Bond order is the number of bonding electron pairs shared between two atoms: one for a single bond, two for a double bond, and three for a triple bond. A higher bond order generally means a stronger, shorter bond. In a resonance hybrid the bond order may be fractional, since the bonding is spread over several structures.

键级是两个原子之间共享的成键电子对数:单键为一,双键为二,叁键为三。键级越高,键通常越强、越短。在共振杂化体中,由于成键分布于多个结构,键级可能为分数。