Electronic Configuration of Transition (d-Block) Elements
Chemistry · D And F Block Elements · NEET
The outer electronic configuration of a transition (d-block) element is (n-1)d^1-10 ns^1-2. This means the inner d orbital of the second-last shell holds 1 to 10 electrons, and the outermost s orbital holds 1 or 2 electrons. For NEET, memory hook: "d fills LATE but LEAVES FIRST" — you add electrons to 4s before 3d, but when you make an ion you always remove the 4s electrons first.
The 4s orbital fills before 3d (it is lower in energy while empty), but when you form an ion the 4s electrons are removed first because 3d drops lower once filled. So Fe is [Ar]3d6 4s2 while Fe2+ is [Ar]3d6.
Your doubts, answered
What is the general electronic configuration of transition (d-block) elements?
It is (n-1)d^1-10 ns^1-2. Here n is the outermost shell number. (n-1)d means the d subshell of the shell just below the outer shell; it can hold 1 to 10 electrons. ns means the outermost s subshell; it holds 1 or 2 electrons. Example: Fe (Z=26) is [Ar] 3d^6 4s^2. NCERT states this general form directly, with one special case: Pd is 4d^10 5s^0.
Why does the 4s orbital fill BEFORE the 3d orbital?
When 3d and 4s are being filled (empty atoms), the 4s orbital has slightly lower energy, so by the Aufbau rule electrons enter 4s first. That is why Sc is [Ar] 3d^1 4s^2 and not [Ar] 3d^3. This lower energy of 4s only applies while the orbitals are empty or filling. Once electrons are in, the 3d level actually drops below 4s — which is why ions behave differently (next doubt).
When making a transition metal ION, which electrons are removed first, 4s or 3d?
Always remove the 4s electrons FIRST, then 3d. This is the point most students get wrong. Once the atom is formed, 3d sits lower in energy than 4s, so the higher-energy 4s electrons leave first. Fe is [Ar] 3d^6 4s^2. Fe^2+ = remove two 4s electrons = [Ar] 3d^6 (NOT 3d^4 4s^2). Fe^3+ = remove one more (a 3d electron) = [Ar] 3d^5. Memory hook: fill 4s first, empty 4s first.
How do I write the configuration of Fe^2+ and Fe^3+ for NEET?
Start from Fe (Z=26): [Ar] 3d^6 4s^2. Fe^2+ : take away both 4s electrons -> [Ar] 3d^6 (4 unpaired electrons). Fe^3+ : take away both 4s AND one 3d electron -> [Ar] 3d^5 (5 unpaired electrons, extra-stable half-filled). Fe^3+ is more stable than Fe^2+ because 3d^5 is a stable half-filled set. This 3d^5 stability is a very common NEET answer.
Why are Cr and Cu exceptions to the general configuration?
Expected Cr = [Ar] 3d^4 4s^2, but actual Cr = [Ar] 3d^5 4s^1. Expected Cu = [Ar] 3d^9 4s^2, but actual Cu = [Ar] 3d^10 4s^1. Reason: exactly half-filled (d^5) and completely filled (d^10) d subshells are extra stable because of symmetry and exchange energy, and the 3d–4s energy gap is tiny, so one 4s electron shifts into 3d. This is such a big NEET topic that it has its own page (linked below).
Does 'transition element' include Zn, Cd, Hg by this configuration rule?
No. A true transition element must have a partly filled d subshell in its atom OR in a common ion. Zn, Cd and Hg have completely filled d^10 in both the atom and their common +2 ions (e.g. Zn is 3d^10 4s^2, Zn^2+ is 3d^10). Since d is never partly filled, they are studied with the d-block but are not counted as transition elements.
⚠️ The NEET trap ✗ Fe^2+ = [Ar] 3d^4 4s^2 (removing two 3d electrons because 3d filled last). ✓ Fe^2+ = [Ar] 3d^6 (remove the two 4s electrons first). Fe^3+ = [Ar] 3d^5. 🧠 NEET repeatedly rewards the fact that 3d^5 (Mn^2+, Fe^3+) is an extra-stable half-filled shell. Remove 4s before 3d, then check for a half-filled/full-filled ion — that decides oxidation-state stability and magnetic-moment questions.
Real NEET questions
NEET 2024
The E° value for the Mn3+/Mn2+ couple is more positive than that of Cr3+/Cr2+ or Fe3+/Fe2+ due to the change of:
A · d5 to d2 configuration
B · d4 to d5 configuration ✓
C · d3 to d5 configuration
D · d5 to d4 configuration
Solution: The reduction is Mn3+ + e- -> Mn2+. Write the configurations: Mn3+ is 3d^4 and Mn2+ is 3d^5. So the change is d^4 -> d^5. The product Mn2+ reaches the extra-stable half-filled 3d^5 shell, so this reduction is strongly favoured, making E°(Mn3+/Mn2+) highly positive. Correct answer: (b) d4 to d5 configuration.
NEET 2020 / NEET 2026
The calculated 'spin-only' magnetic moment of Ti2+ (3d^2) is:
A · 5.92 BM
B · 3.87 BM
C · 2.84 BM ✓
D · 4.90 BM
Solution: First get the configuration: Ti is [Ar] 3d^2 4s^2, so Ti2+ = remove both 4s electrons = [Ar] 3d^2, giving n = 2 unpaired electrons. Spin-only formula: mu = sqrt(n(n+2)) = sqrt(2 x 4) = sqrt(8) = 2.84 BM. Correct answer: (c). Note how writing the ion configuration correctly (removing 4s first) is the key first step.
NEET 2025
Statement II: The number of unpaired electrons in a Cr2+ ion (Z=24) is the same as that of a Nd3+ ion (Z=60). Is Statement II true or false?
A · Cr2+ has 4 unpaired, Nd3+ has 3 unpaired — different, so FALSE ✓
B · Both have 4 unpaired — TRUE
C · Both have 3 unpaired — TRUE
D · Cr2+ has 6 unpaired, Nd3+ has 6 unpaired — TRUE
Solution: Cr (Z=24) is the exception [Ar] 3d^5 4s^1; Cr2+ = remove 4s and one 3d? No — remove the single 4s electron and one 3d electron to lose 2 electrons: Cr2+ = [Ar] 3d^4 = 4 unpaired electrons. Nd3+ = [Xe] 4f^3 = 3 unpaired electrons. 4 is not equal to 3, so Statement II is FALSE. This is why the official key marked 'Statement I true but Statement II false'.
Solved D And F Block Elements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
n is the number of the outermost shell. (n-1) is the shell one below it. So for the 3d series the outer shell is 4 (n=4), and (n-1)d = 3d. The general outer configuration is (n-1)d^1-10 ns^1-2.
Is Sc a transition element by its configuration?
Yes. Sc is [Ar] 3d^1 4s^2 — it has a partly filled 3d subshell, so it is a transition element. (Sc3+ becomes 3d^0, but the neutral atom having partly filled d is enough.)
Why does Fe3+ have 5 unpaired electrons?
Fe3+ = [Ar] 3d^5. Five 3d orbitals each hold one electron (Hund's rule), so all 5 are unpaired. This half-filled 3d^5 is stable, which is why Fe3+ is common and why its magnetic moment is high.
Which single element is the only exception in the NCERT general rule?
NCERT specifically notes Pd, which is 4d^10 5s^0 (empty outer s). Cr and Cu are the famous 3d-series exceptions covered on the next linked page.
How many electrons can the d subshell hold in total?
Ten. There are five d orbitals, and each orbital holds 2 electrons, giving a maximum of 10 — that is the '10' in d^1-10.
