Introduction
“Opposites attract.” Probably one of the best-remembered phrases of science, since it is simple and easily demonstrable by magnets. But, little do people know how much this simple phenomenon constitutes our known world. We are going to assume that you know the basic properties of atoms and their structures. Molecules are the product of two or more atoms binding together through their electrons. Regarding molecules, there are two different types of forces that work in and out of the molecule; intramolecular attraction and intermolecular attraction.
Intramolecular Forces
Intramolecular attractions are forces that occur within a molecule. This can be further distinguished into covalent, ionic and metallic bonds. These bonds are classified by the difference in the behaviour of their electrons.
Covalent Bond
Ionic Bond
▲(Covalent Bond)
The image on the left (not drawn to scale) represents a covalent bond in which two atoms are sharing electrons, while the image on the right (also not drawn to scale) represents an ionic bond in which an atom takes an electron from another atom. The types of these bonds are determined from the combination of atoms that become molecules. An ionic bond is formed between a nonmetal and metal and the covalent bond is formed between a nonmetal and another nonmetal. Electronegativity deserves an article of its own, so let’s understand electronegativity as an element’s strength to pull on electrons for now. Each element has a different number of electronegativity and depending on this number, the location of the electron in the bond is determined. In the covalent bond of H2on the left, since they are both the same element, the electronegativity is the same. Thus, the electrons are pulled equally and end up in the middle where they share the electrons. But, for the ionic bond of NaClon the right, since the electronegativity of the chlorine atom is much higher than that of the sodium atom, the electron is pulled off of the sodium atom into the valence shell of the chlorine atom.
▲(Ionic Bond)
Metallic Bond
▲(Metallic Bond)
Finally, we have the metallic bond (as shown by the diagram on the bottom left which is; by now everyone should know, is not drawn to scale), in which the same metallic elements group together. In this metallic bond, the nucleus of the atoms are relatively rigid, but the electrons in the valence shell are delocalised and can swim around freely in the metal formation. Metallic bonds are strong and that is why metals are hard to break and have high melting and boiling points.
Each bond displays strength in different situations and it cannot be determined unconditionally the order of strength. But, typically their strengths are classified from strongest to least as covalent, ionic, and then metallic.
Intermolecular Forces
Hydrogen Bonding
▲(Hydrogen Bond)
Intermolecular forces are the attraction between two or more molecules. They are hydrogen bonding, dipole-dipole forces and London dispersion forces. The hydrogen bonding is the strongest of these three forces and happens due to the polarity of the molecules regarding hydrogen atoms. Hydrogen bonding occurs when hydrogen bonds with nitrogen, oxygen and fluorine, and then interacts with another nitrogen, oxygen or fluorine atom. Although these bonds are considered covalent bonds, they are things called polar covalent bonds in which the electrons are shared, but are heavily attracted to one atom than the other. As shown in the diagram on the left (simplified actual phenomenon not drawn to scale), since the electrons in the HObond are pulled closer to the oxygen atom, the hydrogen atoms become slightly positive in charge, while the oxygen becomes slightly negative. So, if two such molecules come into contact with one another, the negatively polar part of the molecule creates a bond between the positively polar part of the other molecule; a.k.a. the hydrogen bond.
Dipole-Dipole Forces
The dipole-dipole force is the next strongest force that happens between two molecules with polarity. The diagram on the right shows two HCl molecules interacting with a dipole-dipole force. Although the HCl molecule does not have an ionic bond, the difference in electronegativity is great enough to create something called a polar covalent bond. This bond is an intermediate stage between covalent and ionic, which is represented by the symbol of δ which means partial charge. This dipole-dipole force is similar to that of the hydrogen bond, in which the two molecules with polarity face their positively polar part of the molecule to a negatively polar part of another molecule. This dipole-dipole effect can happen between a molecule and an ion (ion dipole force) or between a polar molecule and a nonpolar molecule (induced dipole force). The ion dipole force is when a polar molecule is attracted to an ion. The induced dipole force is when a polar molecule interacts with a nonpolar molecule and the electrons in the polar molecule move according to the polarity of the polar molecule. So if an HCl molecule (polar covalent) comes into contact with CO2 (covalent) and the H part of the molecule comes near CO2, the electrons of the CO2 will go near the H part of the HCl molecule and create induced polarity; thus the name induced dipole force.
▲(Dipole-Dipole Forces)
London Dispersion Force
▲(London Dispersion Force)
The diagram on the left represents a London dispersion force which is the weakest of the three forces. This attraction happens only momentarily as the electrons of the atoms or molecules are spinning around the atom or molecule. As the electrons in a molecule or atom spin around, it creates momentary partial charge. These momentary partial charges interact with other momentary partial charges to pull and push away from each other momentarily, and that is what is known as the London dispersion force. This force exists in every molecule or atom.
In Real Life
The combination of these forces determine the boiling points, melting points and other characteristics of matter. All matter can exist in the three stages of solid, liquid and gas. Matter that are gases in room temperature like N2 have little to no intermolecular forces that keep them from sticking together, and that’s why they have a very low boiling point. Water with the strongest hydrogen bond have a lot of attraction within themselves. That is what we call surface tension. This surface tension is what prevents bugs like Water striders from falling into the water or your soap bubble from popping! So the next time you look at a cup of water or any object, imagine them as tiny particles that are hugging closely together.
[1] LibreTexts. 2020. 11.2: Intermolecular Forces. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/11%3A_Liquids_and_Intermolecular_Forces/11.2%3A_Intermolecular_Forces. 30th Nov, 2019.
[2] AKlectures. Intramolecular and Intermolecular Forces. https://aklectures.com/lecture/fundamentals-proteins/intramolecular-and-intermolecular-forces. 30th Nov, 2019.
[3] Lumen. The Ionic Bond. https://courses.lumenlearning.com/trident-boundless-chemistry/chapter/the-ionic-bond/. 30th Nov, 2019.
[4] Clark, Jim. 2000. The metallic bond in molten metals. https://www.chemguide.co.uk/atoms/bonding/metallic.html. 30th Nov, 2019.
[5] SOPHIA Learning. Dipole-Dipole Forces. https://www.sophia.org/tutorials/dipole-dipole-forces. 30th Nov, 2019.
[6] Markgraf Bert. What are London Dispersion Forces?. SCIENCING https://sciencing.com/what-are-london-dispersion-forces-13710443.html. 30th Nov, 2019.
[7] BBC. Covalent Bonding. https://www.bbc.co.uk/bitesize/guides/zcvy6yc/revision/1. 30th Nov, 2019.
English 
Japanese