Explain Why 4s Fills Before 3d in Simple Terms

1. The (n+l) Rule (Madelung Rule)

This is the best "simple rule" for determining filling order.

• n is the principal quantum number (the shell number).
• l is the azimuthal quantum number (the subshell shape: s=0, p=1, d=2, f=3).

Rule 1: The orbital with the lowern+l value has lower energy.

Rule 2: If n+l values are equal, the orbital with the lowern value has lower energy.

Let's apply this:

• For the 4s orbital:n=4, l=0. n+l = 4 + 0 = 4
• For the 3d orbital:n=3, l=2. n+l = 3 + 2 = 5

Since 4 is less than 5, the 4s orbital is lower in energy and fills first. This is why Potassium (Z=19) is [Ar] 4s¹ and Calcium (Z=20) is [Ar] 4s².

2. The "Penetration" Effect

In simple terms, while the 4s orbital's average position is further from the nucleus than the 3d, the s-orbital's shape is spherical. This allows a small part of the 4s orbital to "penetrate" or get very close to the nucleus, passing through the 3rd shell. This close approach to the positive nucleus strongly stabilizes the 4s orbital, lowering its energy just below the 3d orbital.

This energy order is only true for filling the orbitals. Once electrons are in the 3d orbital, the 3d orbitals "drop" in energy and become lower than the 4s.

• Filling Order (Aufbau):1s, 2s, 2p, 3s, 3p, 4s, 3d, ...
• Ionization (Losing) Order: When a transition metal forms an ion, it loses electrons from the highest energy level first, which is the 4s orbital.
  • Example: Iron (Fe): [Ar] 4s² 3d⁶
  • Example: Iron (II) ion (Fe²⁺): [Ar] 3d⁶ (it loses the two 4s electrons)