What are Isoelectronic Species in Chemistry?

• Iso is a massive Greek prefix that means 'Equal' or 'Exactly the same'.
• Electronic obviously refers to massive 'Electrons'.
• Definition: Isoelectronic species are massive atoms, ions, or molecules that belong to completely different chemical elements but physically contain the exact same total number of electrons.

Let's look at completely different elements violently becoming 'Isoelectronic' by losing or heavily gaining electrons to reach the magic number 10 (like the noble gas Neon).

• Neon (Ne): A neutral massive noble gas. Atomic number is 10. It has exactly 10 electrons.
• Sodium Ion ($Na^+$): Sodium's normal atomic number is 11 (11 electrons). But it violently loses 1 massive electron to become stable. $11 - 1$ = 10 electrons.
• Magnesium Ion ($Mg^{2+}$): Magnesium's atomic number is 12. It violently loses 2 massive electrons. $12 - 2$ = 10 electrons.
• Fluoride Ion ($F^-$): Fluorine's atomic number is 9. It aggressively steals 1 massive electron. $9 + 1$ = 10 electrons.
• Result: Even though Ne, $Na^+$, $Mg^{2+}$, and $F^-$ are completely different chemicals, they are a massive Isoelectronic Series because they all magically possess exactly 10 electrons.

• Just because they have the exact same number of electrons does NOT mean they are the same physical size.
• The size depends entirely on the massive protons in the nucleus pulling those electrons.
• $Mg^{2+}$ has 12 massive protons violently pulling its 10 electrons inwards, so it physically shrinks and becomes the incredibly smallest.
• $F^-$ only has 9 weak protons trying to pull 10 electrons, so it remains massive and bloated.