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a relatively weak bond formed between molecules without sharing electrons. It can be more helpful to think of them more as attractions than “bonds” because they’re “easily” reversible. Just how easy depends on the strength of the attraction. The attraction comes from charges and/or partial charges liking each other. If the charges are full and “permanent” (eg. Na+ & Cl-) you can get an “ionic bond” (aka salt bridge). These are relatively strong, but still not as strong as covalent bonds, which involve actual sharing of electrons. At the weak end are “London dispersion forces” which involve temporary charges that randomly form because electrons in atoms move around a lot and sometimes they end up clustered together leading to transiently partly charged regions of the (overall neutral) molecule. This partial charge separation situation is referred to as a dipole, and London dispersion forces involve temporary dipoles. Sounds like these wouldn’t have much oomph, but they let geckos walk up walls! Noncovalent bonds may be individually weak, but when you have a lot of them each contributing it can really add up for some serious stickiness. In the middle strength-wise you have hydrogen bonds, which involve a hydrogen sharing electrons with a really greedy sharer (an electronegative atom) like oxygen or nitrogen, which causes it to be partially positive getting attracted to something electronegative that has a “lone pair” of electrons, which O & N often do. H-bonds are similar to those London forces, but they involve permanent dipoles. Dipole-dipole interactions & London forces are collectively referred to as van der Waals interactions, and H-bonds are a special form of dipole-dipole interactions. They’re not “really” special, they’re just defined by where their dipoles come from, and they just get their own name because they come up a lot in biochemistry.