Group VIII (old name) of the extended form of the periodic table consists of nine elements arranged in three triads, including the elements of groups 8, 9 and 10.
| Period | Group | ||
| 8 | 9 | 10 | |
| 4 | Fe | Co | Ni |
| 5 | Ru | Rh | Pd |
| 6 | Os | Ir | Pt |
Though the elements in vertical columns are treated as families, these nine elements are treated in a different manner. Fe, Co, and Ni have marked ‘horizontal’ similarities; therefore, these three are treated as a ‘family.’ The other six elements, properties somewhat different from the first horizontal triad, are studied together. Fe, Co and Ni are called ferrous metals. The members of the second and third triads are collectively known as platinum metals or noble metals.
(i) These three elements have nearly the same atomic masses and resemble closely in their properties. In addition, there is a gradation in their properties.
(ii) Electronic configurations: These atoms have incomplete 3d sub levels.
In each atom, the 4s sublevel is complete, and the d sublevel is incomplete; therefore, these are transition elements.
(iii) Valency: These elements show variable valency; +II (ous) and +III (ic) species are formed.
The stability of the M 2+ cations increases from Fe 2+ to Ni² + . The 3d 5 (half-filled) configuration for Fe³ + is responsible for it being more stable than Fe² + . Removing a 3d electron from the M2+ cations to oxidise them to the M³+ cations becomes more difficult from Fe² + to Ni 2+ . Thus, the stability of the M³ + cations is in the order Fe³ + > Co³ + > Ni³ + . Almost all the stable compounds of nickel are Ni(II) compounds.
Fe is widely present in nature. Its essential ores are hemetite (Fe2O3), magnetite (Fe3O4) and siderite (FeCO3); iron pyrite (FeS2) is another common mineral of iron. The core of the Earth is believed to consist mainly of iron. Cobalt also occurs in considerable amounts in nature. Smaltite (CoAs2) and cobaltite (CoAsS) are its chief ores. Nickel occurs mainly as sulphides and arsenides. Pentlandite [(NiFe)S] is a commercially important nickel ore.
The three metals are hard, greyish-white metals of high densities. They have high melting points. All three metals are ferromagnetic; they are strongly attracted by a magnet and remain magnetized even after removing the magnetic field. The ferromagnetic nature decreases from iron to nickel. Their ionisation energies (I₁) are almost the same; hence, they have the same tendency to form ions.
| Properties | Mn | Tc | Re |
|---|---|---|---|
| Atomic Number | 26 | 27 | 28 |
| Electronic Configuration | [Ar] 3d 6 4s 2 | [Ar] 4d 7 5s 2 | [ [Ar] 4f 14 5d 5 6s 2 |
| Metallic radius (nm) | 0.126 | 0.125 | 0.124 |
| M 2+ radius (nm) | 0.076 | 0.074 | 0.072 |
| M 3+ radius (nm) | 0.064 | 0.063 | - |
| I1 (kJ mol -1 ) | 762 | 758 | 736 |
| I2 (kJ mol -1 ) | 1561 | 1644 | 1752 |
| Melting point (K) | 1808 | 1760 | 1728 |
| Density | 7.9 | 8.9 | 8.9 |
All three metals form their dihalides. Fe forms FeF3, FeCl3, and FeBr3, but not FeI3. FeI3 is unstable and gets reduced to FeI2. Co forms only the trifluorides and not the other trihalides. Ni does not form any stable trihalides.
The oxides formed by iron are
FeO; iron(II) oxide; ferrous oxide
Fe2O3; iron(III) oxide; ferric oxide; hematite
Fe3O4; iron(II) iron(III) oxide; FeO.Fe2O3; ferrosoferric oxide; magnetite
All three are nonstoichiometric.
Cobalt forms only two oxides, CoO and Co3O4, and Nickel forms only one oxide, NiO. However, Ni forms NiO(OH) on oxidation of Ni 2+ with aqueous hypobromite.
The oxides of Fe, Co and Ni combine with oxides of a few other metals to form ferrites, for example, ZnFe2O4, CdFe2O4 and Fe₂NiO4. Ferrites are used in electromagnetic devices.
These three metals form a large variety of complexes. Several theories of coordination chemistry were developed based on studies of complexes of these metals. Some typical complexes of these metals are: