Why a Magnesium Ion Is Smaller Than a Sulfur Ion (And What That Tells Us About Chemistry)
Here's something that trips up a lot of students: magnesium sits to the left of sulfur on the periodic table, yet when you compare their ions, magnesium wins the "smallest" prize. Plus, a magnesium ion (Mg²⁺) comes in around 72 picometers, while a sulfide ion (S²⁻) stretches out to about 184 picometers. On top of that, not by a little — by a lot. That's more than twice the size And that's really what it comes down to. Practical, not theoretical..
It seems backwards at first glance. That's why left-side elements should be bigger, right? So except we're not comparing atoms anymore — we're comparing ions. And that changes everything Worth keeping that in mind. Surprisingly effective..
What Are Ionic Radii, Really?
When chemists talk about ionic radius, they're describing the effective size of an ion in a crystal lattice. On the flip side, here's the thing — you can't actually measure a single ion's size directly. What scientists do is look at how ions pack together in solid compounds (like sodium chloride, NaCl), measure the distances between them, and then do some math to figure out how much space each ion takes up.
The numbers we get aren't perfect — different measurement methods give slightly different results — but they tell us enough to see the patterns that govern the periodic table.
An ion's size depends on three main factors:
- Nuclear charge: More protons in the nucleus pull electrons closer
- Electron count: More electrons mean more electron-electron repulsion, which pushes the electron cloud outward
- Electron configuration: Whether electrons are in the same shell or have been removed from outer shells entirely
Why Magnesium Ion Beats Sulfur Ion in the Size Race
The key insight is this: we're not comparing Mg to S. We're comparing Mg²⁺ to S²⁻.
Magnesium sits in period 3, group 2. Its electron configuration is [Ne]3s². When it forms an ion, it loses those two 3s electrons and becomes Mg²⁺, which has the same electron configuration as neon — a full outer shell of 8 electrons, all in the second energy level.
Sulfur is also in period 3, but in group 16. Consider this: its electron configuration is [Ne]3s²3p⁴. When it gains two electrons to complete its octet, it becomes S²⁻, which has 18 electrons total — still in the third energy level, but with two extra electrons that repel each other and push the cloud outward.
So here's what happens: Mg²⁺ has 10 electrons being pulled by 12 protons. Also, s²⁻ has 18 electrons being pulled by 16 protons. Worth adding: the magnesium ion has fewer electrons in a tighter space, pulled by more nuclear charge per electron. The sulfur ion has more electrons spread out over a larger volume, with less pull per electron.
Counterintuitive, but true.
That's why Mg²⁺ is dramatically smaller.
The Periodic Trend That Explains It All
This isn't an anomaly — it's a predictable pattern across the entire periodic table It's one of those things that adds up..
Within any single period (left to right), cations are always smaller than anions. Sodium (Na⁺) is smaller than chloride (Cl⁻). Worth adding: potassium (K⁺) is smaller than bromide (Br⁻). Always. Beryllium (Be²⁺) is smaller than oxygen (O²⁻) Took long enough..
The trend makes sense when you think about what's actually changing:
- As you move left to right across a period, the nuclear charge increases
- For neutral atoms, electrons are also added to the same shell, so the increase in positive charge is partially canceled out — that's why atomic size only decreases slightly across a period
- But when you form ions, you either remove electrons (making the remaining ones feel a stronger pull) or add electrons (creating repulsion that pushes everything outward)
The result? A cation from the left side of a period will always be smaller than anion from the right side of that same period The details matter here..
What About Isoelectronic Ions?
This gets even more interesting when you compare ions that have the same number of electrons.
Mg²⁺, Na⁺, F⁻, and O²⁻ all have 10 electrons (that's isoelectronic with neon). But their sizes are different:
- Mg²⁺: ~72 pm
- Na⁺: ~102 pm
- F⁻: ~133 pm
- O²⁻: ~140 pm
Same electron count, completely different sizes. Sodium has 11. Magnesium has 12 protons pulling on those 10 electrons. Why? Practically speaking, nuclear charge. Worth adding: fluorine has 9. Oxygen has 8 That's the whole idea..
More protons = stronger pull = smaller ion.
This is exactly the same principle that makes Mg²⁺ smaller than S²⁻, just applied to a series of ions with identical electron configurations That's the whole idea..
