Periodic Trend: Ionic Radius - Video Tutorials & Practice Problems
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Ionic Radius is the distance between an ion's nucleus and its most outer occupied electron shell.
Ionic Radius
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concept
Periodic Trend: Ionic Radius
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For a majority of the periodic trends, looking at the periodic table can tell you if they're increasing or decreasing. In all our periodic trends, we're heading towards the top right corner, and heading in that direction will either increase or decrease that particular periodic trend. Now ionic radius doesn't fit into this idea. Ionic radius does not involve looking at the periodic table. Or let's say ionic radius equals the distance between an ion's nucleus and its outer shell. And the periodic trend is this, the ionic radius increases as the number of electrons of that ion increases. So we're going to say here when we're looking at ionic radius we're looking at ions, and ions can either be positive or negative. For positive ions, which are cations, we're gonna say cations tend to be smaller than their neutral parent form. So what do I mean by this? Well, if we take a look here at Lithium, the lithium atom is 1 s 22 s 1. It has 1 electron in its valence electron shell. Right? Because its second shell only has 1. When I become lithium ion, I lose 1 electron. I lose it from that valence shell. So now the lithium ion no longer has 2 shells, it only has one shell that contains electrons, and we can see that there is a drop in the size for that particular ion. So just remember, the neutral form of the element is bigger than the cation form. So the general trend is losing an electron or electrons causes a decrease in your ionic radius. Now for looking at the anions, anions tend to be larger than their neutral parent forms. Oxygen is 1 s 2, 2 s 2, 2 p 4. Here these electrons are in the second shell, and there's 6 of them total. When we become the oxide ion, we gain 2 more electrons. So now my outer shell has 8 electrons. It may not be as apparent, but the anion is slightly bigger than the neutral form. Because why we said earlier, the ionic radius increases as the number of electrons increases. So basically, more electrons equal larger ionic radius. So we're gonna say gaining an electron or electrons causes an increase in your ionic radius. So just remember, we don't look at the periodic table to determine the trend in ionic radius, instead we look at the total number of electrons that ion has. The more electrons it has, the bigger its ionic radius.
Periodic Trend: Ionic Radius increases as the number of electrons increases.
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example
Periodic Trend: Ionic Radius Example 1
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So here for this example question, it says, which of the following statements is false? Alright. So the cation has a smaller ionic radius than the calcium atom. Remember, atom means it's neutral. Calcium is in group 2 a, so its charge is 2 +. Remember, we said earlier that cations lose electrons, which causes a decrease for the ionic radius of the positive ion. So yes, since calcium is a positive ion it will in fact have a smaller ionic radius than the neutral form, so this is true. Sulfide ion has a larger ionic radius than the sulfur atom. Sulfide I means it's an anion, it's gained electrons. Remember we said that anions are larger than their neutral form because more electrons equals larger ionic radius. So this is true. The cadmium ion has a larger ionic radius than the cadmium atom. So cadmium has a charge of 2 plus, remember cations are smaller than their neutral form because they have less electrons. So this is false in our answer. Bromide ion, I means a negative charge, it's an anion. Anions are larger than their neutral forms because again, more electrons equals larger ionic radius. So the only statement here that's false would have to be option c.
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example
Periodic Trend: Ionic Radius Example 2
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Here we're going to say when ranking ionic radii, you must determine the total number of electrons or atoms and ions. Now, more electrons equals larger ionic radius. But recall, isoelectronic species are atoms or ions that have the same number of electrons. In those cases, what do we do? Well, if we take a look at this example, it says, arrange the atoms and or ions in order of decreasing ionic radius. So that means that we need to go from largest ionic radius to smallest ionic radius. So here if we take a look, in step 1 it says determine the total number of electrons for each element or ion. Just remember, the higher the number of electrons, then the greater the ionic radius. So if we take a look here, iron, when it is neutral, it has 26 electrons because its atomic number is 26. Here, it's lost 2, so it's gonna have 24 electrons remaining. Manganese has an atomic number of 25. So when it's neutral it has 25 electrons. Plus 1 means it's lost 1, so it has 24 electrons as well. Nickel has an atomic number of 28. Here it's just lost 2 electrons, so it has 26 remaining. And then finally, here we have zinc. Zinc has an atomic number of 30, so when it's neutral it has 30 electrons. Plus 2 means now it's lost 2, so it only has 28 electrons remaining. Alright. So we can arrange this somewhat from largest to smallest. We know that zinc 2+ion will be the largest because it has the most electrons, followed by nickel. But how do we break the tie between iron 2+ and Manganese+1? That's where step 2 comes into play. If the atoms or ions have equal electrons, meaning they're isoelectronic, then the more negative the charge, the larger the ionic radius. So we're gonna say for an ionic for isoelectric species, we just said the greater the negative charge, then the greater the ionic radius. So a negative 3 charge would be bigger than a negative 2 charge, bigger than a negative 1, bigger than 0, bigger than plus 1, plus 2, plus 3. Now Now, of course, you could have some species that have a charge, that's outside of negative 3 or beyond plus 3 as well. Same rule would apply. So both Iron 2 +1 and Manganese+1 have 24 electrons, but Manganese+1 is bigger than plus 2. So we'd say that manganese plus 1 ion is bigger than iron 2+ion. So just remember, when it comes to ions, more electrons equals larger ionic radius. If they're tied, then we look at the charge to break that tie. The more negative the charge, the larger the ionic radius will be.
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Problem
Problem
Arrange the following atoms and/or ions in the order of increasing size:Br –, Kr, Rb+, Sr2+.
A
Kr < Br- < Sr2+ < Rb+
B
Kr < Sr2+ < Rb+ < Br-
C
Sr2+ < Rb+ < Kr < Br-
D
Rb+ < Sr2+ < Kr < Br-
E
Br- < Kr < Rb+ < Sr2+
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Problem
Problem
For an isoelectronic series of ions, the ion that is the smallest is always