What does it mean to be isoelectronic?
Having the same configuration of electrons
Examples of particles that are isoelectronic would be:
an argon atom and a (-1) chloride ion. Both have 18 electrons, arranged in the same numbers in the same shells.
a water molecule and a hydronium ion are also isoelectronic. In both cases, we see eight electrons arranged about the oxygen atom, with hydrogen atoms bonded to this central oxygen.