What Most People Get Wrong
The most common mistake is comparing atomic radii instead of ionic radii. If you look at neutral atoms, magnesium (160 pm) is actually slightly larger than sulfur (100 pm). That's the expected trend — more electrons on the same energy level means more electron-electron repulsion, which slightly increases the radius as you move right across a period Not complicated — just consistent. Nothing fancy..
But ionic radii tell a different story because you're fundamentally changing the electron count.
Another misconception: thinking that "more electrons = bigger ion" is always true. Practically speaking, the nuclear charge matters just as much. It's not. That's why S²⁻ (18 electrons) is so much bigger than Cl⁻ (18 electrons) — wait, actually they're the same electron count. Let me correct that: S²⁻ (18 electrons) is bigger than Cl⁻ (18 electrons) because sulfur has only 16 protons pulling on those electrons, while chlorine has 17 Practical, not theoretical..
See how it works? The proton count is the deciding factor.
Why This Matters (Beyond the Exam)
Understanding ionic radii isn't just about memorizing trends — it helps explain real-world chemistry.
Solubility rules depend partly on ion size. Smaller cations tend to form compounds with different solubility characteristics than larger ones. Lithium carbonate (Li₂CO₃) is less soluble than potassium carbonate (K₂CO₃), partly because Li⁺ is a much smaller ion.
Crystal lattice energy — the energy released when ions come together to form a solid — depends on the sizes of the ions involved. Smaller ions can get closer together, which means stronger electrostatic attraction and higher lattice energy. That's why MgO (with Mg²⁺ and O²⁻) has a much higher melting point than NaCl (with Na⁺ and Cl⁻).
Biological systems care about ion size too. Sodium and potassium channels in cell membranes are selective partly based on ion size. The right size fits the channel; the wrong size doesn't.
Practical Tips for Working With Ionic Radii
If you're studying this for a class, here's what actually helps:
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Always specify whether you're talking about atoms or ions — the trends are different, and mixing them up is where most errors happen Small thing, real impact..
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Remember the isoelectronic series trick — when ions have the same electron count, the one with the most protons is smallest.
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Don't memorize every radius — memorize the trends. You can figure out the relative sizes of most ions just by knowing their position in the periodic table and their charge.
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Watch the charge — +1 ions are bigger than +2 ions from the same element. -1 ions are bigger than -2 ions. More charge = bigger change from the neutral atom.
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Period matters — ions from higher periods (like K⁺ vs. Na⁺) are larger because electrons occupy higher energy levels, further from the nucleus That's the whole idea..
FAQ
Why is Mg²⁺ smaller than S²⁻ if magnesium is to the left of sulfur?
Because you're comparing ions, not atoms. Mg²⁺ has lost 2 electrons, so its remaining 10 electrons are pulled tightly by 12 protons. S²⁻ has gained 2 electrons, so its 18 electrons are pulled by only 16 protons with more electron-electron repulsion. Fewer electrons + more proton pull = smaller ion Nothing fancy..
What is the actual radius of Mg²⁺?
Around 72 picometers (pm), though different sources cite slightly different values (typically 66-86 pm depending on the method). The exact number varies because you can't directly measure a single ion's size.
Is Mg²⁺ smaller than all sulfur ions?
Yes. So there's only one common sulfur anion — S²⁻ — and Mg²⁺ is significantly smaller. Even in compounds where sulfur has different oxidation states (like sulfate, SO₄²⁻), the sulfur is covalently bonded, not ionic Simple as that..
Does this trend hold for all periods?
Yes. Within any period, the smallest cation (usually from group 1 or 2) is smaller than the largest anion (usually from group 16 or 17). This holds for periods 2 through 7.
Why do we even care about ionic radius?
Because it predicts solubility, melting points, crystal structure, conductivity, and a host of other chemical properties. It's one of the most practical concepts in inorganic chemistry.
The takeaway is simple: when you move from atoms to ions, the rules change. A cation from the left side of the periodic table will almost always be smaller than anion from the right side — not because of where the element sits, but because of what happens to its electron count when it becomes an ion. Mg²⁺ being smaller than S²⁻ isn't a weird exception. It's the rule working exactly as predicted And it works..
