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Quizzes > Science

Atomic and Ionic Radii Quiz: Test Atomic Size Trends

Quick, free chemistry quiz on atomic size trends. Instant results.

Editorial: Review CompletedCreated By: Jesse PickensUpdated Aug 25, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Coral background layered paper art atom with electrons orbiting and quiz title text on atomic structure

This quiz helps you practice atomic and ionic radii trends, compare ion sizes across periods and groups, and spot patterns in isoelectronic series. You will also review what drives a decreasing atomic radius and how charge affects size. For more practice, try our atomic structure quiz, explore a deeper structure of the atom quiz, or tackle targeted atomic structure quiz questions.

Which species has the smallest ionic radius in the series Na+, Mg2+, Al3+, Si4+?
Na+
Mg2+
Si4+
Al3+
As positive charge increases across this series, the effective nuclear charge on the same number of electrons increases, pulling electrons closer. The Si4+ ion has the highest charge-to-electron ratio, resulting in the smallest ionic radius. Ionic radii decrease with increasing charge if electrons remain the same.
Which of the following elements has the smallest atomic radius in Period 3?
Al
Na
Mg
Ar
Atomic radii decrease across a period due to increasing nuclear charge without additional shielding. As protons are added, electrons are pulled closer to the nucleus, shrinking the atomic size. Argon, being the last element in Period 3, experiences the greatest attraction on its electrons.
Which cation is smaller: Fe2+ or Fe3+?
Fe+
Fe
Fe2+
Fe3+
When iron loses more electrons to form Fe3+, the remaining electrons experience a stronger effective nuclear charge. This stronger pull draws the electrons closer to the nucleus. As a result, Fe3+ has a smaller ionic radius than Fe2+.
Which Group 2 element has the largest atomic radius?
Ba
Mg
Sr
Ca
Atomic radius increases down a group because each successive element has an additional electron shell. The increased shell count outweighs the growing nuclear charge. Barium, at the bottom of Group 2, has the most shells and thus the largest radius.
Which one of the following anions is the largest in size: S2?, Cl?, Ar, K+?
S2?
K+
Ar
Cl?
The S2? anion has gained two electrons, which increases electron-electron repulsion and expands its electron cloud. Although all species in this list are isoelectronic, the ion with the lowest nuclear charge per electron is largest. Sulfide (S2?) has the fewest protons pulling on the same number of electrons.
Among the isoelectronic species S2?, Cl?, Ar, K+, which has the smallest radius?
Ar
K+
Cl?
S2?
All species listed have the same number of electrons but different nuclear charges. The highest nuclear charge pulls the electron cloud in most strongly. Potassium ion (K+) has 19 protons for 18 electrons, making it the smallest among the four.
What is the general trend for atomic radii across a period?
Decrease from left to right
Increase from left to right
Remain constant
First increase then decrease
Across a period, nuclear charge increases as protons are added to the nucleus. The added electrons enter the same principal shell without significant additional shielding. Therefore, the effective nuclear charge increases and pulls electrons closer, decreasing atomic radius.
Which of the following has the larger ionic radius: O2? or F??
F?
They are the same size
O2?
Cannot be determined
O2? has one more added electron than F? but the same nuclear charge of oxygen. The increased electron-electron repulsion expands the electron cloud more in O2?. As a result, oxide ion (O2?) has a larger radius than fluoride (F?).
Arrange these isoelectronic species in decreasing ionic radius: P3?, S2?, Cl?, Ar.
Cl? > P3? > S2? > Ar
S2? > P3? > Cl? > Ar
P3? > S2? > Cl? > Ar
Ar > Cl? > S2? > P3?
All four species share the same electron count but differ in nuclear charge. The lower the nuclear charge, the less tightly electrons are held, so the ion is larger. Phosphide (P3?) has the fewest protons for the same electrons, then sulfide, then chloride, with argon being neutral and smallest.
For the transition-metal ions Sc3+, Ti4+, and V5+, which has the smallest ionic radius?
Sc3+
They are all the same size
V5+
Ti4+
All three ions are isoelectronic with argon, but the nuclear charge increases from Sc3+ to V5+. A higher nuclear charge pulls the electron cloud in more tightly. Thus, V5+ has the greatest pull and the smallest ionic radius.
Why are cations generally smaller than their neutral atoms?
Lower nuclear charge after ionization
Higher electron - electron repulsion
Loss of an electron shell during ion formation
Increased shielding by inner electrons
When a neutral atom loses electrons to become a cation, it often loses an entire valence shell. Fewer shells mean the remaining electrons reside closer to the nucleus. The resulting drop in electron - electron repulsion also contributes to a smaller size.
Which of the following elements has the largest atomic radius?
K
Ca
Sc
Ti
These elements are in the same period (Period 4). Atomic radius decreases from left to right as nuclear charge increases. Potassium (K) is at the far left of this set, so it has the largest radius.
Given the isoelectronic series O2?, F?, Na+, Mg2+, Al3+, rank them by increasing ionic radii.
Al3+ < Mg2+ < Na+ < F? < O2?
Na+ < Mg2+ < Al3+ < F? < O2?
Mg2+ < Al3+ < Na+ < O2? < F?
O2? < F? < Na+ < Mg2+ < Al3+
All species have the same number of electrons but different nuclear charges. Higher positive charge draws the electrons closer, making Al3+ smallest. The radius increases as the net charge decreases, with O2? the largest due to its extra electrons.
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Study Outcomes

