The elements belonging to 3 to 12 group, present in the center of the periodic table are called transition metals. These are actually d- block metals and form a bridge in between the s and p block elements, the metals and non metals. The transition metals are unique among the chemical elements. The elements in which the last electron enters the d- sub shell of their penultimate shell (n-1 shell) are known as d- block elements.Thus, transition metals can be broadly defined as those which either as elements or as ions have partially filled d-subshell. Since d-subshell is partially filled, the elements are called as d- block elements.
The transition metals are classified into four transition series. Each series (Except 6d) consists of 10 elements. (This is because the d- sub-shell can occupy only 10 electrons and they get filled up when we move along a period in a series.)
It is a part of the fourth period and consists of 10 elements from Scandium (At no: 21) to zinc (at no 30) in which 3d - sub-shells are progressively filled.This is also called as 3d series which corresponds to the filling of the 3d orbital.
| Atomic number || Symbol || Electronic configuration |
| Scandium || 21 || Sc || [Ar] 3d1 4s2 |
| Titanium || 22 || Ti || [Ar] 3d2 4s2 |
| Vanadium || 23 || V || [Ar] 3d3 4s2 |
| Chromium || 24 || Cr || [Ar] 3d5 4s1 |
| Manganese || 25 || Mn || [Ar] 3d5 4s2 |
| Iron || 26 || Fe || [Ar] 3d6 4s2 |
| Cobalt || 27 || Co || [Ar] 3d7 4s2 |
| Nickel || 28 || Ni || [Ar] 3d8 4s2 |
| Copper || 29 || Cu || [Ar] 3d10 4s1 |
| Zinc || 30 || Zn || [Ar] 3d10 4s2 |
Second transition series is a part of the fifth period and consists of 10 elements from Yttrium (At no: 39) to cadmium (At no: 48) in which 4d- sub-shells are being progressively filled. This is also called 4d series which corresponds to the filling of the 4d orbital.
|Element ||Atomic Number ||Symbol ||Electronic configuration |
|Yttrium ||39 ||Y ||[Kr] 4d1 5s2 |
|Zirconium ||40 ||Zr ||[Kr] 4d2 5s2 |
|Niobium ||41 ||Nb || [Kr] 4d4 5s1 |
|Molybdenum ||42 ||Mo || [Kr] 4d5 5s1 |
|Technetium ||43 ||Tc || [Kr] 4d5 5s2 |
|Ruthenium ||44 ||Ru || [Kr] 4d7 5s1 |
|Rhodium ||45 ||Rh || [Kr] 4d8 5s1 |
|Palladium ||46 ||Pd || [Kr] 4d10 5s0 |
|Silver ||46 ||Ag || [Kr] 4d10 5s1 |
|Cadmium ||48 ||Cd || [Kr] 4d10 5s2 |
It is a part of the sixth period and consists of 10 elements from Lanthanum (At no: 57) and Hafnium (At no: 72) to Mercury (At no: 80) in which the 5d sub-shell is progressively filled. This is also called as 5d series which corresponds with the filling of 5d orbital.
|Element ||Atomic number ||Symbol ||Electronic configuration |
|Lanthanum ||57 ||La ||[Xe] 5d1 6s2 |
|Hafnium ||72 ||Hf ||[Xe] 4f14 5d2 6s2 |
|Tantalum ||73 ||Ta || [Xe] 4f14 5d3 6s2 |
|Tungsten ||74 ||W || [Xe] 4f14 5d4 6s2 |
|Rhenium ||75 ||Re || [Xe] 4f14 5d5 6s2 |
|Osmium ||76 ||Os || [Xe] 4f14 5d6 6s2 |
|Iridium ||77 ||Ir || [Xe] 4f14 5d7 6s2 |
|Platinum ||78 ||Pt || [Xe] 4f14 5d9 6s1 |
|Gold ||79 ||Au || [Xe] 4f14 5d10 6s1 |
|Mercury ||80 ||Hg || [Xe] 4f14 5d10 6s2 |
It is a part of the seventh period and is an incomplete series starting from actinium (At no: 80) in which 6d sub-shell is being progressively filled.This is also called as 6d series which corresponds with the filling of 6d orbitals. This series contains only 3 elements. They are actinium with atomic number 89 followed by two elements with atomic numbers 104 and 105.
|Element ||Atomic No ||Symbol ||Electronic configuration |
|Actinium ||89 ||Ac ||[Rn] 6d1 7s2 |
The valence electrons of transition metals
are responsible for the charges that these metals carry or show and has been explained below.The valence or the electrons in the ultimate shell enter the 'd'
orbital. The first element in each series stats with one valence electron or 'd' electron and the last element will have 10 'd' electrons.
