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"Arrange in increasing order of ionic radius" is one of the most common one-mark questions in chemistry, and it is easy to get right only most of the time. The trouble comes when the list mixes ions with different numbers of electrons, or slips in a noble gas atom. This post gives a reliable method, real numbers to check it against, and the traps that catch students.
What does isoelectronic mean?
Species are isoelectronic when they have the same number of electrons. NCERT's example in Class 11 Chemistry Unit 3 is O²⁻, F⁻, Na⁺ and Mg²⁺, which all have 10 electrons. Their radii differ, NCERT says, "because of their different nuclear charges". The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus both list atomic and ionic radii under periodicity.
Why does more nuclear charge mean a smaller ion?
Every ion in the ten-electron series has the same arrangement, 1s² 2s² 2p⁶. The electrons repel one another in the same way in each. The only thing that changes is the number of protons pulling on them, from 7 in N³⁻ to 13 in Al³⁺. More pull on the same electron cloud makes it smaller.
You can put numbers on the pull with Slater's rules, from J. C. Slater's 1930 paper on atomic shielding constants, which set out simple rules for estimating the sizes of atoms and ions. For a 2p electron in a 1s² 2s² 2p⁶ ion:
The other seven electrons in the n = 2 shell each shield 0.35: 7 × 0.35 = 2.45.
The two 1s electrons each shield 0.85: 2 × 0.85 = 1.70.
Total shielding S = 4.15, the same for every ion in the series.
Effective nuclear charge Z_eff = Z − 4.15.
That gives Z_eff = 3.85 for O²⁻, 4.85 for F⁻, 6.85 for Na⁺, 7.85 for Mg²⁺ and 8.85 for Al³⁺. Shielding is fixed, so Z_eff rises one-for-one with Z.
Worked example: rank the ten-electron series
Question. Arrange N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ in decreasing order of radius.
Check they are isoelectronic: 7 + 3, 8 + 2, 9 + 1, 11 − 1, 12 − 2 and 13 − 3 all give 10 electrons.
Rank by Z: N (7) < O (8) < F (9) < Na (11) < Mg (12) < Al (13).
Largest Z means smallest ion, so: N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺.
Check it against data. In Shannon's 1976 table of effective ionic radii, read from the Imperial College database for four-coordinate ions, the values are N³⁻ 146 pm, O²⁻ 138, F⁻ 131, Na⁺ 99, Mg²⁺ 57 and Al³⁺ 39 pm. The order matches.
The steps are not equal in size, so do not try to predict exact radii from Z alone. The method gives the order, and the order is what the question asks for.
What about ions from the next period?
The same logic works for the 18-electron series S²⁻, Cl⁻, K⁺ and Ca²⁺. Slater's shielding for a 3p electron is 7 × 0.35 + 8 × 0.85 + 2 × 1.00 = 11.25, again the same for all four. Shannon's six-coordinate radii are S²⁻ 184 pm, Cl⁻ 181, K⁺ 138 and Ca²⁺ 100 pm.
Now the trap. Mix the two series and the answer can surprise you. Rank F⁻, Na⁺, Cl⁻ and K⁺ by radius using six-coordinate values: Cl⁻ 181 > K⁺ 138 > F⁻ 133 > Na⁺ 102 pm. K⁺ is larger than F⁻ even though one is a cation and the other an anion, because K⁺ has a third shell of electrons. "Anions are bigger than cations" is only safe within one isoelectronic series.
Which mistakes cost marks?
Ranking by charge alone. Higher positive charge means smaller only when the electron count is the same. Across different series, the number of shells usually decides.
Including a noble gas atom. Ne is isoelectronic with Na⁺ and F⁻, but NCERT notes that noble gas radii are non-bonded radii and should be compared with van der Waals radii, not with covalent radii. Slotting neon into a list of ionic radii mixes two kinds of measurement, so treat such questions with care and follow your textbook's convention.
Mixing tables. NCERT quotes F⁻ as 136 pm and Na⁺ as 95 pm, while Shannon's six-coordinate values are 133 and 102 pm. Both put F⁻ above Na⁺, but never mix sources in one comparison.
"Full shells are equally stable, so the ions are the same size." In a UK survey of more than 300 college students, Keith Taber (2003) found that many agreed with statements about the "unconditional stability" of species with full shells. A full shell does not fix size; nuclear charge does.
Our post on periodic trends and their exceptions covers the related trends in ionisation enthalpy and electron gain enthalpy.
When you can rank a list but cannot say why, the Fermi Problem Library has a step-by-step solver that explains the reasoning, so you can check the logic and not only the final order.
A practice routine
Write out both isoelectronic series from memory with electron counts.
Calculate Slater's Z_eff for each species in one series.
Make five mixed lists (for example O²⁻, Na⁺, Cl⁻, Mg²⁺) and rank them, counting electrons and shells first.
Check your answers against one table only, and note any surprise in one sentence.
Then try the same reasoning on a harder case, the lanthanoid contraction.
