Simple Chem Query

1)
There are 2 reasons:

i) If we look at the structures of the conjugate bases , ClO4- and ClO-
(@Satrap: Now is a good time to start practicing drawing Lewis structures if u r doing chemical bonding! [1])
we see that there are three double bonds on the ClO4- between three of the O and the Cl and a single bond between the last O and the Cl.

ClO- has no double bonds between Cl and O.

The conjugate base of HClO4, ClO4-, is more stable as it can diffuse out the formal charge on the O through resonance to 4 O by giving 4 equivalent resonance structures.

As the ClO4- is a more stable ion, HClO4 is the stronger of the two acids.

P.S.: the strength of an acid is determined by how easily it releases H+, so the acid with the more stable conjugate base will more easily give up H+ and is the stronger acid.

ii) Electron delocalization: the 3 double bonded O are electronegative and pull electron density towards themselves.

The O in the HClO is doing the same thing, but there is only one O in HClO vs. 4 in HClO4..
so there is less pulling in HClO.
Pulling the electron density in away from the O-H bond weakens the bond, making it easier for the H to dissociate.

I had always been confused about the second since class XI..
Coz I used to think if O pulls the electron cloud, then a negative charge appears on it and then it cannot furnish H+ easily..

But look what actually happens:

The rest 3 O atoms pull away the electron cloud from Cl making it too much positive..
Thus Cl, which is now highly positively charged (though charge is less than unity), it starts attracting the electron cloud from O of O-H.. making that bond weaker!

I hope I am clear and elaborate enough! [1]

Dekho Satrap:

First I will try to explain what is the need of effective nuclear charge and why does it arise in the first place!

Then I will go on (if time permits) to discuss slater's rule! [1]

First of all.. Bohr's theory described a nice approximation of the atomic model for the hydroden-like atoms only..

In it the electron only faced attractions from the nucleus..

Now to extend the bohr's theory in somewhat accurate way we need to consider the repulsion provided by the buddy electrons thriving in the same atom with the electron in question..

Toh due to this the net amount of inward force gets altered (not necessary that for every electron it will be less than the actual nuclear attraction.. the reason y I said altered and not changed!)..

And what we do is free the electron-nucleus system of all other electrons and draw the diagram! [1]
but in doing so .. if we keep the nuclear charge same as original then the altered nuclear attraction won't be taken care of.. and so what we do is assign some new value whch we call "EFFECTIVE NUCLEAR CHARGE" (naming is done just to scary away people with weak hearts [3])

And this WE SAY that this change in the nuclear charge comes about due to the screening effect of electrons..
Again u need to realise that screening effect me kuch bhi nahi hai.. just naam diya gaya hai kamjor dil walo ko darane k liye! [3]

Ab ek simple sa example soch lo...

Ek ladki hai.. And Mr. Topiwala is guarding her.. (this is a code between me and Satrap.. rest can take mr.topiwala to be an arbitrary idiot [3])

And suppose, you want to reach that girl badly!

Lekin topiwala tumhe rok raha hai..

Now what will determine to what extent Mr. Topiwala will stop you?

They are:
1) The net area of access to the girl Mr. Topiwala guards from you.. in simple hindi jise kehte hai kitna bada dhaal ban paya woh...
2) the strength in topiwala! Aisa toh nahi ki koi bhi bachchha tumhe ladki tak pahuch ne se rok paye..

So.. similar is case for s,p,d,f orbitals...

What does the electrons residing in these orbitals do?

They repel the electrons lying in the outskirts...

And s orbital covers the nucleus entirely and has just 2 electrons..
while as we move from s to p,d and f, the orbitals diffuse out covering less area progressively but having more number of electrons progressively!

It has been found experimentally,

s electrons provide more effective guarding than p electrons which offer better guard than d which is further more than f

or in other words..

s electrons repel more than p, p repels more than d, d repels more than f.
(this can be also put as the screening effect is in order: s>p>d>f)

now let the screening effect given by an orbital is given quatitatively by "x"

then we can say: Z_effective = Z_actual - x
(isn't this obvious?)

And this formula forms basis of what is called slater's rules!

Slater gives various values of x related to various electrons in various types of orbitals... and thus gives the quantitative value of the Z_effective ..

I am not sure whether u need to memorise the values of x for IITJEE (to be truthful .. I hadn't)

But nonetheless i ll give u the values for the sake of completion:

http://en.wikipedia.org/wiki/Slater%27s_rules#Rules

Just see the two topics #Rules and #Examples that appear on ur screen..ignore everything else in there! :)

I hope I could convince u more than confuse!

Hope this helped!

Cheers! [1]

I ll give some write up for periodic table tonight surely..

Who took ur periodic table classes??

Satrap: post the maths doubts in a separate post..

under the Maths Forum..

else more often than not.. it gets lost! [1]

Dont worry about Slater's Rule! [1]

Dekho the periodic table filling up is based on the energy of the shell the electron would NORMALLY have gone to (i.e. excluding the exceptions like in Chromium and Copper where the last shell is considered 3d not 4s, Lanthanum and Actinium where the f orbital is considered the last orbital not the d)..

Or in other words, it considers the energy of the orbital the last electron NORMALLY should go into and the electrons are arranged according to that! (this is a result of Aufbau's principle)..

And we know that the energy of any orbital is defined by the following two rules:

1) The orbitals having lower value of (n+l) has lower energy..
2) If two orbitals have same (n+l) value, then the orbital with the lower n value will have lower energy.

Thus, following these two laws first we arrange the orbitals according to increasing energy first..

Then the periodic table is built up according to another rule:
Any period of the periodic table cannot have an element in it which has an electron (in the ground or most stable state) which resides in an orbital with principle quantum number greater than the period number in periodic table! [1]

Thus, suppose we are filling up period number x whenever we encounter the first element in which the last electron should have gone to an orbital with n=x+1, we will immediately stop filling the xth period and start filling (x+1)th orbital and continue doing so until we encounter an atom in which the next electron should normally go to an orbital with n=x+2

Hope I was clear enough! [1]