Two atoms are said to be isoelectronic if they have which of the following?

Isoelectronic atoms are defined by their having the same number of electrons, which allows them to exhibit similar electronic configurations and thus similar chemical properties. This concept is particularly useful in comparing atoms or ions of different elements that may gain or lose electrons to achieve a stable electronic arrangement. For instance, a neon atom (with 10 electrons) and a sodium ion (Na⁺, which has lost one electron and has 10 electrons) are isoelectronic because they both have 10 electrons.

In contrast, the number of protons in isoelectronic species can be different, leading to different elemental identities. The number of neutrons can vary as well, creating different isotopes of the same element without affecting the electronic configuration. Lastly, mass number is a result of both protons and neutrons, meaning isoelectronic atoms do not necessarily have the same mass number. Thus, the defining characteristic of isoelectronic species is that they share the same number of electrons.