In atomic physics, a two-electron atom or helium-like ion is a quantum mechanical system consisting of one nucleus with a charge of Ze and just two electrons. This is the first case of many-electron systems where the Pauli exclusion principle plays a central role.
It is an example of a three-body problem.
The first few two-electron atoms are:
| Z=1: | H− | hydrogen anion |
| Z=2: | He | helium atom |
| Z=3: | Li+ | lithium ion |
| Z=4: | Be2+ | beryllium ion |
| Z=5: | B3+ | boron ion |