47.THE PERIODIC TABLE – Melting point(M.P) , Bond dissociation energy(BDE)&Bond energy.

We start discussing the last important periodic properties in this post – Melting point, Bond dissociation energy and Bond energy.


Melting point(M.P) –

The temperature at which a solid gets converted into liquid state is called its melting point. This temperature is indicative of the energy that is required to break the bonds between the atoms in the solid. The more tightly packed the solid structure is , the more will be its melting point ,as it will require more energy to break the strong bonds in it.

Thus, high bond dissociation energy is directly proportional to the melting point temperature of solids.We have already used this term – bond dissociation energy- in the previous post. Let us study in detail what this actually means.


Bond Dissociation energy(BDE/Ed) –

The energy required for homolytic bond cleavage, for a single chemical bond, at 298 K(which also corresponds to the standard enthalpy change )is called the bond dissociation energy of the molecule.

Homolysis (Homo = equal, lysis = breaking/fission) is a phenomenon wherein both the atoms take their electron(which they used to share during bond formation) with themselves, after cleavage of the bond. Thus, the bond is broken symmetrically and the shared electron pair is split evenly.

e.g.- Cl-Cl bond splits homolytically to form two chlorine radicals ⇒ Cl2→ •Cl + •Cl

Enthalpy is a thermodynamic term , which gives us the total heat content of the system. We will study this parameter in detail when we start our discussion on  thermodynamics.


Bond Energy – 

This term gives us the average of all the bond-dissociation energies of the bonds in a molecule.

e.g.- Bond dissociation energy of H-OH bond is 493.4kJ/mol ( H-OH → H+ + OH).
        Bond dissociation energy of O-H bond is 424.4 kJ/mol ( energy required to break O-H bond)

∴Bond energy for water molecule = (493.4 +424.4)/2 = 458.9 kJ/mol.

For a diatomic molecule, bond dissociation energy and bond energy are same.


Trends in M.P – 

There is no specific trend in M.P of elements of the periodic table.However, we must generally know the pattern.

Melting_Point_Trend_IK
General trend in M.P of elements

As seen in the above figure, the metals (e.g.- B,Be,Sc,Ti,V,Cr,Mn,Cu etc ) have high melting points.

Iron_-melting (1)
Molten Iron

Iron melts at 1538° C !! To have a sense of scale, think of how hot one feels when the summer temperatures rise to 45° C ! The molten iron in the above figure is at such high temperature that it glows with a brilliant yellow color (in the visible range). 

[Note – Generally all substances emit thermal radiation but the wavelength is far less than that visible to human eye (visible range).When the piece of metal is heated is starts to glow(black body radiation) due to thermal excitation of electrons.It first becomes red hot because red color has higher wavelength i.e lower energy.Later it starts turning orange and then yellow.This phenomenon just shows that as the temperature rises, the wavelength of light emitted becomes less.

Temperature (T)∝ Energy(E) ∝ (1/Wavelength ,λ)

Temperature and energy are directly proportional to each and they both are inversely proportional to wavelength of light emitted.This means higher the temperature, lesser will be the wavelength. So, substances which melt at a still higher temperature than iron will emit in the UV range but our eyes cannot see in that range].

Non-metals(H,S,Se etc) generally have lower M.P. However, the non-metal CARBON HAS THE HIGHEST M.P of all elements ! This is due to a property it possess called ‘Catenation’ , where C-C covalent bonds can be formed.We shall talk in detail about this property in our coming posts on Organic Chemistry.

As such M.P depends upon a number of factors and so a proper trend is not seen for all elements.With this post we end our discussion on the diffrent properties of elements and their periodic trends. In our next post on the periodic table lets try to find out how accurate this table is in real sense and if there are certain unexplained aspects to it.Till then,

Be  a perpetual student of life and keep learning..

Good Day !

References and further reading –

1.https://en.wikipedia.org/wiki/Bond-dissociation_energy
2.https://chem.libretexts.org/Core/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends

Image source –

1.https://chem.libretexts.org/Core/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends
2.Wikimedia commons – Melting of Iron

 

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