In a previous article I discussed the following ways atoms bond:

1. Covalent Bonds: bond by sharing valence electrons equally between them, 
2. Polar Covalent Bonds: where the valence electrons are still shared between atoms but electrons will tend to hang around the more electronegative atom creating partial charges for each atom, 
3. Ionic Bonds: wherein the more electronegative atom completely takes away one or more valence electrons from the less electronegative atom creating a negatively charged atom and a positively charged atom that bond together through differences in charge. 

Covalent and polar covalent bonding are very strong, meaning it takes a lot of energy to separate the atoms. Ionic bonds are weaker, as they are only bonded through differences in charge. Ionic bonds could arguably fit into this article on weak interactions. They are broken up by other polar atoms, such as when salt, which is sodium and chlorine, is placed in water. The positively charged sodium will bond to the negatively charged oxygen and the negatively charged chloride (chlorine negatively charged ion, just add the -ide at the end of the name) will bond to the more positively charged hydrogen. There are three other types of bonding interactions that are relatively weaker than the covalent bonds that were not discussed in previous articles. They are the Van der Wall interactions, the Dipole-Dipole Interactions, and the Hydrogen Bonds.

Electrons move around the nucleus. They don’t as much orbit around the nucleus as they do move into areas where they are likely to be found, but as the electrons move around the nucleus there is a chance that all of or most of the electrons accumulate on one side, creating a difference in charge. Let’s say we are dealing with an atom called atom A, atom A has four total electrons and as those electrons move around the nucleus all four of them end up on the eastern side of the atom. This causes the eastern side to become more negatively charged and the western side to become more positively charged. This difference in charge on the atom allows for interactions with other atoms due differences in charge.

 This interaction between atoms is known as Van der Waal Interaction or the London Dispersion Force. It was discovered by Dutch Physicist Johannes Diderik Van der Waal in 1873.

The next weak interaction we will talk about is the Dipole-Dipole Interaction. It is very common knowledge and it was also stated earlier in the article that opposite charges attract and like charges repel. In many molecules, in which electron interactions allow the atoms to bond, the electrons are not shared equally amongst the atoms as one of the atoms is more electronegative than the other atom or atoms. Electronegativity is defined as an atom’s affinity for electrons and many times when atoms bond, electrons are not shared equally as the electrons will hang around the more electronegative atom more than the less electronegative atom. This will create a partial difference in charge between the atoms. On one part of the molecule, there will be a partial positive charge on the other part of the molecule and a partial positive charge on another part of the atom. This will allow for interactions with other molecules. Let’s look at the interaction between two hydrogen chloride (HCL) molecules, which consist of one hydrogen and one chlorine atom in a very polar covalent bond (polar just means that the bond has a partial difference in electrical charge). 

As you can see, the difference in charge on either side of the molecule allows for an interaction between the two molecules.

The last weak interaction to discuss is the hydrogen bond. Hydrogen bonding is yet another atomic interaction that results from differences in electrical charge. It occurs when a slightly positive hydrogen atom of a polar covalent molecule is attracted to a partial negative charge of a nearby polar covalent bond. Now, it may seem as though we have already discussed this in the dipole-dipole interaction and hydrogen bonds are very similar to dipole-dipole interactions, but they allow for very interesting properties of the substances they make up. Look at water for example, water is made entirely of hydrogen bonds. We all know that water is made up of two hydrogens and one oxygen. Oxygen is much more electronegative than hydrogen. That means oxygen wants electrons more than hydrogen, so when the oxygen and hydrogen bond, the oxygen ends up with a partial (denoted with lowercase delta as shown in the figure above) negative charge and hydrogen ends up with a partial positive charge. This difference in charge allows for interactions between other water molecules due to the difference in electric charge. 

Source: https://courses.lumenlearning.com/cheminter/chapter/hydrogen-bonding/

One last thing to mention, I have written about three weak interactions that can occur between molecules. It would be important to note that everything mentioned is a dipole-dipole interaction. London dispersion forces induce temporary dipole-dipole interactions through the random movement of electrons. Hydrogen bonds, due to the electronegativity difference between bonded atoms, are a type of permanent dipole-dipole interactions. The only reason they are written separately from the dipole-dipole interactions paragraph is because they are a very special case of dipole-dipole interactions that are very common in many chemical reactions and have very special properties.