Chemistry of elements of second and third transition series

1. Chemistry of elements
of second and third
Transition series
Charul Somani
M.Sc, Phd*

2. Introduction-
The elements which have partially filled d-orbitals, either in the
atomic state or in the common oxidation state, are known as
transition elements. They are also known as d-block elements.
These elements appear in the middle part of the periodic table
between s and p-block elements. These elements, for the first
time, appear in fourth period. Thus, the elements from Sc(21)
to Cu (29) constitute first transition series. Other transition
series also begin from IIIB group and terminate at IB group.
Second transition series is constituted by the elements from
yttrium (39) to silver 47 and the third from lanthanum (57) and
hafnium (72) to gold (79).

3. Metallic radii
Metallic radii of second transition series elements are as expected. These
elements are larger by ~0.8 to 1.7Å than the first transition series elements.
In a transition series electrons enter the (n - 1)d orbitals due to which on
increasing atomic number by one, there is an increase of only 0.15 in the
effective nuclear charge. Thus, the decrease in metallic radius in a
transition series is small, as expected. Shrinkage in lighter elements in a
transition series is expectedly more because with the decrease in size,later
the electrons come closer and mutual repulsion increases and opposes
shrinkage. Consequently, atomic radii remain almost constant in the
middle but increase at the end of the series due to the attainment of d10
configuration. Gradation of atomic radii in the third transition series is
similar to that in the second series.

6. Atomic weight and density
Sizes of the members of both transition series are equal
but in IV B and the following groups it may be seen that
the atomic weight of a III transition series member is
nearly 1.5 times that of the element of second transition
series placed just above it. This trend is also reflected from
their densities. The ratio of their densities is nearly the
same as that of molecular weights.

7. Melting and boiling points
Transition metal group, the melting and boiling points of the
second element are higher than those of the first element and
the melting and boiling points of the third element are higher
than those of the second element. Melting point of silver is
lower than that of copper and gold. Existence of copper and
gold in higher oxidation state (+2 or +3) in their compounds
indicates participation of d orbitals to a greater extent in these
elements than silver. Thus, it may be concluded that in metallic
state the bonds between the atoms of silver are weaker than in
both copper and gold. This is the reason of the lower melting
point of silver.

8. Ionisation energy
The ionisation energy of second and third transition series
elements is medium order which indicates that these elements
have a tendency to form cations. Ionisation energies of IIIB group
elements are comparatively lower. Hence, they have a greater
tendency to form cations.In a group, generally the ionisation
energy decreases with the increase in atomic number so that the
first and the second transition series elements have nearly equal
values. The ionisation energy of the third series elements is higher
than that of the second which is due to lanthanide contraction. It
may be seen that for the members present at the end of the
series, the ionisation enegy increases rapidly which is due to the
completely filled (d10) configuration.

9. Hardness and mechanical strength
Metal are also melleable, ductile and have good
mechanical strength.

10. Oxidation state
 On comparing the oxidation states of the elements of the first transition series with
those of the second and third series, it is observed that the latter have greater tendency
to exhibit higher oxidation states. It may also be observed that +2 is a common state for
the first transition series elements (except Sc), whereas for the heavier elements it is a
very uncommon state-only Pd(II) and Pt(II) form bipositive species. When we going
down a transition metal group, instability of lower state and stability of higher state
increases. Due to this reason, for the middle elements both the higher and lower states
are equally stable, whereas the first and the third elements exhibit mutually opposite
behaviour. For example,
 cobalt both Co(II) and Co(III) are known, whereas the heavier elements of the group Rh
and Ir exhibit +3 or higher oxidation states.
 chronium +3 state is most important but in this state both molybdenum and tungesten
are strongly reducing. In addition to this, +6 state is common and stable for the latter,
whereas Cr(IV) is strongly oxidising. Ru and Os form compound RuO, and OsO4 in +8
oxidation state but Fe does not.

