Shielding effect

1. Nov 17, 2009

phymatter

What actually is shielding effect ?? In particular my book says that the size of Ga is smaller than Al because of poor shielding effect of d electrons ,
but what is this shielding effect , imean how does it work?

2. Nov 17, 2009

waqarrashid33

shielding effect can be defined as:
"the repulsion b/w the inner shells(electrons)"
you will know that there is repulsion b/w electrons so as atomic number increases i.e no of electroms or shell increases which couse electron-electron repulsion and similarly repulsion on electrons of valece shell produces and so radius of atom is increases.there fore we are saying that atomic radius is directly proportional to shielding effec.and shielding effect is directly proportion to shells.

as i m a student of mathematics so sorry for bad presentation.

3. Nov 17, 2009

pzona

Last edited by a moderator: May 4, 2017
4. Nov 18, 2009

Biophreak

Your Gallium and Aluminum example is either a special case, or incorrect. This is because within a family (elements in the same column) atomic radius increases as you go down, and within a period (elements in the same row) atomic radius decreases.

This can be explained by electron shielding. Electron shielding is when the electrons in the energy levels below the valence electrons attenuate the strength of the positive charge 'emanating' from the nucleus. You can think of shielding as a sort of chipping away at the "positiveness" of the nucleus. The more complete electron orbitals beneath the valence level, the more shielding there is- resulting in a weaker "positiveness" of the nucleus as you move down a column on the periodic table.

With this in mind, one can understand why exactly it is that atoms tend to increase in radius as you go down a column in the periodic table. Each time you jump down one period, you are completing an octet, thereby adding a brand new level of shielding. As mentioned ealier, this increased shielding results in a lowering of the "positiveness" of the nucleus. Because of this attenuation of the strength of the nucleus, valence electrons aren't pulled as strongly by the nucleus, resulting in them having a wider "orbit".

An anology might be to think of the solar system. If the sun were to suddenly lose 1/2 it's mass, the planets wouldn't be held so tightly by the sun's gravity (bc gravity is dependent on mass) and would orbit at a greater distance.

Now, to explain why atoms tend to decrease in size as you move ACROSS a period.
Remember that electron shielding stays the same within a period-- which is to say that valence electrons DO NOT add to the shielding effect. So, then as you move across a period, the number of protons in the nucleus increases while the electron shielding strength stays the same. This has the net effect of there being a greater "positiveness" of the nucleus as you move left to right across a period.

This increased "positiveness" causes the valence electrons to be pulled closer and closer to the nucleus as you add more and more protons. Again, by the analogy I used before, picture the sun suddenly gaining double it's mass-- the orbits of the planets would proportionally shrink, as they would be more strongly attracted by the sun's gravity.

I hope this was helpful, ad you were able to find some clarity despite my wordiness

Last edited: Nov 18, 2009