A hydrogen-like atom/ion (usually called a "hydrogenic atom") is any atomic nucleus bound to one electron and thus is isoelectronic with hydrogen. These atoms or ions can carry the positive charge e ( Z − 1), where Z is the atomic number of the atom.

Examples of hydrogen-like atoms/ions are hydrogen itself, He+, Li2+, Be3+ and B4+. Because hydrogen-like atoms/ions are two-particle systems with an interaction depending only on the distance between the two particles, their (non-relativistic) Schrödinger equation can be solved in analytic form, as can the (relativistic) Dirac equation. The solutions are one-electron functions and are referred to as hydrogen-like atomic orbitals.

Other systems may also be referred to as "hydrogen-like atoms", such as muonium (an electron orbiting an antimuon), positronium (an electron and a positron), certain exotic atoms (formed with other particles), or Rydberg atoms (in which one electron is in such a high energy state that it sees the rest of the atom practically as a point charge).

 

Note:   The above text is excerpted from the Wikipedia article Hydrogen-like atom, which has been released under the GNU Free Documentation License.