Variable oxidation state or variable valency is one of the most striking features of the transition elements. All transition elements, except the first and last member of each series exhibit variable valency, related to its electronic structure. These elements lose both the 's' an the 'd' electrons and thus have a lot of valencies. Consequently, all transition elements in general exhibit variable valencies.Valencies of first transition series are
|| +2, +3
|| +2, +3, +4
|| +2, +3, +4, +5
|| +1 to +6 (+4 and +5 are unstable)
|| +2 to +6(+4, +5 and +6 are unstable)
|| +2 to +6 (+4, +5 and +6 are unstable)
|| +2 to +6 (+4 is unstable)
|| +2, +3, +4
|| +1, +2
The transition metals properties
are described as follows.
Transition elements have partly filled d- orbitals. These elements show several interesting properties like variable oxidation state or variable valency, formation of colored complexes and paramagnetic behavior. These metals and their compounds also exhibit catalytic properties.
Some of the important properties of transition metals are
1. Electronic Configuration
The electronic configuration of transition elements may in general be represented as
(n-1) d 1-10 ns1 or 2
The (n-1) means penultimate or next to the outermost shell and d- orbital may have 1 to 10 electrons and the s- orbital of the outermost shell (n) may have 1 or 2 electrons. The electronic configurations of all the four series of elements are given in a table above.
2. Atomic and ionic radii
The atomic and ionic radii of the elements of a particular transition series decreases from left to right. In the first series, the atomic radii become almost constant for chromium, because of two factors-Increase in nuclear charge and increase in the screening effect which just balance each other.
Atomic radii of Chromium is - 117pm. Mn, the next element has an atomic radii of 117 pm. The next three elements, Co, Ni and Cu has 116. 115 and 117 respectively.
To summarize, atomic and ionic radii decrease with increase in atomic number, due to increase in nuclear charge.
3. Magnetic Properties
Magnetic character is of two types.
Those which are attracted by the magnetic field are termed as paramagnetic and those repelled by the magnetic field are diamagnetic. Transition elements and their compounds are paramagnetic in nature, they conduct electricity.Paramagnetism is due to the presence of unpaired electrons in the d- orbitals of the transition metal atoms, ions or molecules. The greater the number of unpaired electrons, the greater will be its magnetic behavior. So, all transition metals with at least one unpaired electron are paramagnetic.
Most d- block compounds are colored in the solid or in solution states. The color of the transition metal ions is due to the presence of unpaired or incomplete (n-1) d orbitals and the ability to promote an electron from one energy level to another. In these ions d- electrons are promoted to the higher energy levels within the same d- sub-shell. The color exhibited depends upon the complementary color of light observed by these ions.
Colors of some transition metals are
5. Complex formation
Complex formation or complexation is a typical behavior of transition metals. In these complex compounds, the transition metal ions form co-ordinate bonds with a number of neutral or negatively charged ions which are capable of donating electrons to the metal atom.
A few examples are [Fe(CN)6]3-, [Fe(CN)6]4-, [Cu(NH3)4]2+,
6. Catalytic behavior
Most of the transition elements and their compounds are good catalysts. Platinum, cobalt, nickel, chromium, manganese, vanadium pent-oxide, etc are good examples. They are used as catalysts since they have incomplete d- orbitals and can form unstable intermediates which can then change into products.
Inner transition elements are also called as f- block elements. The elements in these group are present below the main table and contains two series.
- The first series is called as Lanthanoid series and starts from Cerium (Ce atomic number -58) to Lutetium (Lu atomic number- 71). There are totally 14 elements in this series.
- The second series is called the actinoid series. This series also contains 14 elements from Thorium (Th - 90) to Lawrencium (Lr- 103).
Most of the actinoids are radioactive isotopes, and do not occur in nature. They are the result of a radioactive decay and thus are not stable.