Krypton atoms cluster together in stable clumps of 13, 55, 147, 309, 561,... atoms.
Ordinarily, krypton, being a noble gas, does not enter into any combinations with other atoms - even other krypton atoms. However, P. Lethbridge and T. Stace, at the University of Sussex, have coaxed krypton atoms to cluster together in large, crystal-like clumps with icosohedral symmetry; that is each clump possesses 20 regular faces. The coaxing occurs when gaseous krypton trickles into a vacuum chamber through a hole only 200 micrometers in diameter. The expansion of the gas cools it so that when krypton atmos collide, relative velocities are low, and the weak Van de Waals forces between the atoms are sufficient to hold the clumps together.
So far, clumps of 147 and 309 atoms have been detected with a mass spectrograph. One theory of atomic "pack ing" predicts clumps should have "magic numbers" of 13, 55, 147, 309, 561, 923 .... So far, the "magic" has been working!
(Baggott, Jim; "Krypton Atoms Cling Together in 'Shells,'" New Scientist, p. 31, March 3, 1990.)
Comment. One would anticipate that the smaller clumps of 13 and 55 atoms would be easier to assemble.