Iron triad physical properties

Table 1. Some properties of iron triad elements

	Fe	Co	Ni
density, g/сm3	7.87	8.90	8.91
tm, oС	1539	1492	1455
Rа, nm	0.125	0.125	0.124
Ionisation energy Е ® Е+, kJ∙mol-1	762.5	760.4	737.1
Е+ ® Е2+, kJ∙mol-1	1561.9	1648	1753.0
Е2+ ® Е3+, kJ∙mol-1	2957	3232	3395
Еo(Е2+ + 2е = Е), V	-0.44	-0.29	-0.25

Iron is a fairly soft metal, existing in different forms according to temperature (Fig. 1):

-Fe -Fe -Fe -Fe.

Figure 1. Phase diagram of pure iron

Alpha iron, also known as ferrite, is the most stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).

Above 912 °C and up to 1400 °C α-iron undergoes a phase transition from bcc to the fcc configuration of γ-iron, also called austenite (Fig. 2). This one is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.

Figure 2. The body centered cubic unit cell (bcc or ccc) of -Fe and face-centered cubic unit cell (fcc) of γ-Fe

Magnetic properties of iron¹. Below t = 770С it’s ferromagnetic substance (this temperature of ferro(ferri-)magnetic-paramagnetic transition is called the Curie point), in the range 770-910 ^оС it’s paramagnetic substance (-Fe). At 910-1401С it’s a paramagnetic (-Fe) with a face-centred cubic lattice. At 1401-1539С (b.p.) it’s -Fe with body-centred cubic lattice.

Table 2. Structure and magnetic properties of iron allotropes

-Fe*	-Fe	-Fe	-Fe
“ferrite” ferromagnetic body-centred cubic lattice	non-magnetic no change of structure	face-centred cubic	body-centred cubic

Some R_a decrease in the series Fe-Co-Ni is the result of d-contraction, i.e. the growth of attraction energy of ns-electrons to the nucleus, whose charge is increased. This phenomenon also leads to the decrease of chemical activity of metals. Boiling point depression in the series is explained by the decrease of the number of electrons that participate in metallic bonding. Fe has lower density compared to Co and Ni because Fe forms body-centred cubic lattice at STP but Co and Ni have a close-packed hexagonal lattice.

Cobalt is a bluish silvery metal that exhibits ferromagnetism, and forms allotropes like iron.

Iron triad trends

• The elements of iron triad are found in nature mostly as sulfides form strong complexes.

• All elements also.

These elements belong to the d block. They have electron configurations:4s²3d^x with x=6,7, and 8 for Fe, Co and Ni, respectively. Therefore, oxidation state +2 is the most common.

In the chemistry of the triad, we observe the continuing tendency for the higher oxidation states to decrease in stability along the first transition series; unlike cobalt and iron, the +3 state of nickel is rare and relatively unimportant and the +2 state is the only important one.

Iron is characterized by two series of compounds: iron(II) and iron(III). The former corresponds to iron(II) oxide FeO, and the latter to iron(III) oxide Fe₂O₃. In addition, salts of ferric acid H₃FeO₄ are known in which the oxidation state of iron is +6.

The simple compounds of cobalt(III) are strongly oxidizing whereas crystalline field of ligands in complexes of Co(III) makes them relatively stable. Cobalt (II) compounds are the most stable.