Periodic Table
24
Cr
51.996
Chromium
Transition MetalConfig exception (3d⁵4s¹); K₂Cr₂O₇ orange oxidiser; +6 strongly oxidising.
Electronic Configuration
[Ar] 3d⁵ 4s¹or1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
Electron shells
2e⁻n=1
8e⁻n=2
13e⁻n=3
1e⁻n=4
Properties
Atomic number
24
Atomic mass
51.996 u
Category
Transition Metal
Block / Group / Period
d · 6 · 4
Valence electrons
1
Oxidation states
+2, +3, +6
Electronegativity
1.66
Atomic radius
166 pm
Ionization energy
653 kJ/mol
Electron affinity
65 kJ/mol
Melting point
2180 K / 1907 °C
Boiling point
2944 K / 2671 °C
Density
7.15 g/cm³
State at 25 °C
solid
JEE / NEET notes
- Electronic config exception: [Ar]3d⁵4s¹ (half-filled stability).
- K₂Cr₂O₇ — orange, oxidiser; turns green (Cr³⁺) on reduction.
- Chromyl chloride test: confirms chloride (red CrO₂Cl₂ vapours).
- +6 (CrO₄²⁻ yellow ⇌ Cr₂O₇²⁻ orange) interconvert with pH.
Exceptions
- • 3d⁵4s¹ configuration (not 3d⁴4s²).
Ores & extraction
Ores: Chromite (FeO·Cr₂O₃)
Uses
Stainless steel · Chrome plating · Pigments · Leather tanning
Important compounds
K₂Cr₂O₇CrO₃Cr₂O₃CrO₂Cl₂
Personal Notes
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