Shielding Electron and Slater’s Rule
Shielding electron or screening or shielding effect decreasing the attractive force between the valence shell electrons and the nucleus of the atom. Hence slater’s rule uses for calculating the shielding or screening constant and effective nuclear charge of an atom or ion in chemical science. For hydrogen atom contain one electron, hence hydrogen atom has no shielding effect. But valence electrons for multi-electron atoms attracted by the nucleus of the atom and repelled by the electrons from inner-shells. These attractive and repulsive force acting on the valence electrons experience less attraction from the nucleus of an atom. Therefore, the inner electrons which shield the higher energy electron are called shielding electron, and the effect is called the shielding effect or screening effect.
In learning chemistry, the larger the number of inner electrons, the lesser will be the attraction between the nucleus and outer orbitals electron. Hence when the number of inner electron increases, shielding or screening constant also increases but effective nuclear charge decreases which affects some chemical properties of atom or molecule of the matter.
Effective Nuclear Charge of an Atom
Effective nuclear change means the net positive charge which affects the attraction of outer electron particles from the nucleus of polyelectronic atom. This term used because the shielding electrons prevent the attraction of outer orbital electrons of an atom. Therefore, the effective nuclear charge (Zeff ) = Z – σ, where σ = shielding constant.
Slater’s Rule for Calculating Shielding Constant
s or p-orbital electrons of an atom or ion
- First we write the electronic configuration of atom or ion by following order and grouping, (1s) (2s 2p) (3s 3p) (3d) (4s 4p) (4d) (4f) (5s 5p), etc.
- Electron in a certain ns, np-level screened only by electrons of the same energy level and by the electrons of lower energy levels. Electrons lying above the energy level do not screen any electron to any extent. Therefore, the higher energy electrons have no screening effect on the lower-energy electrons.
- Electrons of an (ns np) level shield the valence electron in the same group by 0.35 each. This rule also true for the electrons of the nd or nf level for electrons in the same group.
- Electrons belonging to one lower quantum shell or (n – 1) shell shield the valence electron by 0.85 each.
- Electrons belonging to (n-2) or still lower quantum energy level shield the valence electron by 1.0 each.
Shielding Constant for Sodium Atom
For calculating the value shielding constant of inner electrons of the sodium atom, the electron configuration according to Slater’s rule, (1s)2 (2s, 2p)8 (3s)1. Therefore, by using Slater’s rule shielding constant and effective nuclear charge for 3s-electron of sodium,
σ = (2 × 1) + (8 × 0.85) + (0 × 0.35)
∴ Zeff = (11 – 8.8) = 2.2
Question: Calculate the shielding or screening constant for 2p-electron of carbon and oxygen atom.
Answer: Electron configuration of carbon and oxygen according to the Slater’s rule, (1s)2 (2s, 2p)4, and (1s)2 (2s, 2p)6 respectively. Therefore, the shielding constant for carbon atom = (2 × 0.85) + (4 × 0.35) = 3.10 and oxygen atom = (2 × 0.85) + (6 × 0.35) = 3.80.
Shielding Constant for d or f-orbital Electron
The rules for s or p-electron are quite well for estimating the screening constant of s and p-orbital. But for d or f-orbital the four and five rule replaced by new rules for the estimation of screening or shielding effect and effective nuclear charge. The new rule is all electrons below the nd subshell or nf-subshell contribute 1.0 each towards the screening constant.
Screening Constant and Zeff for Vanadium
- Vanadium has atomic number 23 and the electron configuration according to the Slater’s rules, (1s)2 (2s 2p)8 (3s 3p)8 (3d)3 (4s)2. Therfore, the shielding constant for 4s electron = (2 × 1.0) + (8 × 1.0) + (8 × 0.85) + (3 × 0.85) + (1 × 0.35) = 19.7. Hence the effective nucler charge (Zeff) for 4s-electron =23 – 19.7 = 3.3.
- Shielding constant for 3d electron = (2 × 1.0) + (8 × 1.0) + (8 × 1.0) + (2 × 0.35) = 18.70. Hence, the effective nuclear charge (Zeff) for 3d electron = (23 – 18.70) = 4.30.
Screening Constant and Zeff for Chromium
- Chromium has atomic number 24 and the electron configuration according to slater’s rule, (1s)2 (2s 2p)8 (3s 3p)8 (3d)5 (4s)1. Therefore, the screening constant for 4s = (2 × 1.0) + (8 × 1.0) + (8 × 0.85) + (5 × 0.85) + (0 × 0.35) = 21.05. Hence Zeff for 4s-electron = (24 – 21.05) = 2.95.
- But the screening constant for 3d-electron = (2×1.0)+(8×1.0)+(8×1.0)+(3×0.85)+(4×0.35) = 19.40
and Zeff = (24 – 19.40) = 4.60
Ionization Energy and Shielding Effect
Larger the number of electrons in inner-shell, lesser the attractive force holding the valence electron to the nucleus, and lower will be the value of ionization energy. Therefore, when we move down in the group of the periodic table, the number of shielding constant increases and effective nuclear charge decreases, and hence the ionization trends also decrease. Therefore, the ionization trends for the group-II elements Be > Mg > Ca > Sr > Ba. Hence the formation of ionic bonding trends also increases from Be to Ba.