Types of Charges in Chemistry: Explained

[Math Is Hard]

I just started a chemistry class and the prof handed out some nomenclature rules for naming compounds. One of the rules says that... in the case of a metal + non-metal compound where the metal has more than one type of charge, do the following..(and then she lists the procedures).

What I didn't understand is what is a type of charge means. She hasn't covered this yet, but I was just curious about it. I always try to read ahead before the lectures.

Thanks

 

[Math Is Hard]

I found something similar to the instructions I have on this website:
http://users.ixpres.com/~statmech/Chem%201A%20Spring%202004/Basic%20Inorganic%20Nomenclature.htm

 

[honestrosewater]

For some reason, your questions tend to interest me even when I know nothing about the subject. I googled ic ous charge (mentioned in the link: "If the metal can have more than one type of charge, put a Roman numeral in parentheses after the name of the metal to indicate the charge on that metal in that compound. The classical system is may also be used (“ous” & “ic”)"), and it looks like these are suffixes attached to ions with different numbers, perhaps the number of electrons that they've lost or gained. Maybe you know what the numbers mean? Just thought I would mention it. I only know of two 'types' of charge: positive and negative.

http://www.google.com/search?hl=en&lr=&q=ic+ous+charge&btnG=Search

 

[Math Is Hard]

Thanks for the Google, HRW. I've been poking around a little myself, and so far all I've found is a reference that
"Some elements have more than one possible charge or valence. These are referred to as the multi-valent elements. Iron for instance has a +2 charge (Iron II 0r Ferrous) in some compounds and a +3 charge (Iron III or Ferric) in other compounds so we will have to recognize in which state the Irin is in and write the formula accordingly. "
http://members.aol.com/profchm/formwrit.html

so I am obviously missing an understanding of multi-valence in my fundamentals.

 

[Gokul43201]

Haven't looked at any of the links yet, but here's what these folks mean by "type of charge" (which usage, IMO, is bordering of idiotic):

All elements have what is known as a preferred oxidation state. What this means is that, if you take an isolated atom of this element, and allow it to choose from a large selection of charge configurations (by exchanging electrons with a sink or source), it will, in the majority of cases, settle down in a particular charged state.

This charged state is the one that minimizes the energy of that atom (now an ion). There are a bunch of rules that tell you what electronic configurations are most favorable (least energetic). For instance, you may have heard of the Octet Rule : that atoms like to have 8 electrons in their valence shell. The Octet configuration (also called the Noble Gas configuration, because all the Noble Gas elements have these octets, and hence are extremely stable) however, may not always be easily reached if the element is naturally in a configuration that is far from an octet. In that case, the element must choose from one of many "next best" options (such as fully filled or half-filled subshells) that are nearby.

It is for this reason that the transition metals (the pink block at webelements) often have multiple valencies/oxidation states/"types of charge" - because there is no single overwhelmingly stable choice nearby that easily beats out the rest. And what specific choice they make usually depends on what serves as the source or sink of electrons - usually, other elements also wanting to gain or lose electrons (it's kinda like a marketplace where the price is set by supply and demand).

Let's look at three examples, Sodium (Na : [Ne] 3s¹), Oxygen (O : [He]2s² 2p⁴) - which have single oxidation states - and Iron (Fe : [Ar] 3d⁶ 4s²) - which is usually found in either the +2 (ferrous) state or the +3 (ferric) state.

I hope you have a basic knowledge of electronic configurations, but if not, feel free to shoot back more questions. For now, I'm assuming a rudimentary knowledge.

The configuration of Na tells you that it has 1 electron (in the 3s subshell) more than the Noble Gas Argon. To get itself to this stable Argon-like configuration, Na must lose 1 electron, thus getting a single positive charge. Hence, Na is always seen in compounds as Na⁺ - with a single positive charge (or a deficiency of one electron).

Similarly, Oxygen has a configuration that is two electrons short of that of Neon (it is also 4 electron in excess of Helium, but that's farther away, and hence harder to attain), so given the chance, it will suck up a pair of electrons. Thus, in compounds O is seen as O^2-, or we say that the preferred oxidation state of oxygen is -2.

