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] 3s
1), Oxygen (O : [He]2s
2 2p
4) - which have single oxidation states - and Iron (Fe : [Ar] 3d
6 4s
2) - 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] 3d
6 4s
2. It can lose a 4s electron and a 3d electron to become [Ar] 3d
5 4s
1 (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] 3d
5 (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 Fe
2+ or Fe
3+. 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).
) It mentions a 1s subshell and a 2s subshell, but not a 3s.
