Chromium (Cr) - Atomic Structure & Chemical Properties

Atomic Structure

Chromium (Cr) is a transition metal with atomic number 24.

• Atomic Number (Z): 24 (Indicates 24 protons in the nucleus).
• Protons: 24
• Electrons: 24 (in a neutral atom).
• Neutrons: For its most common isotope, Chromium-52 ($^{52}\text{Cr}$), the number of neutrons is 52 - 24 = 28. Other isotopes exist with varying neutron numbers.
• Atomic Mass: Approximately 51.996 u.
• Atomic Radius (Metallic): 128 pm.
• Atomic Radius (Covalent): 127 pm.

Electronic Configuration & Shell Diagram Representation

Chromium exhibits an anomalous electronic configuration due to the enhanced stability of half-filled d-orbitals.

• Ground State Electronic Configuration: $[Ar]3d^54s^1$
  • The expected configuration would be $[Ar]3d^44s^2$. However, to achieve a more stable half-filled $3d$ subshell and a half-filled $4s$ subshell, one electron from the $4s$ orbital shifts to the $3d$ orbital.
• Shell Diagram Representation (Textual):
  • K-shell (n=1): 2 electrons ($1s^2$)
  • L-shell (n=2): 8 electrons ($2s^2~2p^6$)
  • M-shell (n=3): 13 electrons ($3s^23p^63d^5$)
  • N-shell (n=4): 1 electron ($4s^1$)

Periodic Trends

Chromium’s position in the periodic table (Group 6, Period 4) dictates its general periodic trends.

Ionization Enthalpy

The energy required to remove an electron from an isolated gaseous atom.

• First Ionization Enthalpy ($\Delta H_{i1}$): 652.9 kJ/mol
• Second Ionization Enthalpy ($\Delta H_{i2}$): 1590.6 kJ/mol
• Third Ionization Enthalpy ($\Delta H_{i3}$): 2987 kJ/mol
  • The successive ionization enthalpies increase as electrons are removed from increasingly positively charged ions, requiring more energy.

Electronegativity

A measure of an atom’s ability to attract electrons in a chemical bond.

• Pauling Scale: 1.66
  • Chromium is an electropositive metal, but its electronegativity is higher than that of alkali and alkaline earth metals, typical for transition metals.

Electron Gain Enthalpy

The energy change when an electron is added to a neutral gaseous atom to form a negative ion.

• Electron Gain Enthalpy ($\Delta H_{eg}$): -64 kJ/mol
  • The negative value indicates that energy is released when Chromium gains an electron, though it is not as strongly exothermic as non-metals.

Atomic Radius

The size of an atom.

• Metallic Radius: 128 pm
• Covalent Radius: 127 pm
  • Atomic radius generally decreases across a period (due to increasing effective nuclear charge) and increases down a group (due to increasing number of shells). Chromium follows this general trend within its period.

Key Physical Properties

Chromium is known for its hard, lustrous, and corrosion-resistant nature.

• Density: 7.19 g/cm$^3$ at 20 °C.
• State at STP (Standard Temperature and Pressure): Solid.
• Color: Silvery-grey with a high luster.
• Melting Point: 1907 °C (2180 K).
• Boiling Point: 2671 °C (2944 K).