Why do the melting point and boiling point of transition metals high?
Elements that have partially filled inner (n-1)d
sub-shell in the atomic state or in the permanent oxidation state are called
transition metal elements. Most of the transition elements are metal.
Transition metals become very hard except Zn, Cd and
Hg. They are very much harder than s-block and p-block elements. Generally, transition
metal elements have very high melting point
and boiling point.
The melting points of most of the transition metalselements (except Zn, Cd and Hg) are above 900°C. Their melting point and boiling point are very much higher than s-block
and p-block elements.
The transition metals show very high melting point and boiling point due to the presence of very strong metallic bond as well as covalent bond in their metallic crystal lattice.
Besides, transition metal elements are closely packed
and hold together by the strong metallic bonds.
In the transition metals, electrons of ns-sub shell as
well as odd electrons of (n-1)d-sub shell participate in the formation of
inter-atomic metallic bond among the atoms.
As a result, the metallic bond becomes very strong. So
the amount of heat energy required to break such type of bond is very much
high. Hence melting point and boiling
point of transition metals become high.
For example, among all the transition metals, the
tungsten atom have highest melting point. The melting point of tungsten is 3410°C.
Periodic trends of melting point and boiling point of transition metals
The melting
point and boiling point of transition metals gradually increases from left
to right along a particular transition series and reach a maximum value and then
decreases.
For example, the melting point of 3d-series gradually
increases from Sc to V and it reach a maximum value for Cr and after it the
melting point slowly decreases.
Because of with increase in atomic mass and atomic
number, the number of unpaired electrons of transition metals increases.
For
example, when we move from group-3 to group-6 along any
transition series, the number of unpaired electrons gradually increases and it
is maximized for group-6 metal element.
Then the number of unpaired
electrons gradually decreases as the atomic number increases due to coupling of
electrons.
As the number of unpaired
electrons in the valence shell increases or decreases, the strength of the
metallic bond increases or decreases respectively.
As the strength of a metallic
bond increases or decreases, the hardness, melting point, etc., of transition
metal elements increases or decreases, respectively.
For example, in the valence
shell of the element Cr, Mo, etc., there are 6 unpaired electrons, so the
strength of the metallic bond is the highest.
On the other hand, in the
case of Zn, Cd and Hg, the strength of the metallic bond is lowest as there are
no unpaired electrons. So these metals are flexible and
their melting point and boiling point
are very low.
Compare of M.P and B.P of 3d, 4d and 5d series transition metals
It has been experimentally found that the melting point and boiling point of
4d-series transition metals are higher than that of 3d-series transition
metals.
Again, the melting
point and boiling point of 5d-series transition metals are higher than that
of 4d-series transition metals.
That is, the increasing order of melting point and boiling point of 3d, 4d and 5d-series transition
metals are as follows, 3d < 4d < 5d.
This fact can be explained on the basis of their size
and metallic character. We know that the atomic size of elements increases from
top to bottom along a group.
Hence the size of 4d-series transition metal elements
is larger than 3d-series transition metal elements. Similarly, atomic size of
5d-series elements is greater than 4d-series elements.
If the atomic size of elements increases then the force
of attraction of nucleus towards outer most electrons becomes less. Hence the
outer most valence electron is available for metallic bond formation.
That is, with increase in atomic size the metallic
character of the elements increases. Since metallic character of elements
increases hence metallic bond among the metal atom becomes very strong.
Therefore, a large amount of heat energy is required to
break this type of metallic bond. Consequently, the melting point and boiling point increases from 3d-sreies to
4d-series and from 4d-series to 5d-series metal elements.
Why do the m.p and b.p of Mn metal very much less than expected value?
The melting point and boiling point of Mn metal very
much less than expected value. This anomalous behavior of Mn can be explained
on the basis of its electronic configuration.
From the electronic configuration of Mn atom, it has
been found that 4s-orbital of Mn is full filled. So it is stable electronic
configuration.
Again, 3d-orbital of Mn occupied five electrons which
is exactly half filled. Hence it is also stable electronic configuration.
For
this reason, although there are five unpaired electrons in the Mn-atom, they
are not readily available to form metallic bonding. That is, in case Mn atom,
stability factor is more dominating over the number of unpaired electrons.
For
the above said reason, the metallic bond of Mn atom becomes weak and hence the melting
point and boiling point of Mn metal becomes very much less than the expected
value.
Why
do Zn, Cd and Hg have low melting and boiling point?
The
melting point and boiling point of d-block or transition metal elements depends
on the metallic bond presence among the metal atoms. Now, the strength of
metallic bond depends on the number of unpaired present in the outer most
valence shell.
If
the number of unpaired electrons increases then metallic bond becomes stronger
and hence the metal elements show high melting and boiling point.
Since
Zn, Cd and Hg have no unpaired electron then the strength of
their metallic bond is lowest. So Zn, Cd and Hg metals are flexible and their melting point and boiling point are very low.
- Why do the melting point and boiling point of transition metals high?
- Compare of M.P and B.P of 3d, 4d and 5d series transition metals
- Why do Zn, Cd and Hg have less melting and boiling point?
- Why do the m.p and b.p of Mn metal very much less than expected value?
melting point and boiling point of transition metals, m.p
and b.p of 3d, 4d and 5d series transition metals, Zn, Cd and Hg have less
melting and boiling point, m.p and b.p of Mn metal,
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