Intermolecular forces: a basic, ground-up approach

academics chemistry intermolecular forces
By Alex H.

Intermolecular forces (IMFs) are “electrostatic” interactions between molecules – a result of all the charges floating around and interacting in the system. IMFs influence the properties of substances that we can observe and interact with – for example, the phase of the substance or its boiling point. 

IMFs are generally divided into two categories: van der Waals forces (permanent dipole-dipole interactions and London dispersion forces between temporary (instantaneous) dipoles) and hydrogen bonding.

1 – What is a permanent dipole? How do they interact?

Atoms interact and form bonds because they want to become “happier” – that is, more stable. This really boils down to electronegativity – how much an atom wants to get more electrons. If 2 atoms have similar electronegativities, the most stable situation for both atoms is to roughly share the electron density between them. If 2 atoms have very different electronegativities, the most stable situation would be for the less electronegative atom to give its electron density to the more electronegative atom.

Screen Shot 2023-10-11 at 9.41.59 AM

That pull of electron density from a less electronegative atom to a more electronegative atom results in a “dipole”. 

Screen Shot 2023-10-11 at 9.42.36 AM

The presence of dipoles in a molecule doesn’t mean that the molecule is polar! Dipoles can actually cancel each other out, and molecules must have an overall, net dipole moment in order to be considered “polar”. Whether or not the dipoles cancel is dependent on the geometry of the molecule, which can be determined based on VSEPR rules. 

Screen Shot 2023-10-11 at 9.43.06 AM

We can think more about how dipoles can cancel each other out by starting with methane and sequentially substituting H with Cl:

Screen Shot 2023-10-11 at 9.43.33 AM

Permanent dipoles will attract each other like magnets – the δ+ part of one molecule to the δ- part of another. These are the van der Waals dipole-dipole interactions.

2 – What is a temporary (instantaneous) dipole? How do they interact?

Electrons are not static – they’re constantly moving around within the electron cloud densities around the molecule. That’s why we often refer to “electron density” rather than directly saying “electrons”. 

As electrons are moving around, it’s natural for there to be instants in which there is a temporary concentration of electrons. Imagine shaking around a box filled with marbles – if you took a million snapshot images of the box, you’d observe some photos in which all the marbles are concentrated at one part of the box. These temporary densities and lack thereof of electrons (marbles) create temporary (instantaneous) dipoles. Because these temporary dipoles occur simply as a result of electrons moving around and not due to any other more specific structure or character of the molecule, any molecule can experience temporary dipoles.

These instantaneous dipoles in turn can create induced dipoles in other molecules (polar or nonpolar). It can be helpful to think about electron densities as fluffy clouds with negative charge that can be repelled by other negative charges. If the δ- part of a Molecule A possessing an instantaneous dipole approaches the fluffy density cloud of Molecule B, the cloud will be repelled and Molecule B will now experience an induced temporary dipole. Molecule A and Molecule B will then experience a temporary dipole-temporary dipole interaction, termed a “London dispersion force”. Since all molecules can experience temporary dipoles, all molecules experience London dispersion forces. Temporary dipole interactions are the weakest of the IMFs. 

Different molecules will experience different degrees of inducibility, depending on the “fluffiness” of the cloud. The more tightly a nucleus holds on to its electrons, the less fluffy the cloud and the less inducible the dipole. How tightly a nucleus holds on to its electrons is related to how far the electrons are from the nucleus – farther electrons experience less pull from the nucleus thanks to electron shielding and the inverse square nature of Coulomb’s law.

3 – What is hydrogen bonding?

Hydrogen “bonding” is a bit of a misnomer. It is referred to as a “bond” because it is such a strong interaction, but it is still an interaction, not a bond.

Only nitrogen, oxygen, and fluorine are electronegative enough to participate in these types of reactions. However, the presence of N, O, and F are not sufficient. For two molecules to participate in hydrogen bonding, 

  1. the first molecule must have a hydrogen bound to N, O, or F; and
  2. the second molecule must have a lone pair of electrons on N, O, or F.

 

Screen Shot 2023-10-11 at 9.44.43 AM

Because H bonding is such a strong interaction, it can result in large changes in properties such as boiling point. This is, for example, why water has a much higher boiling point than we would expect for a dihydrogen chalcogen. H2O is the only dihydrogen chalcogen that can participate in hydrogen bonding, and that interaction is strong enough that it significantly increases the amount of energy it requires to break the IMFs and boil the water.

Screen Shot 2023-10-11 at 9.45.10 AM

 

Alex is currently pursuing a PhD in Inorganic Chemistry at MIT. She previously earned her BS in Chemistry at CalTech, where she was recognized with the Merck Index Award.

Comments

topicTopics
academics study skills MCAT medical school admissions SAT college admissions expository writing strategy English MD/PhD admissions writing LSAT physics GMAT GRE chemistry biology math graduate admissions academic advice interview prep law school admissions ACT language learning test anxiety premed career advice MBA admissions personal statements homework help AP exams creative writing MD test prep study schedules computer science Common Application mathematics summer activities history secondary applications philosophy organic chemistry economics research supplements grammar 1L PSAT admissions coaching dental admissions law psychology statistics & probability legal studies ESL CARS PhD admissions SSAT covid-19 logic games reading comprehension calculus engineering USMLE mentorship Spanish parents Latin biochemistry case coaching verbal reasoning AMCAS DAT English literature STEM admissions advice excel medical school political science skills French Linguistics MBA coursework Tutoring Approaches academic integrity astrophysics chinese dental school gap year genetics letters of recommendation mechanical engineering units Anki DO Social Advocacy algebra art history artificial intelligence business careers cell biology classics data science diversity statement geometry kinematics linear algebra mental health presentations quantitative reasoning study abroad tech industry technical interviews time management work and activities 2L AAMC DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay athletics business skills cold emails fellowships finance first generation student functions graphing information sessions international students internships logic networking poetry proofs resume revising science social sciences software engineering trigonometry writer's block 3L Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian JD/MBA admissions Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser burnout campus visits cantonese capacitors capital markets central limit theorem centrifugal force chem/phys chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function extracurriculars freewriting genomics harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports letter of continued interest linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics