0
$\begingroup$

There is a reaction given in my book:

$\ce{2FeSO_4 +Heat -> Fe_2O_3 + SO_2 + SO_3}$

I just noticed that on the left $\ce{Fe}$ has valency $2$, while it has $3$ on the right. How can it change valency?

Changing the number of electrons is easy, but this only changes the charge, not the valency. Does this mean that the number of protons changes?

$\endgroup$
2
$\begingroup$

Yes, valency can change, just like that. Some elements (in fact, most of them) can manifest different valencies; that's their nature. If you look again at this very reaction, you might notice that Fe is not the only element to change its valency here; S does the same. Reactions like this one are called redox reactions. There is a huge body of knowledge that explains what valencies (or rather, what oxidation states) an element can have, and which of them are more stable; the answer by @Raghav Gupta touches on that in more detail.

As to the number of protons (if you mean the protons in the element's nucleus), it has nothing to do with valency. Really, Fe contains 26 protons; is its valency 26? Absolutely not, it is 2 or 3. Also, the number of protons in an element can never be changed at all. If you change it, you'll get another element; also, that would be a nuclear reaction, a thing which is out of scope of chemistry textbooks.

$\endgroup$
0
$\begingroup$

The valency of an element is the number of hydrogen atoms that can combine with or replace (either directly or indirectly) one atom of the element. In other words, the valency of an element is the number of electrons an atom of the element uses to combine with atoms of other elements - it is the combining power of an atom of the element. In an atom, the valence electrons are the electrons that can be used in combining with other atoms - these are the electrons in the orbitals of the outermost shell (also called valence shell). That means valency is not dependent on number of proton.

Iron (Fe) Atomic no 26, so 26 electrons to be filled in different orbital. So the electronic configuration is: $\mathrm{1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6}$. This is the way of filling of electrons according to aufbau's principle. But the exact arrangement is $\mathrm{1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2}$

So Iron can lose two electrons from the outermost 4s subshell to show a valency of two. Then it can lose another electron from the 3d subshell (penultimate) two show a valency of three. When it shows the valency of two the electron configuration is $\mathrm{1s^2 2s^2 2p^6 3s^2 3p^6 3d^6}$. When it shows the valency of three the electron configuration is $\mathrm{1s^2 2s^2 2p^6 3s^2 3p^6 3d^5}$

And the +3 valence state is stabler than the +2 valence state, because the d-orbital contains 5 electrons (half-filled). As you know fully filled and exactly half filled orbitals are more stable than the orbitals having different no of electrons, i.e. less than half filled and more than half filled but not fully filled.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.