Question

What is Zeff? How to Calculate Zeff of Sodium and oxygen ?

Answer 1

Normally Zeff.( effective nuclear charge ) is the ratio of number of protons to number of electrons .
e.g $Zeff. = \frac {number of protons }{number of electrons }$
But it is also depends upon screening effect (σ)
Zeff = Z - σ
Where Z is the atomic number of the element.
σ = { 0.35 × number of electrons of nth orbit + 0.85× number of electrons in (n-1)th orbit + 1 × rest of electrons)

Now, for sodium : 1s², 2s², 2p⁶, 3s¹
σ =0.35 ×0 + 0.85 × 8 + 1 × 2 = 0 + 6.8 + 2 = 8.8
So, Zeff = 11 - 8.8 = 2.2

Similarly for oxygen : 1s² , 2s² , 2p⁴
σ = 0.35 ×5 + 0.85 × 2 + 1 × 0 = 1.75+ 1.7 = 3.45
Zeff = 8 - 3.45 = 4.55