Chemistry · Structure Of Atom · NEET
Only ONE thing matters: they must have the same number of electrons. It does not matter if they are atoms, cations, or anions, or which element they came from. If the electron count is equal, they are isoelectronic. Example: N3-, O2-, F-, Ne, Na+, Mg2+, Al3+ all have 10 electrons, so all seven are isoelectronic.
Start with the atomic number Z (that is the number of protons = electrons in a neutral atom). Then adjust for the charge. For a positive ion, SUBTRACT the charge (it lost electrons): Ca2+ = 20 - 2 = 18 electrons. For a negative ion, ADD the charge (it gained electrons): Cl- = 17 + 1 = 18 electrons. Simple rule: cation subtract, anion add.
No, and this is the most common mistake. They have the SAME electrons but DIFFERENT protons (different Z). That is exactly why Na+ (11 protons) and F- (9 protons) can both have 10 electrons yet be different sizes. Same electrons, different nuclear charge.
Yes. Na has Z = 11, and Na+ lost 1 electron, so it has 10 electrons. Mg has Z = 12, and Mg2+ lost 2 electrons, so it also has 10 electrons. Both have 10 electrons, so they are isoelectronic. They match the neon (Ne) configuration.
Yes. You just add up all the electrons in the whole species. CO and N2 both have 14 electrons, so they are isoelectronic. CO3^2- and NO3^- both have 32 electrons, so they are isoelectronic (and also the same shape). NEET sometimes tests this molecular version, so do not think it is only for single ions.
When electrons are equal, size depends only on nuclear charge (protons, Z). More protons pull the electrons in tighter, so a HIGHER Z means a SMALLER ion. In the 10-electron family: N3- (Z=7) is the largest and Al3+ (Z=13) is the smallest. Order of size: N3- > O2- > F- > Na+ > Mg2+ > Al3+.
Which one of the following represents all isoelectronic species?
From the following pairs of ions, which one is NOT an isoelectronic pair?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Isoelectronic species are atoms, ions, or molecules that have the same total number of electrons. For example, O2-, F-, Na+, and Mg2+ each have 10 electrons, so they are isoelectronic.
Yes. Neon (Ne) has 10 electrons, and Na+ = 11 - 1 = 10 electrons. Both have 10 electrons, so they are isoelectronic. Na+ has the neon electron configuration.
Ar, K+, Ca2+, Cl-, and S2- all have 18 electrons, so they are isoelectronic. This 18-electron family is a very common NEET trap set.
Isotopes are atoms of the SAME element with the same protons but different neutrons (like C-12 and C-14). Isoelectronic species are DIFFERENT species with the same number of ELECTRONS but usually different protons. Do not mix the two.
Because they have different numbers of protons (nuclear charge). More protons pull the same electrons closer, making the ion smaller. So higher atomic number means smaller size in an isoelectronic series.
