Chemistry · Structure Of Atom · NEET
The 4s and 3d subshells are very close in energy in chromium. If we filled normally we would get 3d4 4s2. But if one electron moves from 4s to 3d, we get 3d5 4s1 — now BOTH the 3d subshell (half-filled, 5 electrons) and the 4s subshell (half-filled, 1 electron) are half-filled. Half-filled subshells are extra stable, so this arrangement has lower energy. Nature always chooses the lowest-energy (most stable) ground state, so Cr = [Ar]3d5 4s1.
Same idea as chromium. Normal filling gives 3d9 4s2. But shifting one 4s electron into 3d gives 3d10 4s1 — a completely filled 3d subshell (10 electrons) plus a half-filled 4s. A fully-filled subshell is very stable, so this lower-energy arrangement is the real ground state. So Cu = [Ar]3d10 4s1.
NCERT gives two reasons. (1) Symmetry: when a subshell is exactly half-filled or fully-filled, the electrons are spread out evenly (symmetrically). Symmetry means the electrons shield each other less, so the nucleus pulls them more strongly — this lowers energy. (2) Exchange energy: electrons with the SAME spin in different orbitals of the same subshell can 'exchange' places. More possible exchanges means more stability. Half-filled and fully-filled subshells allow the maximum number of exchanges.
Inside one subshell, electrons that have the same spin (same arrow direction) and sit in different orbitals can swap positions with each other. Each possible swap releases a little energy and makes the atom more stable — this released energy is the exchange energy. A half-filled subshell (like 3d5, all 5 electrons same spin) gives the largest number of possible swaps, so it is very stable. You do NOT need to calculate it for NEET; just know 'more parallel same-spin electrons = more exchange energy = more stable'.
Cr and Cu are the two you must remember for NEET from the 3d series. The same half-filled stability also explains other exam favourites: nitrogen (2p3), phosphorus (3p3), and the extra ionisation-enthalpy stability of half-filled shells. In the f-block, europium (Eu = [Xe]4f7 6s2) and gadolinium (Gd = [Xe]4f7 5d1 6s2) keep a half-filled 4f7. So the SAME principle appears in many chapters.
Filling order (Aufbau) fills 4s before 3d because 4s is slightly lower in energy when empty. But once electrons are present, 4s and 3d energies become almost equal in Cr and Cu. So an electron shifts from 4s to 3d only when that shift creates a half-filled or fully-filled d subshell. This shift happens only for Cr and Cu in the 3d series — do not apply it to every element.
Which one is the wrong statement?
The electronic configurations of Eu (Z=63), Gd (Z=64) and Tb (Z=65), respectively, are:
Which among the following electronic configurations belong to main-group elements? A. [Ne]3s1 B. [Ar]3d3 4s2 C. [Kr]4d10 5s2 5p5 D. [Ar]3d10 4s1 E. [Rn]5f0 6d2 7s2
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Chromium (Z=24) = [Ar]3d5 4s1 and Copper (Z=29) = [Ar]3d10 4s1. Both differ from the plain Aufbau prediction because one 4s electron moves into 3d to make a half-filled or fully-filled d subshell.
Yes. NCERT gives two reasons: (1) symmetrical (even) distribution of electrons, which lowers shielding and increases nuclear attraction, and (2) larger exchange energy from more same-spin electrons that can swap places. Both make the atom's energy lower and so more stable.
Yes. Nitrogen is 2p3, a half-filled p subshell, which is why nitrogen has a higher first ionisation enthalpy than oxygen (2p4). The same half-filled stability idea appears across many NEET chapters.
No. You only need the concept: more parallel (same-spin) electrons in a subshell means more exchange energy and more stability. Half-filled (d5, f7, p3) and fully-filled (d10, f14, p6) subshells give the most stability.
Half-filled subshells like 3d5 have the maximum number of unpaired electrons, which gives the highest spin-only magnetic moment. The next topic, spin-only magnetic moment, uses the number of unpaired electrons you get from these configurations.
