If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Mini-video on ion size

Correcting a mistake and learning a bit about ion size. Created by Sal Khan.

Want to join the conversation?

  • blobby green style avatar for user Jade
    What is the difference between atomic radii and ionic radii
    (8 votes)
    Default Khan Academy avatar avatar for user
  • leafers seedling style avatar for user Ruchi Vashist
    I'm confused! In a video, Sal said that as an element gains electrons, it becomes smaller in size due to more attraction. But in this video, he says that as Na loses an electron, it's radius decreases, while the opposite happens when Cl gains an electron. Please help!
    (4 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user lee ho
    In the video, Khan said that the atomic radius of Sodium Ion is going to be a lot like Na, if we ionize Na and Cl each other, because it loses an electron.
    However, doesn't that mean Cl should get smaller, because Cl (as an anion) would gain one electron, which not only makes its atomic radius lot like Ar, but also going to attract those outer electrons because the more positive charges were added on Cl?

    I don't understand how can Cl gets bigger in size-wise after it ionizes with Na?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • mr pants purple style avatar for user Ryan W
      Cl- has the same number of electrons as Ar, but it has less protons, so less of a positive charge pulling those electrons to the nucleus, so will has a larger radius.

      Likewise Na+ has the same number of electrons as Ne, but it has 1 more proton so attracts all those electrons more strongly so has a smaller radius.
      (6 votes)
  • piceratops seed style avatar for user Mckinly.rochon
    what is a ion
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leafers sapling style avatar for user Abhijeet Ghosh
    I understand why the sodium becomes smaller (it loses a whole shell), but why does the chlorine become bigger? Is this because there is a missing proton so there is no added pull towards the center?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Hecretary Bird
      The chlorine gets an electron, which is repelled by the other electrons in the valence shell. These electrons all want to be as far away from each other as they can, but are all attracted to the same nucleus. When you add an extra electron into the mix, the same attracting force of the 17 protons now has to deal with 18 electrons, which makes the outermost electron able to be further away from the nucleus.
      (4 votes)
  • blobby green style avatar for user ch.araghunathan
    Hey guys
    I have an extremely important proposal to make. Based on knowledge, I can surmise that Beryllium's Ionic Radius is the smallest radius possible on the periodic table. This is because the Ionic radii of cations in period 2 (lithium, beryllium) are going to be slightly smaller than that of Helium's atomic radius (because both cations have higher ZEFF). Out of these 2 cations, Beryllium has the highest Effective Nuclear Charge.

    Using Effective Nuclear Charge, we can say that:
    Helium has ZEFF of 2-1.9= +0.1 (barely greater than 0)
    Lithium has a ZEFF of 3-2= +1
    Beryllium has a ZEFF of 4-2= +2

    Beryllium having a nuclear charge of +2 will have the strongest attraction towards its nucleus. EVEN STRONGER ATTRACTION THAN OF LITHIUM (ZEFF of +1).

    The noble gas, Helium, in comparison to the others, will have the smallest atomic radius as it only has a ZEFF of +0.1

    Can you please clarify on that. I want to be sure if my conjecture makes sense.
    (3 votes)
    Default Khan Academy avatar avatar for user
    • aqualine seedling style avatar for user Seongjoo
      Yep, you are correct that Be will have a stronger ZEFF than Li if they are both ions. This is simply due to Be having one more proton than Li, leading to a higher ZEFF when they are both ions. Furthermore, even if both elements are not ions, (i.e. in their neutral state) Be will still have a smaller atomic radii due to its stronger ZEFF.

      Of course, if you manage to knock an electron off of a neutral hydrogen, you will have the smallest atomic radii possible.

      Anyways, great job figuring this all out on your own!
      (2 votes)
  • leaf orange style avatar for user philipp.sylt
    So then why does Sodium have a larger atomic radius than Chlorine in elementary state, since they both have got the 3s Shell and Chlorine even the 3p ontop? Thanks for your help :)
    (2 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Just Keith
      The atomic radius is affected by many factors (and is itself only an estimate). Thus, it is not good to over-generalize about why one element has a different atomic radius than another.

      However, one of the main reasons why Na has a larger atomic radius than Cl is because Cl has more protons, though they both are filling the n=3 shell. Thus, the electrons in the outermost shell of Cl experience a stronger attraction toward the nucleus (because the nucleus of Cl has a greater charge).
      (3 votes)
  • leaf green style avatar for user estherlai16
    Would drawing the sizes different affect the equation at all?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Bella Kanaday
    Is the likelihood of two elements reacting affected by whether or not they are in the same period?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Henry Elicker
    Would it be more correct to say that, since technically when Chlorine gains an octet and becomes a similar size to Argon, it would actually get smaller, specifically 79 pm to 71 pm. Conversely, Sodium is going from 190 pm to 38 pm, a huge difference. And it's that difference that explains the size difference when they become ions. I think that would be better to say that instead of Chlorine becoming larger, which it technically is not. Just clarifying here.
    (2 votes)
    Default Khan Academy avatar avatar for user

Video transcript

In the video on solubility, I draw little pictures of sodium and chloride ions when sodium chloride dissolves or disassociates into water. This is sodium and this is chloride. And my simple brain, when I looked at it, I said, OK, how should I draw these things? I said, well, they're in the same period, and sodium is a Group 1 element. It's an alkali metal, while chlorine is a halogen, so chlorine's going to have a smaller atomic radius. And the logic there, just to review from the atomic table trends, is that both of their valence electrons are in the third shell. Sodium only has 11 protons pulling in the center. It has 11 in the center, and it has only one electron out there in its valence shell. So the attraction isn't as strong as the case of chlorine, which has 17 protons in the center. Although it has more valence electrons-- it has 7 of them-- these protons are going to have a stronger attraction on them. So if you just look at the trend in the periodic table, you'd expect the sodium neutral atom to be bigger than the chlorine neutral atom. Because this guy has more protons pulling everything in. And that's how I drew the ions in that video. I said, oh, when I disassociate in water, I'll have a big sodium ion and a smaller chlorine ion, which is incorrect. Because think about-- and this was pointed out to me by one of the viewers, and they're correct, and I should have realized it. What happens when you ionize these things? This guy will lose an electron, right? He gives the electron to this guy. So his electron configuration is actually going to look a lot more like neon. He now will have no electrons in that third shell, in the third energy state. So now he's going to have an atomic radius that's actually much more similar to neon here, right? Because he's going to have filled up the second shelf. So actually, the sodium ion, this is completely incorrect. The sodium ion is going to have an atomic radius not that different than neon. Actually, it will be even a little smaller than neon because it has the same electron configuration, but it has one more proton. So the sodium ion is actually going to be smaller. Because it gets rid of the electron in that third shell, and the chlorine cation, gained an electron, so it has completely completed its third shell. So here you have where the chlorine ion is going to be bigger. So in that solubility video, I should've actually switched the places between the sodium and the chlorine, at least in size-wise. And, of course, I showed how they disassociate in water, and this would be attracted to the oxygen end of the water, and you have the hydrogen end and all that. But you can watch the solubility video for that. It doesn't change the real takeaway from the video. But I think this is a really interesting point that it brings up, that when you ionize these neutral atoms, it can significantly change, especially significantly change their relative atomic sizes. Anyway, hopefully, you found that interesting.