Osmium (Os) Revision Guide

Osmium (Os)

Osmium (Os) is a chemical element with atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group.

Osmium’s atomic structure is characterized by the following:

Atomic Number (Z): 76
- Indicates 76 protons in the nucleus.
- In a neutral Osmium atom, there are 76 electrons.
Atomic Mass: 190.23 u (average atomic mass)
Neutrons: The most common isotope, Os-192, has 116 neutrons (192 - 76 = 116).
Atomic Radius (Covalent): Approximately 128 picometers (pm). This value reflects the size of the atom in a covalent bond.

The electronic configuration describes the distribution of electrons in the atomic orbitals.

Full Electronic Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d⁶ 6s²
Condensed Electronic Configuration: [Xe] 4f¹⁴ 5d⁶ 6s²
- [Xe] represents the electronic configuration of Xenon, the noble gas preceding Osmium.
Shell-wise Electron Distribution:
- K-shell (n=1): 2 electrons
- L-shell (n=2): 8 electrons
- M-shell (n=3): 18 electrons
- N-shell (n=4): 32 electrons
- O-shell (n=5): 14 electrons (5s² 5p⁶ 5d⁶)
- P-shell (n=6): 2 electrons (6s²)
This distribution follows the order of increasing energy levels and Hund’s rule, filling orbitals within each subshell. The presence of partially filled d-orbitals (5d⁶) is characteristic of transition metals and contributes to Osmium’s unique properties.

Osmium’s position in Group 8, Period 6 of the periodic table influences its periodic trends:

Ionization Enthalpy (First Ionization Energy): Approximately 840 kJ/mol.
- This is the energy required to remove the outermost electron from a gaseous Osmium atom. Being a metal, its ionization enthalpy is relatively lower than non-metals but higher than alkali metals.
Electronegativity (Pauling Scale): Approximately 2.2.
- Osmium has a relatively high electronegativity for a metal, comparable to elements like platinum and gold. This indicates a strong attraction for electrons in a chemical bond.
Electron Gain Enthalpy: Approximately -100 kJ/mol (Electron Affinity).
- This value indicates that Osmium atoms can accept an electron to form an anion, releasing a small amount of energy. However, for most metals, this process is not highly favorable compared to non-metals.
Atomic Radius (Covalent): 128 pm.
- As a d-block element, Osmium’s atomic radius is influenced by the filling of d-orbitals and the lanthanide contraction, which results in elements after the lanthanides having similar or slightly smaller atomic radii than expected.

Osmium possesses distinct physical properties:

Density: 22.59 g/cm³ at 20 °C.
- Osmium is the densest naturally occurring element.
State at STP (Standard Temperature and Pressure): Solid.
Color: Bluish-white.
Melting Point: 3033 °C (3306 K).
- Osmium has one of the highest melting points among elements.
Boiling Point: 5012 °C (5285 K).
- Its boiling point is also exceptionally high.