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🧪 NEET Chemistry❓ Question

Why is Zn not a transition element though it sits in the d-block? NCERT definition and copper confuse me

Nisha Verma12th Sample

23 Sept 2026 · 1 min read

Boards revision for d and f block and I've hit a wall with one definition. NCERT says transition elements are those which have incomplete d subshell either in neutral atom or in their ions. Fine. Then it says Zn, Cd, Hg are not regarded as transition elements.

Zn is 3d104s23d^{10}4s^2, d is full, so excluded. OK. But Cu is 3d104s13d^{10}4s^1 in the neutral atom, also a full d subshell, and Cu is definitely treated as a transition metal. And Sc is 3d14s23d^1 4s^2 but Sc³⁺ has no d electrons. So how are these rules different?

Second thing: the exceptional configurations of Cr (3d54s13d^5 4s^1) and Cu (3d104s13d^{10} 4s^1). My teacher said "half-filled and fully-filled are stable" and that's it. I'd like an actual reason because I got a 1-mark "explain" question wrong in a test with that answer.

Third: when Fe becomes Fe²⁺, which electrons go first, 4s or 3d? I wrote 4s23d44s^2 3d^4 in a test and got no marks.

If someone has a neat summary table please share, my brain needs structure right now.

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7 answers & replies

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  • [+]Tanvi Deshmukh12thSample· 27d ago

    Summary table time!!

    ElementAtomCommon ionTransition?
    Sc3d14s23d^1 4s^2Sc3+Sc^{3+}: 3d03d^0Yes, atom has d electron
    Cu3d104s13d^{10} 4s^1Cu2+Cu^{2+}: 3d93d^9Yes, ion has incomplete d
    Zn3d104s23d^{10} 4s^2Zn2+Zn^{2+}: 3d103d^{10}No, full in atom and ion

    The rule is: incomplete d subshell in the atom OR in any common oxidation state. Sc qualifies through the atom, Cu through its ion.

  • [+]Rahul DubeyDropperSample· 26d ago

    Reason for Cr and Cu: 3d and 4s are very close in energy, so moving one electron from 4s to 3d is cheap. What you gain is extra stability of the half-filled (d5d^5) or fully-filled (d10d^{10}) subshell, from symmetrical distribution and more exchange energy (more parallel-spin electrons can swap positions). That is the "explain" answer worth a mark: close energies + exchange energy + symmetry.

    For ions, 4s electrons leave first. Fe is [Ar]3d64s2[Ar]3d^6 4s^2, so Fe2+Fe^{2+} is 3d63d^6 and Fe3+Fe^{3+} is 3d53d^5. Never write 4s² 3d⁴ for Fe²⁺. I lost marks for the same thing in my first attempt, I know that pain.

    • [+]Nisha Verma12thSample· 26d agoAuthor

      Fe²⁺ = 3d⁶, will not forget. Thank you, writing "close energy + exchange energy" on my formula sheet.

  • [+]Vivek Mishra12thSample· 26d ago

    Zn, Cd and Hg are in group 12. They have d10d^{10} in atom and in the +2 ion, so no incomplete d. Because of the filled d they are different from the usual transition metals: low melting points, and mercury is liquid at room temperature. In other transition metals, unpaired d electrons also take part in metallic bonding, which gives high melting points. In group 12 there are none, so metallic bonding is weaker.

  • [+]Dev Agarwal12thSample· 26d ago

    Cu has full d10 in the atom, same as Zn, so Cu should also be excluded by the same logic, no? I think they are just grouped separately for convenience.

    • [+]Nisha Verma12thSample· 26d agoAuthor

      Dev, see Siddharth's answer. It's the ion that saves Cu. Zn has no way out.

  • [+]Siddharth RaoDropperSample· 26d ago

    Dev, the catch is that the definition covers ions as well. Cu⁺ is 3d103d^{10}, but the stable common state in aqueous solution is Cu²⁺, which is 3d93d^9, with one unpaired electron. That is why copper salts are blue and why Cu counts. Zn only forms Zn²⁺ and that is 3d103d^{10}, always full, and its compounds are colourless. Same story for Cd and Hg. It is not a convenience thing, it follows from the definition.

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