23. Discuss Slater's rule and its application.
24. Write short note on allotropy of sulphur.
25. Explain SHAB principle.
26. Discuss the mechanism of formation of photochemical smog
27. What is electronegativity ? How is electronegativity obtained by the Puuling's method?
28. Write any four methods to control air pollution
29. Write note on "Hydrogen as a next generation fuel"
30. Discuss the following reaction in Inquid So2 a) salvation b) acid-base reaction
31. Discuss the flame colouration of I and II group elements
Answers
23.
The general principle behind Slater's Rule is that the actual charge felt by an electron is equal to what you'd expect the charge to be from a certain number of protons, but minus a certain amount of charge from other electrons. Slater's rules 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" (e.g., to compare the effective nuclear charge and shielding 3d and 4s in transition metals). Slater's rules are fairly simple and produce fairly accurate predictions of things like the electron configurations and ionization energies.
Slater's Rules
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: Identify the electron of interest, and ignore all electrons in higher groups (to the right in the list from Step 1). These do not shield electrons in lower groups
Step 3: Slater's Rules is now broken into two cases:
the shielding experienced by an s- or p- electron,
electrons within same group shield 0.35, except the 1s which shield 0.30
electrons within the n-1 group shield 0.85
electrons within the n-2 or lower groups shield 1.00
the shielding experienced by nd or nf valence electrons
electrons within same group shield 0.35
electrons within the lower groups shield 1.00
These rules are summarized in Figure 2.6.1 and Table 2.6.1 .
Figure 2.6.1 : Graphical depiction of Slater's rules with shielding constants indicated.
Shielding happens when electrons in lower valence shells (or the same valence shell) provide a repulsive force to valence electrons, thereby "negating" some of the attractive force from the positive nucleus. Electrons really close to the atom (n-2 or lower) pretty much just look like protons, so they completely negate. As electrons get closer to the electron of interest, some more complex interactions happen that reduce this shielding.
Table 2.6.1 : Slater's Rules for calculating shieldings
[1s]
0.30
-
-
-
[ns,np]
0.35
-
0.85
1
[nd] or [nf]
0.35
1
1
1
The shielding numbers in Table 2.6.1 were derived semi-empirically (i.e., derived from experiments) as opposed to theoretical calculations. This is because quantum mechanics makes calculating shielding effects quite difficult, which is outside the scope of this Module