effective nuclear charge

1.
the ENC decreases.
ENC = attraction by nucleus - repulsion by non-outer shell electrons.
the attraction is directly proportional to no. of protons, i.e. atomic number. this is easy to understand.

however, repulsion is NOT directly proportional to no. of electrons. more appropriately, it's related to no. of filled inner electron shells.
rule of thumb is, repulsion due to inner-shell electrons is greatest, repulsion due to same-shell electrons is weaker, repulsion due to outer-shell electrons is ZERO. [this concept is useful in determining ENC of an inner-shell electron, which probably not examined in A-Level.]

the point is, increase in number of electron SHELLS will reduce the attraction more greatly than the increase in attraction (as attraction force obey inverse square law).
lithium as one inner shell while sodium has two. thus ENC of lithium is stronger than sodium's.

on the other hand, increase of repulsion due to same-shell electrons is weaker than increase in attraction.
compare sodium and magnesium. both have same no. of inner shells, but ENC of magnesium is stronger than sodium's.

approximate estimation could be like:
shielding effect = (no. of inner electrons)^2 + 0.5 x(no. of same shell electrons)


2.
no.
ENC generally increases across the period, and decreases down the group.

3.
you can do that.
note the specify "the effective nuclear charge with respect to outermost electron".
as ENC increases, the net attraction between nucleus and outershell electron increases, which means more energy is needed to overcome the attraction, promoting the electrons to infinity (ionization).

note that ENC is NOT the only consideration of I.E.
some other factors may also contribute, like half-filled / full-filled orbitals. consider nitrogen VS oxygen.


add supplement if i did not put it clear enough.


2011-01-02 00:07:14 補充：
what?
ENC is examined in AL!
i said, "ENC for inner electrons" is not examined;
common use of ENC is still examined!!

haha thanks!
so i need to learn about ENC but only not the ENC for inner electrons.^_^

我都想知
我搞唔清呢個concept