Question

what is the effective nuclear charge, $z_{eff}$, for the electrons in energy level 4?
2
8
20
10

Explanation:

Step1: Identify nuclear charge

The nucleus has 20 protons, so nuclear charge $Z = 20$.

Step2: Calculate shielding electrons

Electrons in energy levels 1,2,3 shield level 4. Count these: $2+8+8=18$.

Step3: Compute effective nuclear charge

Use $Z_{eff}=Z - \text{shielding electrons}$
$Z_{eff}=20 - 18$

Answer:

2