A structure in which the nuclei of two atoms approach each other closely and their electrons are arranged in atomic orbitals characteristic of a single atom of atomic number equal to the sum of the nuclear charges. Quasiatoms can be formed for short times in atom-atom and ion-atom collisions when the nuclei are much closer than the mean orbital radius of the innermost K-shell electrons. The electrons are then bound in the electric field of both nuclear charges Z₁ and Z₂, which resembles the spherically symmetric 1/r² Coulomb field of a single united atom having charge Z_ua = Z₁ + Z₂.