Why is the size of Al3+ less than that of Li+? - Chemistry Stack Exchange most recent 30 from chemistry.stackexchange.com 2019-09-22T16:12:30Z https://chemistry.stackexchange.com/feeds/question/15034 https://creativecommons.org/licenses/by-sa/4.0/rdf https://chemistry.stackexchange.com/q/15034 6 Why is the size of Al3+ less than that of Li+? Kushashwa Ravi Shrimali https://chemistry.stackexchange.com/users/3794 2014-08-09T03:30:18Z 2015-09-25T19:18:50Z <blockquote> <p>Why is it so that the size of $\ce{Al^{3+}}$ is lesser than $\ce{Li+}$? </p> </blockquote> <p>I'm a bit confused about this one as the aluminium(III) ion is composed of 10 electrons and 13 protons, while the lithium ion has only 2 electrons and 3 protons. </p> <p>Electronic configuration of $\ce{Al^{3+}}$ is $\mathrm{1s^2 2s^2 2p^6}$ and that of $\ce{Li+}$ is $\mathrm{1s^2}$. </p> <p>Now, it seems obvious that the size of $\ce{Al^{3+}}$ should be greater than that of $\ce{Li+}$, but it is actually the other way around. <strong>Why is this?</strong> I think that it is a kind of exception, but if anyone can explain the reason behind this, it will help me in my upcoming tests.</p> https://chemistry.stackexchange.com/questions/15034/why-is-the-size-of-al3-less-than-that-of-li/15042#15042 8 Answer by permeakra for Why is the size of Al3+ less than that of Li+? permeakra https://chemistry.stackexchange.com/users/485 2014-08-09T13:45:43Z 2015-09-25T19:18:50Z <blockquote> <p>Now, it seems obvious that the size of Al3+ should be greater than that of Li+</p> </blockquote> <p>No it does not. It is true that electrons repulse each other, however, they all are attracted towards the nucleus. The real 'size' of an atom or ion is a result of compromise between electron-electron repulsion and electron-nuclear attraction.</p> <blockquote> <p>I think that it is a kind of exception</p> </blockquote> <p>Obviously, it is not.</p> <blockquote> <p>Why is this? </p> </blockquote> <p>Commonly, there are three consideration, used to rationalize basic trends in atomic/ionic radii. </p> <ul> <li>electron-electron repulsion (negative charges repulse each other)</li> <li>electron-nucleus attraction (negative charges are attracted towards positive ones)</li> <li>electron shielding (electrons in shells closer to nucleus 'shield' outer electrons from some charge of the nucleus.</li> </ul> <p><strong>While electrons repulse each other, increasing nuclear charge more then compensates it; so if not for shell organisation of electrons, atoms would monotonically decrease in size toward the end of the periodic table, as it is observable in each period 1—3.</strong></p> <p>However, inner electronic shells effectively shield the outer electrons from the nucleus, so they 'see' a nucleus that is a) larger in size and b) has charge equal to nuclear charge minus number of electrons in inner shells. Given that, outer electrons 'see' a nucleus with low charge density, so outer shells progressively increase in size within the same column. </p> <p>In addition to that, electrons still repulse each other, so positive ions are smaller than neutral atoms, and neutral atoms are smaller than negative ions.</p> <p>Now, consider $\ce{Li+}$ and $\ce{Al^{3+}}$. For the former, 2 outer electrons feel the charge of three protons. For the latter, 8 outer electrons feel the charge of 11 protons in the nucleus. Such great charge easily beats all other factors, so the latter cation is much smaller than the former. </p>