Group 9: Cobalt Family

Group 9 is one of the most important, as it includes Cobalt (Co), Rhodium (Rh), Iridium (Ir) and Meitnerium (Mt). The group is also home to a handful of transition metals. Moreover, it’s just one among the least common elements, accounting for 0.0029 % points of such crust’s total mass. Another important group member is iridium, it has only recently been found to be a biologically important element. Rhodium is a typical silvery transition metal. It is also one of the least known members of the group. It has no known biological role, but it does have a notable chemical property. It deviates from the usual electron configuration in some ways. It is also a popular pesticide, but it is banned in the U.S. since 1975. Group 9 elements also exhibit a wide range of oxidation states (Gang, et al., 2002).


1. Electronic Configuration

Groups nine of the modern Periodic Table contains transition metal components with comparable electronic structures. As a rule, these contain d-block features. General electronic configuration of group 9 is (n-1) ns2 nd7, while the electronic configuration of group 9 members is as follows (Gross & Marques, 2016).


Co [27] 1s2 2s2 2p6 3s2 3p6 4s2 3d7 or [Ar] 4s2 3d7
Rh [45] 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1.4d8 or [Kr] 5s1 4d8
Ir [77] 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d7 or [Xe] 6s2 4f14 5d7
Mt [109]1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d7 or [Rn] 7s2 5f14 6d7


2. Trend in the physical properties

They are hard, ductile, and malleable. These elements are also good conductors of electricity and heat. These elements show trend in their physical properties (Ono, 2020).


2.1. Atomic and ionic radii

Ionic radius of group 9 elements increases from Co3+ to Ir3+ While atomic radius show irregular behavior as atomic radius of rhodium is smaller than Iridium. Similarly, radius of Meitrenium is unknown (Gang, et al., 2002).

Atomic radius group 5 periodic table

Figure 1: Atomic radius of Group 9 elements

Ionic radius group 5 periodic table

Figure 2: Ionic radius of Group 9 elements


2.2. Ionization energy

Following value of ionization energy for group 9 elements indicate that there is irregular change in this regard (Gross & Marques, 2016).

First ionization energy group 5 periodic table

Figure 3: First Ionization Energy of the Group 9 elements


2.3. Melting and boiling point

Melting and boiling point of group 9 elements increases gradually from cobalt to Iridium while melting and boiling point of Mt is unknown (Ono, 2020).

melting and boiling points group 5 periodic table

Figure 4: Melting and Boiling Points of the Group 9 elements


3. Coordination chemistry of group 9

Group 9 elements share the same valence electrons as their predecessors, they do not share the same chemical properties. They are also important in a number of enzymatic processes, and can be the launching pad for some of the most exciting chemical reactions on earth. For example, Vitamin B12 (sometimes called cobalamin, because it contains Cobalt) is a water-soluble vitamin essential for the normal functioning of the brain and nervous system, and for the formation of blood and various proteins. Furthermore, the pharmaceuticals, foods, including environmental purifying sectors may all benefit greatly from the use of such molecules. Interestingly, they also come in handy for other tasks such as oxygen transport in the blood. In fact, coordination chemistry has its own category in the chemical sciences (Gross & Marques, 2016).


4. Uses of group 9 elements

Cobalt is used in aerospace, machinery manufacturing, electrical and electronic, chemical, ceramic and other industrial areas, due to its resistance to high temperature, magnetic capabilities and high resistance to corrosion.
The physical aspect of Rhodium (shiny and white silver) and robustness make it an outstanding tool for plating metal that soft such us Copper or Gold. Also, Rhodium is used for dental mirrors because it does not alter images and it can be autoclaved to sanitize dental tools.
Iridium is mainly used to harness platinum alloys. Iridium is also used to make equipments that are used for heat at high temperatures.
The international standard kilogram was made from Iridium 10% alloy together with 90% platinum.

You want to know about all the Groups of the Periodic Table?


Several elements were known as early as the 17th century, but it wasn’t until the 20th century that the periodic table was discovered. It helps scientists learn about the properties of the individual elements. Usually classified as metals, the elements of Group 9 of the periodic table have similar properties. They are usually used as conductors of heat and electricity. In addition, they are usually nonreactive. Some are also classified as noble gases, which are odorless and colorless



1. Glawe, H., Sanna, A., Gross, E. K. U., & Marques, M. A. (2016). The optimal one dimensional periodic table: a modified Pettifor chemical scale from data mining. New Journal of Physics, 18(9), 093011.
2. Ono, S. (2020). Dynamical stability of two-dimensional metals in the periodic table. Physical Review B, 102(16), 165424.
3. Raiola, F., Migliardi, P., Gang, L., Bonomo, C., Gyürky, G., Bonetti, R., … & Winter, J. (2002). Electron screening in d (d, p) t for deuterated metals and the periodic table. Physics Letters B, 547(3-4), 193-199.