[Rayan]

hello.

Arrange the following in the decreasing order of polarizibility:

Li⁺  Be ²⁺     Br^-  Cl^-    Se ^2-

Arrange the following in the decreasing order of polarizing:

Al ³⁺     Cs⁺    Cr ³⁺       Rb⁺      Sr ²⁺      Xe

[Borek]

Google.

http://notes.chem.usyd.edu.au/course/masters/Chem1/Special%20Topics/Polarizability/Polarizability%20Applications.htm

[Rayan]

i'm still confused about it.
in arranging the elements in decreasing polarizing:

Cr > Sr  > Rb > Cs

but who is more polarizing; Al or Xe ?

and in arranging the elements in decreasing order of polarizability:

Se > Br > Cl . what about Li and Be?

am i even close to correct?

[jdurg]

Okay.  Basically, an ion is more polarizeable if it has a large electron cloud hanging around the nucleus.  For a big atom with a negative charge, it will be polarized very easily because it will have a lot of electrons around a big central nucleus.  Those outer electrons will feel a bunch of pull from any other nucleus out there.

Positive ions decrease in polarizability as their nucleus size gets smaller.  This is because the outer electrons feel a VERY strong pull from their own nucleus and are less likely to feel the pull of another element's nucleus.

So when you are arranging the atoms/ions in terms of their polarizability, the first thing you must do is determine how many electrons the atom/ion has and what its atomic number.  If you have two atoms/ions with the same number of electrons, but one has a bigger nucleus, based upon the information above which one do you think would be more polarizable?

[Rayan]

the one with smaller nucleus will be more polarizable because then they will be more likely to feel the pull of another element's nucleus.
now letme get this straight:
an atom/ion is more polarizable if it is large and has a high charge. correct?
and the atomic radius decrease as we go from left to right in the periodic table, and increases down a group. thus Li is bigger than Be but Be has a higher charge. what is more important? being larger or having a higher charge??
and an atom is more polarizing if it is small. Xe being in group 18 has small radius, but it is in the fifth row while Al is in the third. who is more polarizing?

[jdurg]

Almost.  A large atom with a large number of electrons around it will feel the effect of another atom's nucleus on it moreso than a small atom with a low number of electrons.  Li+ is not very polarizeable at all because it has a small nucleus and a small number of electrons.  The Li+ atom holds onto its electrons very tightly and basically doesn't feel the pull of any other atom.  Cs+, however, has a large number of electrons around it so it will become polarized much more readily as the electrons do a good job of blocking the pull from its own nucleus.

So let's look at Al3+ and Xe.  Al3+ has 10 electrons floating around its more compact nucleus.  Xe, meanwhile, has 54 electrons floating around it's larger nucleus.  So Xe has far more electrons hanging around it than aluminum does, and it has two more 'levels' of electrons.  Because of the fact that Al is in a +3 state, it is electronically the same as a neutral Ne atom.  So based on the rules outlined in this thread, what would be more polarizeable, the Neon (Al3+) atom or the Xenon atom?

For positive ions:  The smaller the atomic radius, the smaller the degree of polarizability.

For negative ions:  The larger the atomic radius, the higher the degree of polarizability.

So if we were to compare K+ and Cl-, which one do you think would be more polarizeable?

[Rayan]

 "large atom with a large number of electrons around it will feel the effect of another atom's nucleus on it moreso than a small atom with a low number of electrons" thus Xe is more polarizable then Al.
but why does Li has a small nucleus? shouldnt be somehow large being in group I (based on the fact that atomis radii increased from right to left in the periodic chart). and if it's true, then K+ has a large radius while Cl- has a small one.  (atomic radius for K is 275 pm according to van der waals and for Cl it is 175 pm). how should i compare?

[jdurg]

Li has a small nucleus, and hence a 'small' atomic radius, because it has but 3 protons in there and about 4 neutrons.  The outer electron in Lithium feels a good deal of pull from the nucleus as it has but two other electrons to 'shield' it from that pull.  Therefore, Li has what is considered a 'small' nucleus since the effective nuclear charge felt by the outer electron is fairly large.  (Remember, atomic radii tend to decrease as you move towards the top of the periodic table and to the right.  So Fluorine has the smallest radius and francium would have the largest if it existed for more than a few minutes at a time).

[Rayan]

and what about comparing K+ and Cl- ?
and please, about Li once again, it has a small radius cz it is at the top, but it is to the very left!! it is not making sense to me   . i mean it should have a somewhat big atomic radius..
sorry to bother you..

