Answer:
There are three major types of inter molecular forces: London dispersion force, dipole-dipole interaction, and ion-dipole interaction.
Explanation:
All around us we see matter in different phases. The air we breathe is a gas, while the water you drink is a liquid and the chair you are sitting on is a solid. In this chapter we are going to look at one of the reasons that matter exists as solids and liquids.
In the previous chapter, we discussed the different forces that exist between atoms (inter atomic forces). When atoms are joined to one another they form molecules, and these molecules in turn have forces that bind them together. These forces are known as inter molecular forces.
Inter molecular forces allow us to determine which substances are likely to dissolve in which other substances and what the melting and boiling points of substances are. Without inter molecular forces holding molecules together we would not exist.
Note that we will use the term molecule throughout this chapter as the compounds we are looking at are all contently bonded and do not exist as giant networks (recall from grade \(\text{10}\) that there are three types of bonding: metallic, ionic and convalescent). Sometimes you will see the term simple molecule. This is a convalescent molecular structure.
Inter atomic (between atoms) forces are also known as intramuscular (within molecules) forces. You can remember this by thinking of international which means between nations.
4.1 Inter molecular and inter atomic forces (ESBMM)
Inter molecular forces
Inter molecular forces are forces that act between molecules.
You will also recall from the previous chapter, that we can describe molecules as being either polar or non-polar. A polar molecule is one in which there is a difference in electromagnetically between the atoms in the molecule, such that the shared electron pair spends more time close to the atom that attracts it more strongly. The result is that one end of the molecule will have a slightly positive charge (\(δ^{+}\)), and the other end will have a slightly negative charge (\(δ^{-}\)). The molecule is said to be a dipole.
A dipole molecule is a molecule that has two (di) poles. One end of the molecule is slightly positive and the other is slightly negative. We can depict this very simply as an oval with one positive side and one negative. In reality however, the molecules do not look like this, they look more like the images in Figure 4.1.
Figure 4.1: A different representation of dipole molecules. The red region is slightly negative, and the blue region is slightly positive.
It is important to remember that just because the bonds within a molecule are polar, the molecule itself may not necessarily be polar. The shape of the molecule may also affect its polarity. A few examples are shown in Table 4.1 to refresh your memory. Note that we have shown tetrahedral molecules with all the terminal atoms at \(\text{90}\)\(\text{°}\) to each other (i.e. flat or 2-dimensional), but the shape is really 3-dimensional.
Molecule
Chemical formula
Bond between atoms
Shape of molecule
Polarity of molecule
Hydrogen
\(\text{H}_{2}\)
Non-polar convalescent
Non-polar
Hydrogen chloride
\(\text{H Cl}\)
Polar convalescent
Polar
Carbon tetra fluoride
\(\text{CF}_{4}\)
Polar convalescent
Non-polar
Fluorite-methane
\(\text{CHG}_{3}\)
Polar convalescent
Polar
Table 4.1: Polarity in molecules with different atomic bonds and molecular shapes.
Types of inter molecular forces (ESBMN)
It is important to be able to recognize whether the molecules in a substance are polar or non-polar because this will determine what type of inter molecular forces there are. This is important in explaining the properties of the substance.
Ion-dipole forces As the name suggests, this type of inter molecular force exists between an ion and a dipole (polar) molecule. You will remember that an ion is a charged atom, and this will be attracted to one of the charged ends of the polar molecule. A positive ion will be attracted to the negative pole of the polar molecule, while a negative ion will be attracted to the positive pole of the polar molecule. This can be seen when sodium chloride (\(\text{Na Cl}\)) dissolves in water. The positive sodium ion (\(\text{Na}^{+}\)) will be attracted to the slightly negative oxygen atoms in the water molecule, while the negative chloride ion (\(\text{Cl}^{-}\)) is attracted to the slightly positive hydrogen atoms. These inter molecular forces weaken the ionic bonds between the sodium and chloride ions so that the sodium chloride dissolves in the water (Figure 4.2).
