IIT JEE , AIEEE Forum - Chemistry , Inorganic Chemistry - Well, never thought of this, why are the s-orbital electrons transferred to d-orbital ?

zane3004

just wanted to know, why do the s-orbital electrons end up in d-orbital in some cases ?

As far as i knew, i thought (and was told by prof.) that this happens since d-orbital is more stable half/full filled.
But this explains the case for the Cr, Cu exceptions

What about the exceptions of group 5 and 6 ?
for eg:
Nb-4d4,5s1
Mo-4d5,5s1
Ru-4d7,5s1
Pd-4d10

i never knew about these exceptions.
need help, why do these ones happen ?
half/full-filled d-orbital config. isnt achieved here.

chimanshu_007

palladium i think.....

fully filled 4d orbital is more stable than 4d8......and whether its 5s2 or 5s0....both are stable

srujana

Two important points u need to remember here.

(1)s-orbital electrons end up in d-orbital only when they are of equal energies.

(2)Exchange energy also plays an important role is deciding the stability of atoms.

it is given by N=n(n-1)/2

n=no. of electrons of the same spin.

Greater the exchange energy, greater is the stability of the atoms.

The reason for which half filled and completely filled subshells are more stable is that they have greater exchange energy.

Let us take the example of ,Nb-4d4,5s1

the exchange energy is 10

If the configuration would have been 4d3,5s2

the exchange energy is 6

srujana

There is another criteria for stability-colombic energy, i have read about it a long ago in CHEMISTRY TODAY, it is a really big article which explains all these but i need some time to type it out here

srujana

Heres the article,

The author of this article is Ankit Agarwal [B .Tech IIT-Delhi]

"We quite often assume that Hunds rule of filling orbitals is correct.However elements of 4d series , which do not follow Hunds rule are counted as exemptions.

Let us try to understand why half filled and completely filled configurations are more stable.

"When i appeared for JEE in 2000, the total selections were nearly 2500.Every year the intake has been consistently increasing.In the year 2007 the intake has been announced to be nearly 7000.

However the infrastructure was not upto the mark.In fact in the year 2000, when i joined IIT Delhi, I got a single room in my hostel.

The next year the intake was more, but the number of rooms were same, so the single room became double room.The next year there were four beds.

After that there was no space in the rooms.So in last two years there were new hostels built for the new entrants.

Now let us extend this story to an atom.

Electrons will also prefer to fill in the orbitals singly first, only after then pairing will occur(after all electrons need their privacy !)

But when there is no space for other electrons to fill there, electrons enter the new orbit.

Again coming back to our example of IITians, variety is the spicy of our life.

When we were in IIT, we used to periodically exchange our rooms,just for fun with our luggage in our own rooms.

I would not like to exchange my room on the ground floor with some one on the fourth floor(imagine sleeping and studying in the fourt hfloor and running to ground to change your clothes after bathing).

I would exchange it with the person on the first floor so that my energy wouldnt be wasted.So,the exchange for the electrons is possible only in the degenerate orbitals(i.e orbitals of same energies)

Technically we call this whole phenomenon as exchange energy.

If exchange energy is high greater is the stability.

There is another criterion for stable electronic configuration, coloumbic energy

Two electrons in the same orbital will repel each other, just like if you place two women in a room they start fighting with each other.
This repulsive force is called coloumbic energy.

Lesser is the coloumbic energy, greater is the stability of an electronic configuration.

This is one of the reasons why electrons are singly filled in orbitals, before there is no other option but pairing up of electrons.

A stable electronic configuration has two components

(1) High exchange energy and(2) low coloumbic energy.

If someone says full filled configurations may be the least stable, because coloumbic energy is very high, it is wrong.

There is another factor which u have forgotten to consider , exchange energy. The exchange energy is much higher than coloumbic energy so full filled configurations are extremely stable."

Phew !!!

harsha_27

Ankit Sir rocks................

Thanks for writing it here srujana.I did not read this article before.

srujana

@ Harsha, i have edited it a bit so as to save my hands....but ill present the complete 1 in a short while.

Ankit sir sure does rock .......!

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