Isoelectronic Ion Flashcards: Electron Counts & Radius Ranking

Question 1

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

Accepted Answer

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

Question 2

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

Accepted Answer

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

Question 3

Na⁺, Z = 11: electron count?

Accepted Answer

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

Question 4

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

Accepted Answer

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

Question 5

O²⁻, Z = 8: electron count?

Accepted Answer

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

Question 6

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

Accepted Answer

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

Question 7

Cl⁻, Z = 17: electron count?

Accepted Answer

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

Question 8

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

Accepted Answer

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

Question 9

Al³⁺, Z = 13: electron count?

Accepted Answer

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

Question 10

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

Accepted Answer

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

Question 11

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

Accepted Answer

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

Question 12

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

Accepted Answer

Negative. The excess electrons give a net negative charge.

Question 13

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

Accepted Answer

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

Question 14

N³⁻, Z = 7: electron count?

Accepted Answer

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

Question 15

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

Accepted Answer

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

Question 16

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

Accepted Answer

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

Question 17

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

Accepted Answer

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

Question 18

S²⁻, Z = 16: electron count?

Accepted Answer

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

Question 19

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

Accepted Answer

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

Question 20

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

Accepted Answer

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

Question 21

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

Accepted Answer

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

Question 22

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

Accepted Answer

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

Question 23

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

Accepted Answer

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

Question 24

Ca²⁺, Z = 20: electron count?

Accepted Answer

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

Question 25

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

Accepted Answer

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

Question 26

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

Accepted Answer

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

Question 27

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

Accepted Answer

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

Question 28

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

Accepted Answer

2−. It has two more electrons than protons.

Question 29

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

Accepted Answer

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

Question 30

K⁺, Z = 19: electron count?

Accepted Answer

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

Question 31

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

Accepted Answer

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

Question 32

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

Accepted Answer

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

Question 33

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

Accepted Answer

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

Question 34

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

Accepted Answer

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

Question 35

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

Accepted Answer

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

Question 36

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

Accepted Answer

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

Question 37

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

Accepted Answer

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

Question 38

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

Accepted Answer

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

Question 39

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.

Accepted Answer

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

Question 40

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

Accepted Answer

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

Question 41

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

Accepted Answer

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

Question 42

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

Accepted Answer

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

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Isoelectronic Ion Flashcards: Electron Counts & Radius Ranking