair. I don't understand why this puts a -1 charge on the atom. What happens?

I've attached an image which explains what I'm trying to figure out:

http://i.imgur.com/GWnM9FK.jpg

See those two red dots? What do they mean? I understand that they symbolize the lack of a single electron needed to complete an octet. But why does it give the atom a negative charge?

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The two red dots indicate the presence of two electrons, not the absence, if there were no electrons there it would be balanced. The colours of the electrons in the image indicate their origin: blue electrons originate with oxygen, black from sulfur and red from an outside source (eg. Potassium or H2). Ions have a charge because they have more or fewer electrons than the number of available protons. Sulfate has gained two extra electrons. The arrows in the second image indicate a 'dative covalent bond' where the electrons in the covalent bond all come from one atom rather than being shared, while the lines indicate a normal covalent bond (ie. an electron from each atom. The extra electrons added to the oxygen atoms to form a full orbital level in the normal covalent bond are responsible for the extra charge.

There's a nice visual depiction here: http://chem-net.blogspot.co.uk/2013/02/electron-dot-structure-of-sulfate-ion.html

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The way this is normally taught (introduced) is as the concept of formal charge.

Think about the neutral oxygen atom, how many valence electrons does it have? Now look at the oxygen bonded to the Sulfur, how many electrons does it have if you consider the bonding ones to be shared?

-lone oxygen = 6

-bonded oxygen = 7

As it has one more electron than the neutral atom this gives it a negative charge. Each oxygen is -1 and if you do the same process on the sulfur you get +2 and the total (+2 + -4) is -2 which is the charge on the ion.

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