Fullerene or C₆₀ is soccer-ball-shaped or I_h with 12 pentagons and 20 hexagons. According to Euler's theorem these 12 pentagons are required for closure of the carbon network consisting of n hexagons and C₆₀ is the first stable fullerene because it is the smallest possible to obey this rule. In this structure none of the pentagons make contact with each other. Both C₆₀ and its relative C₇₀ obey this so-called isolated pentagon rule (IPR). The next homologue C₈₄ has 24 IPR isomers of which several are isolated and another 51,568 non-IPR isomers. Non-IPR fullerenes have thus far only been isolated as endohedral fullerenes such as Tb₃N@C₈₄ with two fused pentagons at the apex of an egg-shaped cage or as fullerenes with exohedral stabilization such as C₅₀C_l10 and reportedly C₆₀H₈.