Answer:

IsoElectronicSpecies;−

☆ Those having same number of electrons \bf{(e^-)}(e−) but different nuclear charge forms iso-electronic series.

Examples,

1) O2−,F−,Mg2+ have 10 electrons. Hence, they are iso-electronic species.

2) K+,Ca2+,S2−,Cl− have 18 electrons. Hence, they are iso-electronic species.

3) Ne,F− have 10 electrons. Hence, they are iso-electronics.

4) N3−,Al3+,Mg2+ have 10 electrons. Hence, they are iso-electronics.

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♕ Now we take one the above example and clarify it.

✒ We take second example.

SpeciesK+Ca2+S2−Cl−Z19201617e−18181818

Z denotes the total number of electrons are present in that element.

e− denotes the number of electrons are present after gain/loss of valency electrons