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Isoelectronic Ion Flashcards: Electron Counts & Radius Ranking

Count electrons, find matching ion configurations, rank ionic radii, and repair common charge and ordering errors with 42 short chemistry flashcards.

By author Open Study Contributor

About this deck

Practice isoelectronic ions with 42 short English flashcards. Work with familiar monatomic ground-state ions in the 10-electron [Ne] and 18-electron [Ar] groups: N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺, P³⁻, S²⁻, Cl⁻, K⁺, and Ca²⁺. Atomic number Z is supplied whenever counting is the task.

The cards practice ion and Z → electron count, small list → matching-configuration group or outlier, confirmed isoelectronic pair or set → qualitative radius comparison, and constrained proton/electron counts → charge or ion symbol. Short error-repair cards address sign mistakes, equal-count/equal-size confusion, incorrect groupings, reversed orders, and skipped configuration checks. A few evidence-limit questions test what this rule can establish. Answers start with the result and give a brief explanation.

The sequence starts with charge bookkeeping and unchanged nuclei, introduces the 10-electron group, then brings in the 18-electron group before interleaving comparisons, constrained inverses, and repairs. Closely related variants are separated. Numerical radii, neutral noble-gas radius comparisons, full electron-configuration derivation, transition-metal and coordination cases, molecular species, broad periodic trends, isotope drills, and full-course or exam coverage are excluded to keep each decision short and well defined. Unconstrained radius → ion identification and exhaustive reverse/permutation cards are excluded because they do not provide one clear answer.

Original questions, explanations, organization, and metadata were written with AI assistance from common scientific facts, with factual checks against Purdue’s isoelectronic glossary, Purdue’s ionic-size lesson, and NIST’s atomic-number table. The cover is an original AI-generated conceptual illustration, not a particle-count or radius-scale diagram. CC0 1.0 applies to the original deck text, organization, metadata, and cover to the extent applicable rights exist. Scientific facts are not owned; referenced works retain their own rights. No source or competitor prose, exercises, tables, or diagrams are reproduced.

An independent learning resource with no claimed curriculum or exam alignment, affiliation, or endorsement.

Cards in this deck

  1. Card 1

    Question

    For a monatomic ion, how do atomic number Z and signed charge q give its electron count?

    Answer

    Electron count = Z − q. Use q = +2 for a 2+ ion and q = −2 for a 2− ion.

  2. Card 2

    Question

    When an atom forms a monatomic ion by gaining or losing electrons, what happens to its proton count?

    Answer

    It stays the same. Ion formation changes the electron count, not the element’s atomic number.

  3. Card 3

    Question

    Na⁺, Z = 11: electron count?

    Answer

    10 electrons. The 1+ charge means one fewer electron than the 11 protons.

  4. Card 4

    Question

    What does isoelectronic mean for the closed-shell monatomic ions compared here?

    Answer

    They have the same electron configuration. For these ground-state 10- or 18-electron ions, matching electron totals gives the matching configuration.

  5. Card 5

    Question

    O²⁻, Z = 8: electron count?

    Answer

    10 electrons. The 2− charge means two more electrons than the eight protons.

  6. Card 6

    Question

    A magnesium ion has 12 protons and 10 electrons. What is its signed charge?

    Answer

    2+. There are two more protons than electrons: 12 − 10 = +2.

  7. Card 7

    Question

    Cl⁻, Z = 17: electron count?

    Answer

    18 electrons. A 1− ion has one more electron than its proton count.

  8. Card 8

    Question

    O²⁻ (Z = 8) and Mg²⁺ (Z = 12) both have 10 electrons. Which has the smaller ionic radius?

    Answer

    Mg²⁺. With the same electron configuration, its greater proton count pulls the electron cloud closer.

  9. Card 9

    Question

    Al³⁺, Z = 13: electron count?

    Answer

    10 electrons. Subtract the positive charge: 13 − 3 = 10.

  10. Card 10

    Question

    Which ion has the same electron configuration as Na⁺ (Z = 11): F⁻ (Z = 9), K⁺ (Z = 19), or Ca²⁺ (Z = 20)?

