On Arrangements of Six, Seven, and Eight Spheres: Maximal Bonding of   Monatomic Ionic Compounds
  Let $C(n)$ be the solution to the contact number problem, i.e., the maximum number of touching pairs among any packing of $n$ congruent spheres in $\mathbb{R}^3$. We prove the long conjectured values of $C(6)=12, C(7)=15$, and $C(8)=18$. The proof strategy generalizes under an extensive case analysis to $C(9)=21, C(10) = 25, C(11) = 29, C(12) = 33$, and $C(13) = 36$. These results have great importance for condensed matter physics, materials science, crystallography, organic and physical chemistry of interfaces. 
Samuel Reid
math.MG