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.