Isoelectronic species are:

Atoms, ions, or molecules that have the same number of electrons.

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🔹 Examples

• N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺

All of these have 10 electrons → so they are isoelectronic

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How to identify?

Just count total electrons

Example:

• O²⁻ → 8 + 2 = 10 electrons
• F⁻ → 9 + 1 = 10 electrons
• Na⁺ → 11 − 1 = 10 electrons

✔ Same electrons → Isoelectronic

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🔹 Important Trend (VERY IMPORTANT for JEE/NEET)

In an isoelectronic series:

Size decreases as nuclear charge (Z) increases

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Example Order of Size

For species with 10 electrons:

N³⁻ > O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺

Reason:

• All have same electrons
• But increasing protons (Z) pull electrons closer → size decreases

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🔹 Key Exam Line

“Isoelectronic species have same number of electrons but different nuclear charge.”

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