Introduction
The attracting and repellent forces that develop between the molecules of a material are referred to as intermolecular forces (IMF), which is sometimes shortened. These forces operate as a mediator between a substance's individual molecules.
Explanation:
Bulk characteristics, such as the boiling and melting temperatures of liquids, are determined by intermolecular forces. When molecules in a liquid have enough thermal energy to overcome the forces holding them together, called intermolecular forces, bubbles of vapour are created inside the liquid. Similar to liquids, solids melt when their molecules gather sufficient heat energy to dislodge the intermolecular interactions holding them in place.
Electrostatic in nature, intermolecular forces result from the interaction of positively and negatively charged entities. Intermolecular interactions are the total of both attracting and repulsive elements, similar to covalent and ionic connections. Intermolecular interactions are especially significant for solids and liquids, where the molecules are close together, because electrostatic interactions rapidly decrease with increasing distance between molecules. Only at extremely high pressures do these interactions become significant for gases; at these pressures, they are what causes the observable departures from the ideal gas law.
Three different intermolecular interactions are specifically taken into account in this section. You are already aware of two other forms of electrostatic interactions: ion-ion interactions, which are the basis for ionic bonding, and ion-dipole interactions, which take place when ionic compounds dissolve in polar solvents like water. The first two are frequently referred to as van der Waals forces as a whole.
Ion-Ion Interactions:
Ions with opposing charges are drawn to one another through ion-ion interactions. They are also known as ionic bonds, because they are what keep ionic compounds together.
Two oppositely charged particles (such a sodium cation and a chloride anion) will be attracted to one another in a vacuum, and the force will grow as the two objects get closer to one another. They will eventually congregate, and it will take a lot of work to break them apart again. They combine to create an ion-pair, a new particle with a positively and negatively charged region. These ion pairs and free ions interact very strongly, which causes the clusters to expand. For instance, sodium cations will begin to attract chloride anions to form solid salt crystals when a saturated solution of sodium chloride has more ions than the solution can maintain.
Van der Waal Forces:
In gases, liquefied and solidified gases, and nearly all organic liquids and solids, neutral molecules are drawn to one another by van der Waals forces, which are relatively weak electric forces. The intermolecular forces are named after the Dutch scientist Johannes Diderik van der Waals, who initially proposed them in 1873 while formulating a hypothesis to explain the characteristics of actual gases. Van der Waals forces hold together solids, which are often softer and have lower melting temperatures than solids kept together by stronger ionic, covalent, and metallic connections.
Types of Intermolecular Forces
Following are three types of intermolecular forces;
• Dipole-Dipole Forces
• Hydrogen Bonding
• London Dispersion Forces
Dipole-Dipole Forces
The behavior of polar covalent bonds is that of localized fractional charges that are equivalent but opposite for the bound atoms (i.e., the two bonded atoms generate a dipole). A molecule possesses a total dipole moment if its structure inhibit the individual bond dipoles from cancelling one another. The positive end of one dipole tends to be close to the negative end of another, and vice versa, in molecules with net dipole moments.
Hydrogen Bonding
It is common for molecules to have extremely strong intermolecular interactions when the hydrogen atoms are bound to electronegative atoms like O, N, and F (and to a much lesser degree, Cl and S). As demonstrated by the covalent hydrides of elements in groups 14–17, these lead to much higher boiling points than are observed for substances in which London dispersion forces predominate. Methane and its heavier congeners in group 14 form a series whose boiling points increase smoothly with increasing molar mass. In nonpolar molecules, when London dispersion forces are the only intermolecular interactions, this is the predicted tendency.
The boiling temperatures of the hydrides of the lightest members of groups 15–17, however, are more than 100°C higher than what would be expected based on their molar masses. The boiling point of water is expected to be around 130°C if we extend the straight line linking the points for H2Te and H2Se to the line for period 2! Think about the effects on life on Earth if water boiled at 130°C as opposed to 100°C.
In order to create an open, cage-like structure, each water molecule takes two hydrogen bonds from two other water molecules and gives two hydrogen atoms to make hydrogen bonds with two more water molecules. Liquid water has a structure that is very similar to that of solid water, but because of the fast molecular mobility in the liquid, hydrogen bonds are constantly being made and destroyed.
London Dispersion Forces
We have solely taken into account interactions between polar molecules up to this point. It is necessary to take into account other considerations in order to understand why some nonpolar compounds, like iodine and naphthalene, are solids at room temperature while others, like bromine, benzene, and hexane, are liquids. Even the noble gases can liquefy or solidify under conditions of high pressure, high temperatures, or both.
What sort of attraction is possible between nonpolar molecules or atoms? Fritz London (1900–1954), a German physicist who subsequently worked in the United States, provided a solution to this query. London postulated in 1930 that transient changes in the electron distributions within atoms and nonpolar molecules can lead to the production of fleeting instantaneous dipole moments, which create attractive interactions known as London dispersion forces between normally nonpolar substances.
Example:
Think about a pair of He atoms that are close to one another. Each He atom has two electrons, which are typically evenly spaced out around the nucleus. However, since the electrons are always moving, it is possible that their distribution within a single atom is asymmetrical at any one time, which leads to an instantaneous dipole moment. By drawing them toward the positive end of the instantaneous dipole or repelling them from the negative end, the instantaneous dipole moment on one atom can interact with the electrons of an atom. Overall, this results in the temporary creation of a dipole, known as an induced dipole, in the second atom as a result of the first atom. Atoms are drawn near one another by interactions between these transient dipoles.
Polarizability
How a material interacts with ions and species that have permanent dipoles is also influenced by its polarizability. Thus, in a homologous sequence of compounds, such as the alkanes, the overall tendency toward higher boiling temperatures with increased molecular mass and greater surface area is caused by London dispersion forces.
The extent to which one molecule may interact with its neighbours at any one time is determined by the shape of the molecule, which has a substantial impact on the strength of London dispersion forces. For instance, n-pentane and 2,2-dimethylpropane, both of which having the empirical formula C5H12. While n-pentane has an extended shape that enables it to come into close contact with other n-pentane molecules, n-opentane is practically spherical and has a large surface area for intermolecular interactions. Neopentane has a boiling point that is more than 25°C lower than n-(36.1°C), pentane's which is 9.5°C.
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