[defencegrid]

Can someone please explain how the Permanganate ion MnO4- has a negative charge?

Mg has seven valence electrons.  Three oxygen atoms pull six Mn electrons towards themselves, giving them each a -2 charge.  The fourth oxygen atoms pulls the seventh Mn electron towards itself giving it a -1 charge.  So the four oxygen atoms are -7.  The Mg atom is +7 which makes the molecule neutral.  What am I doing wrong?

[XeLa.]

EDIT: Simply put, the -1 charge refers to the fact that one of the oxygens in the ion has only 7 electrons in its valence shell. It must gain another electron in order to become neutral.

[Enthalpy]

If you claim to know the oxidation state of every atom, you can deduce the charge of the polyatomic ion. Here 1×(+7) and 4×(-2) hence the ion has -1.

But... One shouldn't imagine that the electrons have left the Mn and are far away, because ripping 7 electrons from a Mn atom alone in space would require 38MJ/mol or 396eV per atom, ouch
https://www.webelements.com/manganese/atoms.html
As compared, the affinity of an oxygen atom for a first additional electron is 141kJ/mol only
https://www.webelements.com/oxygen/atoms.html
or 6eV for the first four electrons to four oxygen atoms.

So ions are possible only in extreme proximity of other ions with the opposite charge (in a molecule) or of atoms of solvent like water that carry a partial charge.

[defencegrid]

As such, an external electron is "donated" to this fourth oxygen giving it a noble gas configuration as well.

I don't understand what the "external electron" is.  Where did it come from?

[defencegrid]

If you claim to know the oxidation state of every atom, you can deduce the charge of the polyatomic ion. Here 1×(+7) and 4×(-2) hence the ion has -1.

Yeah I did the calculation as you described above, but I wanted to know exactly what was happening with the electrons and why there was a negative charge.

[XeLa.]

If you claim to know the oxidation state of every atom, you can deduce the charge of the polyatomic ion. Here 1×(+7) and 4×(-2) hence the ion has -1.

Yeah I did the calculation as you described above, but I wanted to know exactly what was happening with the electrons and why there was a negative charge.

There's a negative charge because it signifies that the radical requires one extra electron in order to become electrically neutral.

[defencegrid]

I'm sorry, but I still don't understand.  For the fourth oxygen to have a -2 charge, it would need to have eight electrons around it.  Currently it only has seven electrons.

[XeLa.]

Correct, that's why the ion is given a negative one charge. It signifies that only one more electron is necessary in order to achieve a favourable electronic configuration. It obtains this electron through ionic bonding. Such an understanding is satisfactory for high school chemistry course.

Think of why the fluoride ion is given a -1 charge. It's because fluorine only needs one more electron in order to achieve a favourable electronic configuration. The same is the case here in an elementary sense.

[defencegrid]

Correct, that's why the ion is given a negative one charge. It signifies that only one more electron is necessary in order to achieve a favourable electronic configuration. It obtains this electron through ionic bonding. Such an understanding is satisfactory for high school chemistry course.

So are we saying that the MnO4 molecule is neutral overall, but one of the oxygen atoms is one electron short of an octet?

[XeLa.]

Neutral, yes. But that wouldn't make it a polyatomic ion.

[XeLa.]

Correct, that's why the ion is given a negative one charge. It signifies that only one more electron is necessary in order to achieve a favourable electronic configuration. It obtains this electron through ionic bonding. Such an understanding is satisfactory for high school chemistry course.

So are we saying that the MnO4 molecule is neutral overall, but one of the oxygen atoms is one electron short of an octet?

Correct. It becomes electronically favourable when it forms the ion MnO₄^-. Does that make sense? I would recommend seeing if you can write out the structural formulae of common ions like sulfate, carbonate, nitrate, phosphate, etc. The strongest links are often drawn this way.

[defencegrid]

I will do that.  Thank you for your help and patience.

[Enthalpy]

So are we saying that the MnO₄ molecule is neutral overall, but one of the oxygen atoms is one electron short of an octet?

I'm not aware of an MnO₄ molecule. What exists is an MnO₄^- ion, where the "-" indicates that this ion carries one negative charge, that is, has already gained one electron, which comes from one or several other atom(s) that have lost it, becoming an ion, and bear a positive charge.

For instance, KMnO₄ can be written as K⁺ [MnO₄]^-.

[MnO₄]^- exists only in the presence and near vicinity of a positive ion, in the same crystal or solvent.

The gained electron isn't local to one oxygen atom: it's spread over the whole ion, essentially the four oxygens. This makes the ion more stable: for instance, more water molecules have room to dock to it, easing the dissolution. The notation [MnO₄]^- insists on that.

[defencegrid]

So are we saying that the MnO₄ molecule is neutral overall, but one of the oxygen atoms is one electron short of an octet?

I'm not aware of an MnO₄ molecule. What exists is an MnO₄^- ion, where the "-" indicates that this ion carries one negative charge, that is, has already gained one electron, which comes from one or several other atom(s) that have lost it, becoming an ion, and bear a positive charge.

So basically it's taken the electron from K