Transition element

(Under construction)

A transition element is a chemical element element whose atomic electron configuration of the ground (lowest energy) state has an incompletely filled d sub-shell. Here d stands for an atomic orbital with angular momentum quantum number ℓ = 2. The electron configuration of free transition element atoms contains (nd)^k, with 1 ≤ k ≤ 9, and where n is a principal quantum number, n = 3, 4, 5. The incomplete electronic d subshell gives rise to some characteristic magnetic properties and brightly colored solutions of transition metal complexes.

Although the atoms copper (Cu), silver (Ag), and gold (Au) have in their lowest energy state a filled d sub-shell, after ionization (loss of one or more electrons) they obtain an incomplete d sub-shell; hence, they are usually seen as transition elements. As seen in the table of electron configurations below, Cu, Ag, and Au have the outer configuration: (nd)¹⁰(n+1)s¹, with n = 3, 4, and 5 for copper, silver, and gold, respectively. In the past the group 12 elements zinc (Zn), cadmium (Cd), and mercury (Hg), that are included in the "d-block" of the periodic table, have often been considered as transition elements, but they are nowadays rarely considered as such, because their compounds lack some of the characteristic properties. Because scandium (Sc), yttrium (Y), and lanthanum (La) actually do not form compounds analogous to those of the other transition elements and because their chemistry is quite homologous to that of the lanthanoids (previously known as lanthanide), they are often excluded from the group of transition elements. Also a strict application of the definition would describe lutetium (Lu) as a transition element as it has a singly occupied 5d orbital in its ground state, but according to IUPAC it is a lanthanoid. It appears most commonly as a positive ion without d-electrons in the valence shell and without the characteristic properties of a transition element.

The first three series of the transition elements are shown in the two tables. The elements in the fourth series (period 7 of the periodic table), are formally transition elements. They are man-made [except for Actinium (Z = 87)], not much is known about their compounds and accordingly they are not shown in the tables.

Rows and columns of the Periodic Table of Elements containing transition elements

The atomic number Z is between brackets

Group	3	4	5	6	7	8	9	10	11
1st series	Sc (21)	Ti (22)	V (23)	Cr (24)	Mn (25)	Fe (26)	Co (27)	Ni (28)	Cu (29)
2nd series	Y (39)	Zr (40)	Nb (41)	Mo (42)	Tc (43)	Ru (44)	Rh (45)	Pd (46)	Ag (47)
3rd series	La (57)	Hf (72)	Ta (73)	W (74)	Re (75)	Os (76)	Ir (77)	Pt (78)	Au (79)

Electron Configurations

Z	Symbol	Element	Core	Configuration
21	Sc	Scandium	[Ar]	(3d)1 (4s)2
22	Ti	Titanium	[Ar]	(3d)2 (4s)2
23	V	Vanadium	[Ar]	(3d)3 (4s)2
24	Cr	Chromium	[Ar]	(3d)5 (4s)1
25	Mn	Manganese	[Ar]	(3d)5 (4s)2
26	Fe	Iron	[Ar]	(3d)6 (4s)2
27	Co	Cobalt	[Ar]	(3d)7 (4s)2
28	Ni	Nickel	[Ar]	(3d)8 (4s)2
29	Cu	Copper	[Ar]	(3d)10(4s)1
39	Y	Yttrium	[Kr]	(4d)1 (5s)2
40	Zr	Zirconium	[Kr]	(4d)2 (5s)2
41	Nb	Niobium	[Kr]	(4d)4 (5s)1
42	Mo	Molybdenum	[Kr]	(4d)5 (5s)1
43	Tc	Technetium	[Kr]	(4d)6 (5s)1
44	Ru	Ruthenium	[Kr]	(4d)7 (5s)1
45	Rh	Rhodium	[Kr]	(4d)8 (5s)1
46	Pd	Palladium	[Kr]	(4d)10
47	Ag	Silver	[Kr]	(4d)10(5s)1
57	La	Lanthanum	[Xe]	(5d)1 (6s)2
72	Hf	Hafnium	[Xe*]	(5d)2 (6s)2
73	Ta	Tantalum	[Xe*]	(5d)3 (6s)2
74	W	Tungsten	[Xe*]	(5d)4 (6s)2
75	Re	Rhenium	[Xe*]	(5d)5 (6s)2
76	Os	Osmium	[Xe*]	(5d)6 (6s)2
77	Ir	Iridium	[Xe*]	(5d)7 (6s)2
78	Pt	Platinum	[Xe*]	(5d)9 (6s)1
79	Au	Gold	[Xe*]	(5d)10(6s)1

[Ar] stands for:	(1s)2(2s)2(2p)6 (3s)2(3p)6	[18].
[Kr] stands for:	[Ar](3d)10(4s)2(4p)6	[36].
[Xe] stands for:	[Kr](4d)10(5s)2(5p)6	[54].
[Xe*] stands for:	[Xe](4f)14	[68].

Properties

The most striking similarities shared by the transition elements is that they are all metals and that most of them are hard, strong, and shiny. They have high melting and boiling points, and as metals, are good conductors of heat and electricity. Several of the elements are technologically important: iron, nickel, and, platinum are used in heterogeneous catalysts. The transition elements form many useful alloys, among themselves and with other metallic elements. Most of these elements can be dissolved and form complexes in solution, although the "noble" metals platinum, silver, and gold are difficult to dissolve.

The elements exhibit variable valence and form stable compounds in two or more formal oxidation states (formal charges). For instance the chromium (Cr) in the complex Cr(H₂O)₆³⁺ is trivalent and is denoted by the oxidation state Cr(III). This cation with formal charge +3 has electronic structure [Ar](3d)². The doubly charged chromium in Cr(CN)₆⁴⁻ is divalent, denoted by Cr(II), and has electronic structure [Ar](3d)⁴. Chromate (Cr(O⁻)₄)²⁻ contains Cr(VI).