Now Iron is different. The nearest Noble Gas configurations (Argon and Krypton) are too far (8 and 10 electrons respectively) away. It takes too much work to add or pluck out that many electrons, and the resulting reduction in energy is found to be "not worth the effort". So, Fe settles down at one the the nearby "valleys" (if you plot the energy as a function of the configuration, you will see peaks and valleys; the valley for the octet being deepest, but other valleys existing as well), or local energy minima. So, whcih are these nearby configurations ? Fe has the configuration [Ar] 3d⁶ 4s². It can lose a 4s electron and a 3d electron to become [Ar] 3d⁵ 4s¹ (that's two half-filled subshells; the s-subshell can hold up to 2 electrons and the d-subshell can take as many as 10) or it can lose both the 4s electrons and one 3d electron to become [Ar] 3d⁵ (that's one half filled subshell). While the latter case is a little more stable than the former, it also takes more to get there (you have to pull out one more electron). So, on average, you find that iron can exist either as Fe²⁺ or Fe³⁺. The latter (3+) is more likely to form if there's powerful sinks (atoms like oxygen that crave electrons) in the vicinity, else the former is more likely.

The upshot of all this is that Fe can assume more than one oxidation state ("type of charge" ), and the choice depends on the environment (what other elements are nearby, as well as conditions like temperature and pressure).

 

[Math Is Hard]

Hey Gokul, thanks for taking the time to explain all that to me. I printed this out and was reading it on my lunch hour today. I think I am pretty close to catching on, but one thing I wasn't clear on (and I have seen this before and not understood it then) is why you had written Sodium (Na : [Ne] 3s1). It was the [Ne] part that I don't quite get. Why is that there?

I had some more stuff I was going to write but someone just walked into my office and now I am being dragged upstairs to look at somebody's dang printer problem. :grumpy: More later!

 

[Math Is Hard]

OK, I think I figured out where I am getting confused..

The configuration of Na tells you that it has 1 electron (in the 3s subshell) more than the Noble Gas Argon. To get itself to this stable Argon-like configuration, Na must lose 1 electron, thus getting a single positive charge. Hence, Na is always seen in compounds as Na+ - with a single positive charge (or a deficiency of one electron).

I would have thought that Na has one electron more than Ne (rather than Argon), since Ne has 10 electrons and Na has 11. And then I could understand Sodium written as this (Na : [Ne] 3s1). The [Ne] would be like shorthand for the configuration of the first 10 electrons?

And I understand that a Na atom would love to unload that extra electron and if there were say, a Cl atom nearby, it would happily do so. That would result in Na+ having eight outer electrons and would be mutually beneficial for Cl- to give it eight outer electrons as well. (Octet rule is the only one I know so far, so this part I am OK with).

But the problem may be that I know so precious little about orbitals - my textbook doesn't even mention them, but I have been able to glean a little bit of info from my copy of Chemistry for Dummies. (yes, I know- go on- laugh it up ) It mentions a 1s subshell and a 2s subshell, but not a 3s.

Maybe you (or someone) could just point me toward a website on this where I could read up more, and then I might be able to focus my questions.

 

[Gokul43201]

OK, I think I figured out where I am getting confused..

I would have thought that Na has one electron more than Ne (rather than Argon), since Ne has 10 electrons and Na has 11. And then I could understand Sodium written as this (Na : [Ne] 3s1). The [Ne] would be like shorthand for the configuration of the first 10 electrons?

That's correct. Sorry for the screw up...it is Neon, not Argon.

I'll see if I can dig up some useful links on electronic configurations.

...here's a nice one : http://antoine.frostburg.edu/chem/senese/101/electrons/index.shtml

 

[Math Is Hard]

ooh, I think I am actually starting to understand this, then! Thanks, Gokul! I am going to go look at that link.

And oh, I so totally agree with you about "types of charges" being confusing. Honestrosewater and I had the same initial reaction, I think: "Types of charges? There's only two!" "Preferred oxidation states" is much more clear to me.