General Characteristics of Transition Metals

The elements that lie in between S-block and P-block are the d-block elements. These elements are called transition elements as they show transitional properties between s and p-block elements. These elements contain partially filled d-orbitals and hence they are called as d-block elements. The general electronic configuration of d-block elements is (n-1)d^1-10ns^1-2.

Transition metals

The elements belonging to d-block are metals. The d-block elements are classified into four transition series. These 4 series corresponds the filling of 3d, 4d, 5d and 6d orbitals.

First transition series

This is also called as 3d series which corresponds the filling of 3d orbital. It starts from scandium whose atomic number is 21 and includes 10 elements till zinc whose atomic number is 30.

First transition series (or) 3d series

Element	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

This is also called as 4d series which corresponds the filling of 4d orbital. It starts from yttrium whose atomic number is 39 and includes 10 elements till cadmium whose atomic number is 48.

Second transition series (or) 3d series

Element	Atomic Number	Symbol	Electronic configuration
Ytterium	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

Third transition series

This is also called as 5d series which corresponds the filling of 5d orbital. The first element of this series is lanthanum whose atomic number is 57 and includes 9 elements from hafnium whose atomic number is 72 to mercury whose atomic number is 80.

Third transition series (or) 3d series

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
Tungstun	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

Fourth transition series

This is also called as 6d series which corresponds 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. This is an incomplete series.

Element	Atomic No	Symbol	Electronic configuration
Actinium	89	Ac	[Rn] 6d1 7s2

Electronic Configuration

The general electronic configuration of d-block elements is (n-1) d^1-10ns^1-2. All the d-block elements except zinc, cadmium and mercury have partially filled d-orbitals. But, zinc, cadmium and mercury have completely filled d-orbitals and they exhibit common oxidation state. So, they do not come under transition elements but are studied along with d-block elements. In all the other transition elements the last electron enters the (n-1)d orbital which is called the penultimate shell. The electronic configurations of 4 transition series are given below

Characteristics of Transition Metals

All the transition metals except Zn, cd and Hg exhibit several physical and chemical properties. Some of their properties are discussed below:

Variable oxidation states

By the study of electronic configuration of transition metals it is understood that variable oxidation state can be formed as there are both ns and (n-1)d electrons in bonding. The participation of ns electrons in bonding leads to +2 oxidation state which is a lower oxidation state. The participation of (n-1)d electrons in bonding leads to higher oxidation states like +3, +4, +5, +6 etc. These oxidation states depend upon the nature of combination of transition metals with other elements. The oxidation state increases with atomic number. This increase is related to groups. The most common oxidation state of the elements of first transition series is +2. Ionic bonds are formed in lower oxidation state transition elements whereas covalent bonds are formed in higher oxidation states.

Magnetic properties

By the study if electronic configuration of transition metals it is understood that they generally contain one or more unpaired electrons in the (n-1)d orbital. Due to these unpaired electrons they behave as paramagnetic substances. These substances are attracted by the magnetic field. The transition elements that contain paired electrons behave as diamagnetic substances. These substances are repelled by the magnetic field. The paramagnetic character increases as the number of unpaired electrons increases.

Formation of colored compounds

Most of the transition elements form colored compounds both in solid state as well as in aqueous solution. It is already studied that the transition metals have incomplete d-orbital. The electrons are to be promoted from a lower energy level to a higher energy level. Some amount of energy is required for this process and the radiations of light are observed in the visible region. The compounds absorb a particular color from the radiation and the remaining ones are emitted.

For e.g., Cu²⁺ are bluish green in color due to absorption of red light wavelength.

As Zn has completely filled d-orbitals it cannot absorb radiation and hence Zn²⁺ salts are white.

Formation of complexes

Transition metals form many complex ions. They are the electrically charged complexes with a metal ion in the center which is surrounded and linked by a number of neutral molecules or negative ions. These neutral molecules or negative ions are called as ligands. As the transitions metals are small in size they form large number of complexes.

Electrode potential and low reactivity

The electrode potential is a measure of the total enthalpy change (DH_T) when a solid metal, M is brought into aqueous medium in the form of M⁺ (aq).

The total enthalpy change depends on sublimation energy, ionization energy and hydration energy of the metal.

The stability of the oxidation state of metal depends on the electrode potential. When electrode potential is less the stability is more.

General trends in the chemistry of first row transition series

Metallic character

Most of the transition elements of the first row form metallic bonds due to the presence of incomplete outermost energy level. So, all the transition elements exhibit metallic characters. The strength of the metallic bond depends upon the number of unpaired d-electrons. As the number increases the strength also increases. Due to the absence of unpaired electrons 'Zn' is not a hard metal.

Ionization energy

The ionization energies of first row elements gradually increases with increase in atomic number. The ionization energy of Zn is very high than all the other metals which is due to its fully filled d-orbital. The third ionization energy of Mn is very high than the others.

Electrode potential

The electrode potential is a measure of the total enthalpy change (DH_T) when a solid metal, M is brought into aqueous medium in the form of M⁺(aq).

The total enthalpy change depends on sublimation energy, ionization energy and hydration energy of the metal.

Oxidation State

The first row transition elements show variable oxidation states. Zn is an exception among them. As it has fully filled d-orbital, it exhibits only +2 oxidation state. The oxidation states of first row transition metals are shown below.

Oxidation states of first row transition metals

Ionic radii

In the first row transition elements the ionic radii decreases with increase in atomic number. The value of ionic radii also depends on the oxidation state of metals. As the oxidation state increases the ionic radii decreases and as the oxidation state decreases the ionic radii increases.

Catalytic property

The first row transition elements exhibit catalytic properties due to the presence of unpaired electrons which can form complexes. Iron and vanadium are the most important catalysts. Iron is used as catalyst in the manufacture of ammonia. Vanadium is used in the form of vanadium pentoxide in the manufacture of sulphuric acid.

Colored ions

In the first row transition elements all the elements except Zn form colored ions. As these elements have incomplete d-orbital, some amount of energy is required to promote the electrons from lower energy level to higher energy level. This process exhibits radiations from which the compounds absorb a particular color. But some elements other than Zn also appear colorless depending on their oxidation state. For e.g., Sc³⁺, Ti⁴⁺ and Cu⁺ have completely filled d-orbitals and hence they appear colorless.

Complex formation

All the first row transition elements form complexes. These complexes contain negative ions or neutral molecules linked to a metal ion. These are called as ligands. Some examples of the complex compounds formed by first row transition elements are:

[Fe(CN)₆]^4-, [Cu(NH₃)₄]²⁺, [Ni(CN)₄]^2-, [Zn(NH₃)₄]^2+32+2+ .

Magnetic property

The transition metals generally contain one or more unpaired electrons in the (n-1)d orbital. Due to these unpaired electrons they behave as paramagnetic substances. These substances are attracted by the magnetic field. The transition elements that contain paired electrons behave as diamagnetic substances. These substances are repelled by the magnetic field. The paramagnetic character increases as the number of unpaired electrons increases.

Interstitial compounds

All the first row transition metals form interstitial compounds with the elements of the S and P-blocks. The elements that occupy the interstitial sites in their lattices are H, C and N. Both the elements combine and form bonds which are hard.

Alloy formation

When one metal mixes up with another metal alloys are formed. As the d-block elements have same atomic sizes they can easily take up positions of one another. This causes alloy formation. For example: cr, V, Mn are used in formation of alloy steels.