[jdurg]

Well, the question at the start of the thread there was a question on the radius of the Li+ ion.  The atomic radii of positively charged monatomic ions are ALWAYS much smaller than that of the neutral atom, and vice-versa for negatively charged ions.

Once again, let's just go over the trend of atomic radii.  As you move from left to right on the periodic table, the atomic radius gets smaller and smaller.  Why is this?  Well, as we go across a row of the periodic table, an extra proton is added each time you move right, and an extra electron is added.  However, this extra electron is added to the exact same energy level as the electrons before it.  Because those electrons are in the same energy level, it means that they are approximately the same distance from the nucleus.  So the cloud of electrons just becomes more 'full' and not necessarily 'bigger'.  At the same time, we're adding another proton and its positive charge to the nucleus.  This causes the nucleus to exert a greater 'pull' on those outer electrons so the radius of the atom decreases.  (Since the inward pull increases, but the energy level of the outer electrons remain the same).  This is why the atomic radius decreases as you move across the group.  Those same energy level electrons feel a tighter pull from that increasingly larger charge from the nucleus.  (When you get to the noble gases, however, there is suddenly a large increase in atomic radii.  This could be a combination of how the radius is measured and the completely filled outer shell that the atoms have).

Now we'll examine why the radius increases as you move down the column.  Because of the added protons and higher nuclear charge, you'd expect potassium to have a smaller atomic radius than sodium with its lower number of protons, correct?  Well, if all of those electrons were in the same energy level then yes, it would be smaller.  However, when you go into the next row of the periodic table, any electrons present are in a higher energy level than the row before it.  These electrons have more energy and are able to reside further away from the nucleus.  As a result, the atomic radius INCREASES because of these higher energy level electrons floating around.  So as you go down the periodic table, the radii of the atoms gets bigger as more electrons are added.  Also, those higher energy electrons feel less of a pull from the nucleus because of the shielding provided by the inner shell electrons.

So when you're talking about ions, you have to think about the energy level of the electrons in those ions and the size of the nucleus in those ions.  With Li+, you have an atom with two electrons in the 1s shell and a nucleus with 3 protons.  So the electronic configuration is like that of helium, but with a stronger nuclear charge.  So you would expect the radius of the Li+ ion to be smaller than that of the neutral helium atom due to the non-existant shielding of electrons and the higher nuclear charge.

Now let's compare K+ and Cl-.  Both of those ions have an electronic configuration like that of Argon.  They have full outer shells in the 3rd period of the table.  K+, however, is a positively charged ion so you know right away that it's atomic radius is much smaller than that of the neutral K atom, and Cl- is a negatively charged ion so you know right away that it's atomic radius is much bigger than that of a neutral Cl atom.  Now, take a look at the number of protons each ion has.  K+ has 19 protons and Cl- has 17.  They both have the same number of electrons in the same energy levels as each other.  Based purely on that information, which one would have a smaller atomic radius?  Because it has a smaller atomic radius, meaning that the electrons are held closer to the nucleus, do you think it would be more polarizeable or less polarizeable than an atom/ion with a larger atomic radius?

[Rayan]

K+ will have smaller atomic radius because it has more protons then it can attract its electrons more. and  having a small radius will make it less polarizable.
correct?

[Rayan]

so what if an atom is highly polarizing or has a high polarizability? then what?

[jdurg]

K+ will have smaller atomic radius because it has more protons then it can attract its electrons more. and  having a small radius will make it less polarizable.
correct?

Yes.  You are correct.    K+ will be holding onto it's electrons a lot more tightly than Cl- can because K+ has more protons to grab onto the electrons with.  As a result, Cl- is more succeptable to being polarized since the outermost electrons don't feel as strong of a pull from the nucleus.

If an atom is highly polarizable, then it means that it has a large electron cloud and those outer electrons don't feel as strong a pull from the nucleus as the outer electrons do on atoms with small electron clouds.  This is why the polarizability of a netural atom increases as you move down a group and decreases as you move to the right in a period.  So of all the neutral atoms, Cesium would tend to be the most polarizeable.  (Radioactive atoms tend to be disregarded as it's pretty difficult to get an accurate measurement on their atomic radius and electron cloud).  So I- would be highly polarizable since it has a large electron cloud and those outer electrons feel little pull from the central nucleus.

If an atom is highly polarizing, then it means that it is a relatively small atom with a small electron cloud.  As a result, the pull of the nucleus is able to be felt by anything that comes close.  Let's take a look at Li+.  It has only 2 electrons and 3 protons in the nucleus.  It's a very small atom and the nuclear attraction it puts on surrounding electrons is fairly strong.  As a result, the Li+ ion is highly polarizing.  It is able to induce polarization in other atoms/ions.