Wikipedia
Not to be confused with Isoelectric Point/ Focusing or the baseline of the Electrocardiogram the Isoelectric Line
Isoelectronicity is when Two or more molecular entities ( atoms, molecules, or ions) Have the same number of electrons or a similar electron configuration and the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.
The term valence isoelectronic is used when these molecular entities have the same number of valence electrons or a similar electron configuration, but may have a different number of atoms or a different bonding.
The statement "These compounds or molecules are isoelectronic" is not just an implementation of the above definition. It has significance by the fact that calculations on molecules and electron density, and therefore capability of reaction, have been performed on many common substances. Identifying a new, rare or odd compound as being isoelectronic with an already known one offers clues to possible properties and reactions.