Out of 3d and 4s orbitals, which is filled first ?

Explanation

To determine which orbital, between 3d and 4s, is filled first, we can use the N + L rule. Here’s a step-by-step solution:

### Step 1: Understand the N + L Rule
The N + L rule states that the order of filling orbitals is determined by the sum of the principal quantum number (N) and the azimuthal quantum number (L). The orbital with the lower N + L value is filled first. If two orbitals have the same N + L value, the one with the lower N value is filled first.

### Step 2: Calculate N + L for 3d Orbital
For the 3d orbital:
- The principal quantum number (N) is 3.
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