幻数（英语：Magic Number）是指原子核中质子数和中子数的某个特定数值，当质子数或中子数为幻数，或者二者取值均为幻数时，原子核显示出很高的稳定性。

目前已经确认的幻数有：2、8、20、28、50、82、126这七个幻数。（OEIS中的数列A018226）

自然界广泛存在的氦、氧、钙、镍、锡、铅元素的质子或中子数分别与2到82的幻数相对应。质子数为126的元素目前尚未发现。当原子核之质子数和中子数皆为幻数时，这样的情况称为双幻数。例如自然界存在质子数82、中子数126的铅同位素Pb，就具有双幻数，显得异常稳定。

In nuclear physics, a magic number is a number of nucleons (either protons or neutrons) such that they are arranged into complete shells within the atomic nucleus. The seven most widely recognized magic numbers as of 2007 are 2, 8, 20, 28, 50, 82, and 126 (sequence A018226 in OEIS). Atomic nuclei consisting of such a magic number of nucleons have a higher average binding energy per nucleon than one would expect based upon predictions such as the semi-empirical mass formula and are hence more stable against nuclear decay.