  1. Understand Ion Size Determinants -

    Identify how effective nuclear charge, electron shielding, and electron - electron repulsions combine to influence the sizes of cations and anions.

  2. Compare Isoelectronic Species -

    Analyze ionic radii within an isoelectronic series to rank species by size and explain the underlying nuclear charge effects.

  3. Predict Atomic Radii Trends -

    Use periodic table positions to forecast how atomic radii change across periods and down groups, recognizing the impact of principal quantum shells.

  4. Evaluate Anomalies in Trends -

    Detect exceptions to general atomic radii trends caused by subshell configurations and variations in electron shielding.

  5. Apply Quiz Feedback for Improvement -

    Leverage instant scoring and detailed feedback from the atomic structure quiz to identify knowledge gaps and refine chemistry skills.

Cheat Sheet

  1. Ion Size vs. Charge -

    When you tackle an ion sizes quiz, remember that losing electrons (forming cations) reduces electron - electron repulsion and shrinks the radius, while gaining electrons (forming anions) expands it. For example, Na❺ (102 pm) is smaller than neutral Na (154 pm), which in turn is smaller than F❻ (136 pm). This relationship is well-documented in Purdue University's general chemistry resources.

  2. Isoelectronic Series Ordering -

    Isoelectronic species have the same electron count but different nuclear charges, so radii decrease with increasing proton number. For instance, in the O²❻, F❻, Na❺, Mg²❺ series, radii follow Mg²❺ < Na❺ < F❻ < O²❻, as confirmed by Royal Society of Chemistry data. A handy mnemonic is "More protons, more pull, more small" to remember this trend.

  3. Periodic Atomic Radius Trend -

    On an atomic radii trend test, you'll see radii decrease across a period due to rising effective nuclear charge (Z_eff) and increase down a group as additional electron shells are added. For example, atomic radius drops from K (220 pm) to Ca (197 pm), then jumps from Ca to Sr (215 pm). Cambridge University chemistry lectures cover these patterns in detail.

  4. Effective Nuclear Charge (Z_eff) Calculation -

    Understanding Z_eff is crucial for both ion sizes and atomic radii questions: Z_eff ≈ Z − S (where S is the shielding constant from Slater's rules). For sodium, Z=11 and S≈10, so Z_eff≈1 for the 3s electron, explaining its larger size compared to magnesium's 3s electrons (Z_eff≈2). This calculation is a staple in chemistry atomic structure practice materials.

  5. Smart Quiz Practice Techniques -

    When prepping for an atomic structure quiz, combine ion sizes quiz items, isoelectronic species questions, and atomic radii trend test problems, using active recall and spaced repetition. Flashcards with sample data (e.g., compare Cl❻ vs. Ar) and instant feedback tools help reinforce concepts quickly. The American Chemical Society suggests timed quizzes to build speed and accuracy.

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