    Answer

    F⁻. Both F⁻ and Na⁺ have 10 electrons; K⁺ and Ca²⁺ each have 18.

  11. Card 11

    Question

    A learner writes: “P³⁻ has 15 − 3 = 12 electrons.” With Z = 15, repair the calculation.

    Answer

    15 + 3 = 18 electrons. A negative charge adds electrons to the neutral count.

  12. Card 12

    Question

    A monatomic ion has more electrons than protons. What sign must its charge have?

    Answer

    Negative. The excess electrons give a net negative charge.

  13. Card 13

    Question

    N³⁻ (Z = 7) and Na⁺ (Z = 11) both have 10 electrons. Which has the larger ionic radius?

    Answer

    N³⁻. Its smaller proton count holds the same electron configuration less tightly.

  14. Card 14

    Question

    N³⁻, Z = 7: electron count?

    Answer

    10 electrons. Add three for the 3− charge: 7 + 3 = 10.

  15. Card 15

    Question

    Does the isoelectronic radius rule justify ranking F⁻ (Z = 9) and Cl⁻ (Z = 17) by proton count alone?

    Answer

    No. F⁻ has 10 electrons and Cl⁻ has 18, so their configurations differ. This rule alone does not establish their order.

  16. Card 16

    Question

    Which aluminium ion has 13 protons and 10 electrons? Give its symbol and charge.

    Answer

    Al³⁺. The three-electron deficit gives a 3+ charge.

  17. Card 17

    Question

    All three ions have 10 electrons. Rank O²⁻ (Z = 8), Mg²⁺ (Z = 12), and F⁻ (Z = 9) by increasing ionic radius.

    Answer

    Mg²⁺ < F⁻ < O²⁻. Increasing radius follows decreasing proton count within this series.

  18. Card 18

    Question

    S²⁻, Z = 16: electron count?

    Answer

    18 electrons. Add two for the 2− charge: 16 + 2 = 18.

  19. Card 19

    Question

    A learner says Na⁺ (Z = 11) and Al³⁺ (Z = 13) have equal radii because both have 10 electrons. Repair the conclusion.

    Answer

    Al³⁺ has the smaller radius. Equal electron configurations do not mean equal sizes; Al³⁺ has the greater nuclear charge.

  20. Card 20

    Question

    Which ions form the 18-electron group: P³⁻ (Z = 15), Mg²⁺ (Z = 12), Cl⁻ (Z = 17), and S²⁻ (Z = 16)?

    Answer

    P³⁻, Cl⁻, and S²⁻. Each has 18 electrons; Mg²⁺ has 10.

  21. Card 21

    Question

    O²⁻ (Z = 8) and F⁻ (Z = 9) both have 10 electrons. Which has the larger ionic radius?

    Answer

    O²⁻. Its eight protons attract the same electron configuration less strongly than the nine protons in F⁻.

  22. Card 22

    Question

    A phosphorus ion has 15 protons and 18 electrons. What is its signed charge?

    Answer

    3−. Three excess electrons give a charge of −3.

  23. Card 23

    Question

    For 10-electron ions, a learner gives this increasing-radius order: N³⁻ < Na⁺ < Al³⁺. Z values are 7, 11, and 13. Repair the order.

    Answer

    Al³⁺ < Na⁺ < N³⁻. The greatest proton count gives the smallest radius in this series.

  24. Card 24

    Question

    Ca²⁺, Z = 20: electron count?

    Answer

    18 electrons. Subtract two for the 2+ charge: 20 − 2 = 18.

  25. Card 25

    Question

    Do an ion’s proton and electron counts give its exact ionic radius in picometres?

    Answer

    No. They support a qualitative comparison within a matching-configuration series, not an exact radius value.

  26. Card 26

    Question

    K⁺ (Z = 19) and Ca²⁺ (Z = 20) both have 18 electrons. Which has the smaller ionic radius?

    Answer

    Ca²⁺. Its extra proton attracts the same electron configuration more strongly.

  27. Card 27

    Question

    A learner groups F⁻ (Z = 9) with Cl⁻ (Z = 17) as isoelectronic because both are 1−. Repair the grouping claim.

