In simple words, Van Der Waals Forces are those bonds that play the role of attracting both molecules and atoms. These interactions include weak electrostatic forces lying in a close range within molecules lacking charges. Moreover, they are the weakest intermolecular forces, consisting of dipole-dipole and dispersion forces.
As a segment of molecular physics, these forces came into existence from the name of a Dutch Scientist, Johannes Diderik Van Der Waals. In 1873, he first discovered the Van Der Waals bond while working on a theory on real gasses.
Let us explore to study more about them!
Define Van Der Waals Forces
According to the Van Der Waals forces definition, they are comparatively weaker electrostatic forces that attract uncharged or neutral molecules towards each other in almost all organic liquids, gasses, and solids.
In solids, these forces hold each other having lower melting points, and are relatively softer than the ones held by ionic or metallic bonds. In comparison to most of the ionic and covalent bonds, these interactions do not arise from an electronic bond.
The Van Der Waal forces include attractions within various atoms, resulting from influenced dipoles. However, they also involve a repulsive interaction within molecules, arising from the overlapping of more than two atomic electronic clouds situated closer to each other. Further, they are known as a universal interaction between various particles, divided by mediums of air or vacuum.
Some Van Der Waals forces examples are hydrogen bonding, dipole-dipole interactions, and dispersion forces.
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Equation of Van Der Waal Bonds
The Van Der Waals equation is a state that shows two properties of gases, such as the excluded volume of real gases and its attractive forces. It gets expressed as:
(P+n2a/V2) (V-nb)= nRT
Where a = magnitude of attraction within molecules/ atoms
And, b = excluded volume.
Do It Yourself: Study the equation thoroughly, and find out some Van Der Waals equation examples.
Properties of Van Der Waals
The below pointers show some of the characteristics of Van Der Waals forces.
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They comprise relatively weaker, electric forces compared to ionic, metallic or covalent bonds.
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The interactions are addictive when a large number of molecules are present. These are still present when molecules get placed afar.
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The Van Der Waal force is omnipresent and responsible for the attraction of atoms and molecules within each other.
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They remain unaffected due to temperature changes except in situations of dipole-dipole interactions.
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Although they are present in most of the materials, yet their effects get overpowered by the primary bonds.
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Moreover, they cannot get saturated.
Types of Van Der Waals Interactions
Initially, there were three types of Van Der Waals forces. These include:
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London Dispersion Forces: These bonds are the weakest attractive bonds, resulting from temporary and induced dipoles present in various atoms and molecules. They form when electrons present in two adjacent atoms occupy temporary positions. They are also known as dipole-induced dipole attraction.
These forces are also responsible for the condensation of non-polar substances into liquids. They often freeze into solids when the temperature falls quickly. According to the Van Der Waals definition, these forces depend on the polarization ability of the molecules or atoms.
The dispersion interactions are also present within two molecules, even the polar ones when they are extremely close to each other. The strength of the Waals forces depends on the number of electrons present in the molecule. Moreover, they occur due to the movement of the electrons.
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Figure 2: The diagram shows the London dispersion interactions in helium atoms.
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Dipole-Dipole: These interactions result in the attractive forces within the constant dipoles of two polar molecules. Dipoles arise from the differences among the electronegative effects of the atoms near each other. When it occurs, a polar molecule’s partially negative portion interacts with a partially positive portion of another one.
The opposite-charged dipoles have stronger interactions among them. A molecular dipole arises when there is an unequal sharing of electrons within atoms.
A Van Der Waals example of dipole-dipole forces is visible in hydrogen chloride (HCl) as a positive end of an element attracts the negative end of the other.
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Hydrogen Bonds: It is a special type of dipole-induced dipole interaction within hydrogen atoms. They are comparatively much stronger bonds than dipole-dipole interactions and London forces. These forces arise due to the attractive forces within a hydrogen atom, sharing a covalent bond with two highly electronegative atoms, such as N, O, and F, etc.
The strength of a hydrogen bond ranges between 4 kJ/mol and 50 kJ/mol. A hydrogen atom in a molecule gets attracted to other N, F, and O atoms. However, only N, O, and F atoms in one molecule can form hydrogen bonds.
A Vanderwaal Force example of hydrogen bonds is the interaction of water molecules.
Components of These Forces
Van Der Waals dispersion forces are close-knit interactions depending on distance resulting in intermolecular attractions or repulsions. These bonds get stronger when they lie in a range of 0.4 kilojoules per mole (kJ/mol) and 4 kJ/mol. Moreover, they are active within a distance of fewer than 0.6 nanometers (nm).
However, within a distance of 0.4 nanometers, the effect of these attractions tends to be repulsive for electron clouds. Read some of the components below that result in formations of these secondary bonds:
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They consist of a negative component that restricts molecules from collapsing. It occurs because of the Pauli Exclusion Principle.
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Polarization is present in this interaction, which is also known as the Debye force after Peter J.W. Debye. It is a force responsible for generating attractions within a permanent polarity of a molecule and an induced polarity of the
other.
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Known as a Keesom force, named from William Hendrik Keesom, is another contributor to the Waals force. There is either an attractive or repulsive interaction existing between dipoles, constant charges, multi-poles or quadrupoles, etc.
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London dispersion force, named after Fritz London, is a component of Waals force. It occurs as an attraction within various molecules as a result of immediate polarization. However, some non-polar ones also experience this force.
Factors Affecting the Strength of Van Der Waal
The factors affecting Van Der Waals forces are as follows:
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The Number of Electrons Present in an Atom: The amount of electrons present is responsible for the creation of temporary dipoles. The strength of the Waals forces depends on the number of dipoles. Therefore, an increase in the number of dipoles increases the bonds of Van Der Waals.
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Size of the Atoms: The strength of attractive bonds of these forces varies with changes in the size of atoms. An intermolecular force increases as the size of atoms increases, such as helium, radium, krypton, etc. Relatively, an element’s boiling and melting point vary because of the change in these forces.
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Nature of the Elements: An element’s or a non-metals nature has a relation with the strength of the Waals forces. Most non-metals present in a liquid or gaseous state consist of these forces, whereas some metals comprise strong, cohesive forces.
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The Shape of Atoms: The form of an atom has a direct relation with the strength of these forces. A thinner molecule has the potential of developing more temporary dipoles compared to short, fat ones.
Importance of Van Der Waal Force
There are multiple applications of Van Der Waals forces in molecular science. Some of these are:
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These intermolecular forces enable Gecko lizards to move on surfaces efficiently. Similarly, a few other species of spiders have these biological patterns too.
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These forces are responsible for the interactions of proteins with other atoms.
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They also affect various characteristics of gases, adhesion, and colloidal stability.
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These forces play a fundamental role in the study of supramolecular chemistry, nanotechnology, surface, and polymer science, etc.
Rack Your Brains
Here are some questions on the Van Der Waals intermolecular forces.
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In the equation of the Waals forces, the value of ‘a’ varies with an increase in
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Which of the following constant has a high value in a chlorine gas
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‘b’
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‘c’
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‘a’
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None of these.
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Characteristics of Van Der Waals Forces
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Both Covalent bonds and ionic bonds are much stronger than van der Waals forces
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These forces can be added up.
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These forces cannot be saturated
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Directional characteristics cannot be assigned to these forces
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They are temperature independent (except for dipole-dipole interactions)
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Van der Waals forces are stronger if the atom/molecule is in close proximity, the further away they go, the weaker they get.