Question

In the Si4- anion, what is the effective charge felt by a valence electron?

Answer 1

Effective charge is calculated using :

Z_eff = Z – S

Where Z_eff = effective charge, Z = atomic number, S = screening constant

For Si, atomic number = 14.

To find screening constant :

• Write the electronic configuration. Si has atomic number = 14. Si^4- has 4 extra electrons = 14 + 4 = 18 electrons. Electronic configuration : 1s2 2s2 2p6 3s2 3p6 .

Here, electron under consideration – valence electron. So, an electron in 3p is considered. The shielding is due to the remaining electrons. Hence, electrons which shield are: 1s2 2s2 2p6 3s2 3p5

• Add the value corresponding to shielding from each electron on the valence electron :
  • Electrons in same n level contribute 0.35 ( unless this level is 1s. Then 0.30 is contributed)
  • If s or p is the valence electron ( which is the case here):

Electron in ( n-1) level contributes 0.85.

Electrons in ( n-2), ( n-3) and so on contribute 1.00

• If d or f electron is considered ( not the case here)

Each electron in (n-1) , ( n-2) …contribute 1.00

So, the electron in the question is in 3p orbital. The electrons shielding it are: 1s2 2s2 2p6 3s2 3p5

Number of electrons in same n level ( n = 3) = 2 + 5 = 7. Contribution = 7(0.35) =2.45

Number of electrons in ( n-1) level ( n = 2) = 2s + 6 p = 8 . Contribution = 8(0.85) =6.8

Electrons further away = 2 electrons in 1s. Contribution = 2(1.00) = 2.

So , S = 2.45 + 6.8 + 2 = 11.25

Hence, Z_eff = Z – S = 14 – 11.25 = 2.75