13. Tendency to form coordination
compounds
Heavy transition elements also have a tendency to form
coordination compounds, In the spatial distribution of d-
orbitals. 3d- orbitals do not have any node and hence are
compact, whereas 4d and 5d-orbitals have one and two
radial nodes respectively and hence have more extension
in space i.e. they are diffused. Due to large size, heavy
transition elements exhibit 6 coordination number,
although, a number of compounds with still higher
coordination numbers are known.

14. Comparison with 3d analogues
Ionic radii - It has already been discussed that in between the 4d and
5d levels is interposed the 4f shell which fills after lanthanum. The
occupation of this level is accompanied by a gradual decrease in
atomic and ionic radii from La to Lu and the total decrease in size
within the lanthanide series, known as lanthanide contraction is
approximately equal to the normal increase in size between one
period and the next. The result is that in the transition groups there is
the normal increase of about 0.2 Å in radius between the first and
the second transition series members (filling the 3d and 4d
subshells) but the expected increase between the second and third
members is just balanced by the lanthanide contraction so that these
two elements are almost identical in size.

16. Oxidation state -
Higher oxidation states for heavier transition elements in general
much more stable than for the elements of the first transition series.
The stability of higher oxidation state is exhibited by their very little
tendency to undergo reduction. The compounds of lighter elements
in higher oxidation states are easily reduced. In fact, the number of
compounds of heavier transition elements in higher oxidation states
is so large that these states appear to be the common oxidation states
for these elements.

17. Magnetic behaviour
Magnetic moment may be determined experimentally and the
number of unpaired electrons may be calculated from the magnetic
moment. In transition metal compounds, the knowledge of the
number of unpaired electrons, that are present in a molecule, is used
to interprete their many features like struture, stereochemistry and
spectra. Number of unpaired electrons in metal ions may be
calculated by the following relation.

18. Where u is magnetic moment which is measured in Bohr magneton
(BM) unit, g is gyromagnetic ratio and the total spin quantum
number of all the electrons is S which is equal to nx1/2=- N /2 where
n is the number of unpaired electrons. For the first transition series
elements, g=2. substituting these values of g and S, we get the
following relationship between μ and the number of unpaired
electrons n.μ = 2 +1μ = √n(n+2)

19.  One important characteristic of the heavier transition elements is that they tend to form low spin
compounds (∆> π), whereas the elements of the first transition series form both low spin and high
spin compounds. Low spin compounds are those in which the number of unpaired electrons is
minimum. In contrast to this, in high spin compounds the number of unpaired electrons is
maximum. In transition metal is ions, all the five d-orbitals are of same energy i.e. they are
degenerate. But when they form compounds, they split into different sets of energy for octahedral
geometry. The energy difference between the lower energy and higher energy orbitals is known as
splitting energy and is represented by ∆. The energy required to pair two electrons is known as
pairing energy which is represented by л. The value of ∆ is lower than л (∆<л) for high spin
complexes, whereas for low spin compounds ∆ value is comparatively higher (∆>T). In the
heavier elements, greater tendency of pairing of spins is due to two reasons. First, pairing energy
(л) for the heavier elements is lower than that for the lighter elements. This is because the 4d and
5d orbitals are larger than 3d orbitals. Due to more space the interelectronic repulsion between two
electrons is significantly less. As a result, the pairing energy will be lower in comparison to 3d
orbital
 Second the splitting of d orbitals

21. Stereochemistry -
Transition metal complexes can have different shapes depending on
its coordination number. The shapes that are common for transition
metal complexes formed using monodentate ligands (ligands which
only form one bond to the central metal ion or atom) are tetrahedral,
square planar and octahedral, as shown below. 6-co-ordinated
complex ions, in which the central metal is attached to six
ligands, have an octahedral shape . On the other hand, 4-co-
ordinated complex ions, in which the central metal is attached to
four ligands, can either have a tetrahedral or square planar shape.