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Interactive Periodic Table

All 118 elements with properties, electron configurations, and JEE/NEET relevance. Free, no login required.

Showing 118 of 118 local elements.

NCERT Class XII Chapter 4 - The d- and f-Block Elements

Interactive 3d Transition Series Oxidation State, Aqueous Ion Color & Magnetic Moment Board Inspector

Select any first-row 3d element (Sc Z=21 through Zn Z=30) to inspect neutral vs cation configurations, exchange-energy anomalies (Cr: [Ar] 3d5 4s1 and Cu: [Ar] 3d10 4s1), the 5-box 3d orbital diagram, spin-only magnetic moment mu = sqrt(n(n+2)) BM, and hydrated ion colors.

First-Row 3d Transition Series Selector (Z = 21 to Z = 30)

Selected Element

Cr - Chromium

Z = 24 | Group 6

Oxidation State Toggle (4s electrons removed first upon ionization)

Neutral Atom Config:[Ar] 3d5 4s1
Active Ion (Cr3+):[Ar] 3d3
Common Oxidation States:+2, +3, +6
Max Oxidation State:+6

Exchange-energy anomaly: Cr: [Ar] 3d5 4s1 (half-filled 3d5 subshell maximizes parallel-spin exchange stabilization over 3d4 4s2).

Exchange-Energy & Ionization Board Rule

Ground-state exceptions: Cr: [Ar] 3d5 4s1 (half-filled d5) and Cu: [Ar] 3d10 4s1 (fully-filled d10). Across all 3d metals Sc-Zn, 4s electrons removed first upon ionization before any 3d electron is lost.

Ion Species

Cr3+

3d Electron Count

3d3

Unpaired e- (n)

n = 3

Spin-Only Moment

3.87 BM

3d Orbital Box Diagram (5 Degenerate 3d Orbitals - Hund's Multiplicity Rule)

mu = sqrt(n(n+2)) BM = sqrt(3(3+2)) = 3.87 BM
↑3d_1
↑3d_2
↑3d_3
·3d_4
·3d_5

Aqueous Hydrated Ion Color Swatch & d-d Transition Verdict

Cr3+ 3d3 Violet

Paramagnetic (3.87 BM)

d-d transition active; stable t2g3 configuration in [Cr(H2O)6]3+ (n = 3, mu = 3.87 BM).

NCERT Table 4.8 Hydrated Ion Color Reference Strip (click to inspect):

NCERT Chapter 4 Board Reasoning Callout (Cr)

Cr2+ (3d4) is strongly reducing because oxidizing to Cr3+ (3d3) achieves a half-filled t2g3 subshell in octahedral aqueous coordination, whereas Mn3+ (3d4) is strongly oxidizing to reach half-filled Mn2+ (3d5).

  • Why Zn, Cd, Hg are not regarded as true transition elements: Zn (3d10 4s2), Cd (4d10 5s2), and Hg (5d10 6s2) have completely filled d10 orbitals in both their ground state and their common (+2) oxidation state.
  • Why Mn shows maximum oxidation state +7 in MnO4-: Manganese ([Ar] 3d5 4s2) uses all seven valence electrons (five 3d + two 4s) in covalent-ionic bonding with oxygen, stabilized by p-pi to d-pi multiple bonding in tetrahedral MnO4-.
  • Why Cu+ disproportionates in aqueous solution (2Cu+ -> Cu2+ + Cu): High negative hydration enthalpy of Cu2+(aq) outweighs the second ionization enthalpy of copper.
Practice Chapter 4 d-Block Board Questions