单原子气体是物理及化学上的名词，是指只由一个原子组成的气体，所有的化学元素在相当高温下都是单原子气体。

单原子气体的运动只需考虑移动（电子激发在室温不需考虑），因此在绝热过桯下，单原子气体的理想热容比（C_p/C_v）为5/3，和双原子气体需考虑旋转（但室温不需考虑振动）的情形不同。对于理想的单原子气体：

其中R为气体常数。

In physics and chemistry, monatomic is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a monatomic gas is one in which atoms are not bound to each other. All chemical elements will be monatomic in the gas phase at sufficiently high temperatures.