What is a common characteristic of an isoelectronic species?

Isoelectronic species are defined as atoms or ions that have the same number of electrons, which means they share an identical electron configuration. This characteristic is fundamental because it allows for comparisons of their chemical behavior and bonding properties despite differing atomic numbers, or numbers of protons.

For example, when comparing nitrate ion (NO₃⁻) and molecular nitrogen (N₂), both have a total of ten electrons, making them isoelectronic. Despite being entirely different substances with unique chemical properties, their electron configurations are the same.

The other options do not accurately define isoelectronic species. Having the same number of protons refers to elements or different ions of the same element, but does not encompass the broader range of isoelectronic species. Atomic mass and the number of neutrons can vary significantly between isoelectronic species depending on their identity and their isotopes. Thus, the defining feature of isoelectronic species lies solely in their shared electron count.