Why does an element show variable valency?

Answer 1

The "valence" of an element is its "combining power". "Combining power" is the old definition of "valence". it refers to the number of other atoms which it can combine with to make a compound. For instance, Na has a combining power of 1, as does chlorine. Therefore, we get one Na and one Cl in a compound, NaCl. From Na2O we can see that oxygen has a combining power, or valence, of 2.

There are two combinations of Fe and Cl, FeCl2 and FeCl3. Therefore, iron must have two valences - two combining powers - 2 and 3. Somewhere in history, the valance was assigned a + or - to correspond to cations and anions, and along with that came the notion that the sum of the valences for a compound is zero.

Valences are NOT charges unless the substance actually exists as an ion. Today, the term "valence" has pretty much gone by the wayside, and we use "oxidation number" instead. "Oxidation number" is the new "valence". In FeCl2 and FeCl3 we say that iron has oxidation numbers of +2 and +3.

So why does iron have two oxidation numbers? That's because there are two stable combinations of Fe and Cl. Reactions occur because the products will be a lower energy than the reactants. If you place Fe metal in a container of Cl2 gas you will get a reaction which forms either FeCl2 or FeCl3 depending upon the relative amounts of the two reactants. Given sufficient Cl2, the reaction will go to FeCl3 since that product is a bit lower in energy than FeCl2.

We can relate the oxidation number (or valence) to the number of electrons available to react (the valence electrons). Sodium forms only NaCl when sodium metal is placed in Cl2 gas since sodium has only 1 valence electron and can form only one stable compound with chlorine.

How to pick which oxidation number (valence) to use? I might ask my students to simply write all the possible formulas for compounds of iron and chlorine, and I would expect to see FeC2 and FeCl3. There is no way to pick one or the other as the products, unless you are also given the conditions of the reaction.

This is why the names of compounds which exhibit multiple oxidation states include a Roman numeral to specify which oxidation state the metal is in. I.e. iron(II) chloride and iron(III) chloride.

Answer 2

Iron, being a d-block element, shows transition in its electronic configuration levels.

Now, according to Aufbau principle, iron should have 1s2,2s2,2p6,3s2,3p6,4s2,3d6 to get all its 26 electrons filled up.

This can also be written as 1s2,2s2,2p6,3s2,3p6,3d6,4s2.

Now, the d-orbital which can hold a maximum of 10 electrons is filled up with only 6 electrons. It is understood that half-filled orbitals are more stable than partially filled orbitals. Hence, iron can stay relatively stable at 3d5 orbital rather than 3d6. Also depending on the neighbouring atom it interacts, the 2 electrons in the outermost 4s subshell can also be withdrawn.

Considering both these cases, the configuration can be 1s2,2s2,2p6,3s2,3p6,3d6 ( iron donated 2 of its outer 4s electrons in this case, and it's valency is +2) or 1s2,2s2,2p6,3s2,3p6,3d5 ( iron donated 3 of its electrons: 1 electron from 3p and 2 electrons from 4s, valency is now +3). Thus, iron exhibits variable valency and transition electronic states.

Giridharan L M

Answer 3

because of the electrons shells around the necleus.

Answer 4

because they lose electrons from penultimate shell

Answer 5

Transition metals can lose electrons not only from external shell of electrons but also from the penultimate shell.