Chemistry · Periodic Classification Of Properties · NEET
Isoelectronic means 'equal electrons'. Two or more species are isoelectronic if they contain the exact same number of electrons, even though their atoms are different. Example: Na+ has 10 electrons (11 protons minus 1 lost electron) and F- also has 10 electrons (9 protons plus 1 gained electron). Same electron count = isoelectronic. The number of protons does NOT have to match.
Start from the atomic number (that gives the neutral atom's electrons), then adjust for charge. For a positive ion (cation), SUBTRACT the charge because it lost electrons: Mg2+ = 12 - 2 = 10 electrons. For a negative ion (anion), ADD the charge because it gained electrons: O2- = 8 + 2 = 10 electrons. If two species land on the same number, they are isoelectronic.
All of them have the same electrons, so the only thing that changes the size is the number of protons (nuclear charge Z). More protons pull the same electron cloud inward, making a smaller ion. So write them in order of Z. The species with the FEWEST protons is the LARGEST, and the one with the MOST protons is the SMALLEST.
These are all isoelectronic (each has 10 electrons, the Neon configuration). Now compare protons: N=7, O=8, F=9, Na=11, Mg=12, Al=13. Fewest protons = largest, most protons = smallest. So size order (largest to smallest): N3- > O2- > F- > Na+ > Mg2+ > Al3+. The anion N3- is biggest; the cation Al3+ is smallest.
Because cations come from metals with MORE protons and anions come from non-metals with FEWER protons. In an isoelectronic set the electron count is fixed, so the species with more protons (the cations) hold the electrons more tightly and shrink, while the species with fewer protons (the anions) hold them loosely and stay large. That is why anions sit at the big end and cations at the small end.
Yes. Count electrons: Ar = 18, K+ = 19 - 1 = 18, Cl- = 17 + 1 = 18, Ca2+ = 20 - 2 = 18, S2- = 16 + 2 = 18. All have 18 electrons (the Argon configuration), so they are isoelectronic. This exact set was tested in NEET 2025.
No, and this is a common trap. Charge only helps AFTER you confirm the species are isoelectronic. If the electron counts are different, you cannot just compare charges. First check they have the same electrons, THEN order by proton number (Z). A 3+ ion in one set is not automatically smaller than a 1- ion in a different set.
The element expected to form the largest ion to achieve the nearest noble gas configuration is:
In which of the following options the order of arrangement does NOT agree with the variation of property indicated against it?
Which of the following statements are true? A. Unlike Ga (very high melting point), Cs has a very low melting point. B. On the Pauling scale, electronegativity of N and Cl are not the same. C. Ar, K+, Cl-, Ca2+ and S2- are all isoelectronic species. D. First ionization enthalpy order of Na, Mg, Al, Si is Si > Al > Mg > Na. E. Atomic radius of Cs is greater than that of Li and Rb.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. They only need the same number of electrons. Their charges are usually different, for example O2-, F-, Na+ and Mg2+ all have 10 electrons but carry charges from -2 to +2.
More nuclear charge (more protons) means a smaller ion. So arrange by proton number Z: fewest protons is the biggest, most protons is the smallest.
Yes. A neutral atom counts too. For example, neutral Ar (18 electrons) is isoelectronic with K+, Cl-, Ca2+ and S2-, which also have 18 electrons each.
NEET repeatedly asks you to order ions by size or spot the isoelectronic set (2016, 2023, 2025). It is a fast, guaranteed mark if you remember: same electrons, then count protons. No calculation is needed.
Yes. CN- has 6 + 7 + 1 = 14 electrons and CO has 6 + 8 = 14 electrons, so they are isoelectronic. Molecules and polyatomic ions can be isoelectronic too, not just single atoms.