    Answer

    They are not isoelectronic. F⁻ has 10 electrons and Cl⁻ has 18; the same ionic charge does not guarantee the same configuration.

  28. Card 28

    Question

    A sulfur ion has 16 protons and 18 electrons. What is its signed charge?

    Answer

    2−. It has two more electrons than protons.

  29. Card 29

    Question

    All three ions have 18 electrons. Rank Cl⁻ (Z = 17), S²⁻ (Z = 16), and Ca²⁺ (Z = 20) by increasing ionic radius.

    Answer

    Ca²⁺ < Cl⁻ < S²⁻. Within the shared configuration, radius increases as proton count decreases.

  30. Card 30

    Question

    K⁺, Z = 19: electron count?

    Answer

    18 electrons. A 1+ charge removes one electron from the neutral count.

  31. Card 31

    Question

    A learner says Mg²⁺ has 12 electrons because magnesium keeps its 12 protons when it ionizes. Repair the electron count.

    Answer

    10 electrons. The 12 protons stay, while a 2+ charge means two fewer electrons.

  32. Card 32

    Question

    Which nitrogen ion has 7 protons and 10 electrons? Give its symbol and charge.

    Answer

    N³⁻. Three excess electrons give a 3− charge.

  33. Card 33

    Question

    Are N³⁻ (Z = 7) and F⁻ (Z = 9) isoelectronic in their ground states?

    Answer

    Yes. Each has 10 electrons and the same closed-shell configuration.

  34. Card 34

    Question

    All three ions have 18 electrons. Rank S²⁻ (Z = 16), K⁺ (Z = 19), and P³⁻ (Z = 15) by decreasing ionic radius.

    Answer

    P³⁻ > S²⁻ > K⁺. Within this series, the ion with the fewest protons has the largest radius.

  35. Card 35

    Question

    A calcium ion has 20 protons and 18 electrons. A learner labels it Ca²⁻. Repair the charge sign.

    Answer

    Ca²⁺. The ion has two more protons than electrons, so the charge is positive.

  36. Card 36

    Question

    Can you apply the isoelectronic radius rule to Mg²⁺ (Z = 12) and Ca²⁺ (Z = 20) just because both have charge 2+?

    Answer

    No. Mg²⁺ has 10 electrons and Ca²⁺ has 18. A different comparison is needed; sharing a charge does not make this rule applicable.

  37. Card 37

    Question

    Which ion breaks the 10-electron group: O²⁻ (Z = 8), Ca²⁺ (Z = 20), F⁻ (Z = 9), or Mg²⁺ (Z = 12)?

    Answer

    Ca²⁺. It has 18 electrons; the other three each have 10.

  38. Card 38

    Question

    A chlorine ion has 17 protons and 18 electrons. What is its signed charge?

    Answer

    1−. One excess electron gives a charge of −1.

  39. Card 39

    Question

    A learner says O²⁻ is smaller than F⁻ because it is more negative. Both have 10 electrons; Z is 8 for O and 9 for F. Repair the comparison.

    Answer

    O²⁻ is larger. Within the same configuration, its smaller proton count gives a weaker nuclear pull and a larger electron cloud.

  40. Card 40

    Question

    Al³⁺ (Z = 13) and Mg²⁺ (Z = 12) both have 10 electrons. Which has the smaller ionic radius?

    Answer

    Al³⁺. It has more protons pulling on the same electron configuration.

  41. Card 41

    Question

    A solution ranks S²⁻ (Z = 16) and Na⁺ (Z = 11) by proton count alone. Which required check was skipped?

    Answer

    Whether the electron configurations match. S²⁻ has 18 electrons and Na⁺ has 10, so the isoelectronic radius rule cannot justify that ranking.

  42. Card 42

    Question

    Can a qualitative ionic-radius rank, with no element, charge, or candidate list, uniquely identify an ion?

    Answer

    No. A size rank alone does not fix an element or charge. Identification needs further constraints, such as proton and electron counts.

Three blue electron-cloud spheres decrease in size from left to right around glowing amber centers on a dark background.

42 cards

Isoelectronic Ion Flashcards: Electron Counts & Radius Ranking

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