Introduction to Iridium
Iridium (Ir) is a chemical element distinguished by its high density and corrosion resistance. It is classified as a transition metal and belongs to Group 9, Period 6 of the periodic table. Its elemental symbol, Ir, is derived from the Greek goddess Iris, due to the varying colors of its compounds. Iridium is one of the rarest elements in Earth’s crust, found in very low concentrations, often alongside platinum-group metals.
Basic Atomic Composition
The atomic number of Iridium is 77. This number defines the element and indicates the following for a neutral atom:
- Number of Protons: Iridium atoms possess 77 protons within their nucleus. The number of protons determines the element’s identity.
- Number of Electrons: In a neutral Iridium atom, the number of electrons orbiting the nucleus is equal to the number of protons, totaling 77 electrons.
- Number of Neutrons: The number of neutrons can vary among isotopes of Iridium. For the most abundant stable isotope, Iridium-193 (Ir-193), the mass number is 193. The number of neutrons is calculated by subtracting the atomic number from the mass number: 193 - 77 = 116 neutrons. Other isotopes exist with different neutron counts.
Electron Configuration
Electron configuration describes the distribution of electrons of an atom in atomic orbitals. For Iridium, with 77 electrons, the full electron configuration is:
$1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^7$
This configuration can be condensed using the noble gas notation, representing the electrons up to Xenon (Xe), which has 54 electrons. The condensed electron configuration for Iridium is:
$[Xe] 4f^{14} 5d^7 6s^2$
This notation indicates that the electrons are arranged in the same way as Xenon, followed by 14 electrons in the 4f subshell, 7 electrons in the 5d subshell, and 2 electrons in the 6s subshell.
Valence Electrons
Valence electrons are the electrons located in the outermost shell of an atom, which are primarily involved in chemical bonding. For transition metals like Iridium, the valence electrons include those in the highest principal energy level’s s orbital and often the electrons in the partially filled d subshell of the preceding energy level.
For Iridium, the outermost principal energy level is the 6th shell, which contains 2 electrons in the 6s orbital. The 5d subshell is partially filled with 7 electrons. Therefore, the valence electrons for Iridium typically include the 2 electrons from the $6s^2$ subshell and the 7 electrons from the $5d^7$ subshell, allowing for various oxidation states and bonding behaviors.
Common Applications and Occurrence
Iridium’s extreme hardness, brittleness, and high melting point make it challenging to work with, but these properties also make it valuable for specialized applications. It is utilized in spark plugs for aircraft, high-temperature crucibles for crystal growth, electrical contacts, and as an alloying agent to harden platinum. Its resistance to corrosion makes it ideal for surgical tools and pen tips.
Globally, Iridium is most commonly obtained as a byproduct of nickel and copper mining, with major sources located in South Africa, Russia, and Canada. Iridium is also found in meteorites, and its elevated concentration in the Earth’s crust at the Cretaceous-Paleogene (K-Pg) boundary layer provides geological evidence supporting the theory of an asteroid impact contributing to the extinction of dinosaurs.