The
quantitative prediction of specific energy level is not possible if an atom
have more than one electron (multi-electrons system). In multi-electron system
each electron acts as a shield for electron further away from the nucleus,
reducing the attraction between the nucleus and distant electron. The Slater provides
a set of rules which help to predict the extent of shielding.
In
the Slater rule determine the actual charge felt by an electron and also allow
you to estimate the effective nuclear charge Zeff from the real
number of protons in the nucleus and the effective shielding of electrons in
each orbital "shell"
“Slater defined Z* or Zeff (effective nuclear charge) as a measure of nuclear
attraction for an electron. Or The magnitude of positive charge “experienced” by an electron
in the atom.
Thus Z* =Z-S
Where Z is the (nuclear charge)= The
number of protons in the nucleus of an atom, and S represent the shielding constant and value is greater than zero but less than Z.
Step 1: Write the electron configuration
of the atom in the following form:
(1s) (2s, 2p) (3s, 3p) (3d) (4s,
4p) (4d) (4f) (5s, 5p) . . .
Step (2): Electron in higher group do not shield the
electrons in the lower group.
Step (3): For ns and np
valence electrons:
(A) The electrons in same ns and np group
contribute 0.35, except the 1s which shield 0.30
(B) Electrons within the n-1 group
contribute 0.85
(C) Electrons within the n-2 or
lower groups contribute (shield) 1.00
Step (4): For nd and nf
valence electrons:
(A)
Electrons in same nd and nf group contribute 0.35
(B)
Electrons in the group to the left contribute 1.00
Shielding
constant (S) obtained is subtracted from Z to get Z*
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