Iridium (Ir) - Atomic Structure & Chemical Properties

Atomic Model Details

Iridium (Ir) is a d-block transition metal.

Fundamental Particles

• Atomic Number (Z): 77
• Number of Protons: 77
• Number of Electrons: 77 (in a neutral atom)
• Average Number of Neutrons: Approximately 115 (derived from its average atomic mass of 192.217 u, where 192.217 - 77 ≈ 115.217). Common isotopes include Ir-191 (114 neutrons) and Ir-193 (116 neutrons).

Atomic Size

• Metallic Radius: Approximately 136 picometers (pm). This value reflects the radius of an iridium atom within a metallic crystal lattice.

Electronic Configuration & Shell Diagram Representation

Electronic Configuration

The ground state electronic configuration of Iridium is determined by filling electrons into atomic orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund’s rule.

[Xe] 4f¹⁴ 5d⁷ 6s²

• Expanded form: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d⁷ 6s²
• Valence Electrons: The outermost electrons, primarily 5d⁷ and 6s², are involved in chemical bonding.

Shell Diagram Representation

The distribution of electrons in different principal energy shells (n) for Iridium is as follows:

• n=1 (K-shell): 2 electrons (1s²)
• n=2 (L-shell): 8 electrons (2s² 2p⁶)
• n=3 (M-shell): 18 electrons (3s² 3p⁶ 3d¹⁰)
• n=4 (N-shell): 32 electrons (4s² 4p⁶ 4d¹⁰ 4f¹⁴)
• n=5 (O-shell): 15 electrons (5s² 5p⁶ 5d⁷)
• n=6 (P-shell): 2 electrons (6s²)

This distribution totals 77 electrons, matching the atomic number of Iridium.

Periodic Trends

Ionization Enthalpy

• First Ionization Enthalpy (ΔiH₁): Approximately 870 kJ/mol.
  • Iridium exhibits a relatively high first ionization enthalpy for a metal due to its strong nuclear charge, compact atomic size, and the stability imparted by the filled 4f subshell and partially filled 5d subshell, requiring significant energy to remove the outermost electron.

Electronegativity

• Pauling Scale Electronegativity: Approximately 2.20.
  • Iridium has a comparatively high electronegativity for a metal, indicative of its ability to attract electrons in a chemical bond, although less than non-metals. This is characteristic of the heavier transition metals.

Electron Gain Enthalpy

• Electron Gain Enthalpy (ΔegH): Values for transition metals are generally low, often positive (endothermic), or close to zero, meaning they have a weak tendency to accept additional electrons. Specific reported values can vary, but generally, metals do not readily gain electrons to form stable anions.

Atomic Radius

• Atomic Radius: As stated above, the metallic radius is approximately 136 pm.
  • Moving across a period in the d-block, atomic radius generally decreases slightly due to increasing effective nuclear charge. Down a group, atomic radius generally increases. However, the lanthanide contraction preceding the 5d series elements (like Iridium) causes the 5d elements to have atomic radii very similar to their 4d counterparts.

Key Physical Properties

• Density: 22.56 g/cm³ (at 20 °C). Iridium is one of the densest known elements, surpassed only by osmium.
• State at Standard Temperature and Pressure (STP): Solid.
• Color: Silvery-white. It possesses a lustrous appearance.
• Melting Point: Approximately 2446 °C (2719 K). Iridium has a very high melting point, indicative of strong metallic bonding.
• Boiling Point: Approximately 4428 °C (4701 K). Its extremely high boiling point is consistent with its strong metallic bonds and high stability.