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

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?

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?

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).

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?

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