Answer :
The effective nuclear charge seen by the 3s valence electron is 2.5.
The effective nuclear charge can be can be calculated by the following formula :
Zeff = Z - s
where,
Z = atomic number
s = screening constant
The electronic configuration of Na i given as :
₁₁Na = (1s²) (2s² 2p⁶) ( 3s¹ )
screening constant cab be calculated as :
s = ( 0.35 ( no. of electrons remaining in nth shell) + 0.85 ( electrons in (n-1) + 1 ( no. of e;ectron in ( n-2 ) )
s = ( 0.35 ( 0) + 0.85 ( 8) + 1 ( 2)
s = 6.8
Zeff = 11 - 6.8
Zeff = 2.5
Thus, The effective nuclear charge seen by the 3s valence electron is 2.5.
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