Periodic Trend: Ranking Ionic Radii - Online Tutor, Practice Problems & Exam Prep

Periodic Trend: Ranking Ionic Radii Concept 1

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 equal 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 the largest ionic radius to the 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, has 26 electrons because its atomic number is 26. Here, it's lost 2, so it's going to 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 going to say for an ionic for isoelectronic 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, of course, you could have some species that have a charge, that's outside of negative 3 or beyond plus 3 as well. The 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 equal a 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.

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⁺

Arrange the following isoelectronic series in order of decreasing radius:F^–, O^2–, Mg²⁺, Na⁺.

• A. O^2- > F^- > Na⁺ > Mg²⁺
• B. F^- > O^2- > Na⁺ > Mg²⁺
• C. O^2- > F^- > Mg²⁺ > Na⁺
• D. F^- > O^2- > Mg²⁺ > Na⁺
• E. O^2- > Na⁺ > F^- > Mg²⁺

For an isoelectronic series of ions, the ion that is the smallest is always 

• A. The ion with the fewest protons.
• B. The least positively charged ion.
• C. The ion with the highest atomic number.
• D. The ion with the most neutrons.
• E. The ion with the most electrons.