How can atoms get very close together even without direct covalent bonds between them? [closed]








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Generally speaking, because of the Pauli exclusion principle, it’s difficult for the electron clouds of two atoms to overlap unless they are covalently bound together. The degree of overlap can be estimated with the van der Waals(vdw) radius. When the distance between two atoms is smaller than the sum of their vdw radii, the repulsion between them increases steeply.


However, atoms within the same molecules can be much closer than the vdw radii implied. For example, the half distance between the hydrogen atoms in CH4 is 0.89 angstrom, which is 74% of the vdw radius of hydrogen atoms. The half distances between the fluorine atoms and chlorine atoms in CF4 and CCl4 are 73% and 82% of their vdw radii, respectively (data sheet: C-H: 1.09, C-F: 1.32, C-Cl: 1.77, vdw H: 1.2, vdw F: 1.47, vdw Cl: 1.75). So how can these atoms get so close together despite the lack of covalent bonds between them? My guess is that vdw radius measures the distance between atoms of separate molecules. Because neutral molecules are bound together by the very weak dispersion force while in CX4 the four terminal atoms are tightly bound to the central carbon atom, they can overcome much larger repulsion between them.


My second question is whether there is a lower limit on how close atoms can get together. For example, C60F60 is much larger than CF4, which means the angles between adjacent C-F bonds is much smaller in C60F60. If the lengths of C-C bonds and C-F bonds are maintained, the distance between fluorine atoms may be unacceptably small, which may slightly stretch the C-C and C-F bonds to accommodate the fluorine atoms. Can we predict the stability of perhalogensted compounds (such as the instability of polytetrachloroethylene) by the atomic distances?


