Isoelectronic

When an atom, radical, or ion has an equal number of electrons or has the same number of electrons when compared to the atoms of another element, the two species are called isoelectronic.

Examples:

• Sodium is isoelectronic to neon when it loses one electron

When Na⁺=[He]2s²2p⁶ it is isoelectronic to Ne=[He]2s²2p⁶

• Chlorine is isoelectronic to argon when it gains an electron

When Cl^-=[Ne]3s²3p⁶ it is isoelectronic to Ar=[Ne]3s²3p⁶