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

When ranking ionic radii, consider the total number of electrons; more electrons result in a larger ionic radius. For isoelectronic species, the ionic radius increases with more negative charges. For example, zinc 2+ has the largest radius due to having the most electrons, followed by nickel. To differentiate between ions with equal electrons, the charge is crucial: a more negative charge indicates a larger ionic radius. Thus, understanding electron configuration and charge is essential for predicting ionic sizes.

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.

Here’s what students ask on this topic:

What factors determine the ionic radius of an ion?

The ionic radius of an ion is primarily determined by two factors: the total number of electrons and the charge of the ion. More electrons result in a larger ionic radius. For isoelectronic species (ions with the same number of electrons), the ionic radius increases with a more negative charge. This is because additional negative charge increases electron-electron repulsion, causing the ion to expand. Conversely, a more positive charge results in a smaller ionic radius due to increased attraction between the electrons and the nucleus.

How do you rank ionic radii for isoelectronic species?

To rank ionic radii for isoelectronic species, first ensure that the ions have the same number of electrons. Then, compare their charges. The more negative the charge, the larger the ionic radius. For example, among N^3-, O^2-, and F^-, all of which have 10 electrons, N^3- has the largest radius, followed by O^2-, and then F^-. This is because N^3- has the most negative charge, leading to greater electron-electron repulsion and a larger radius.

Why does a more negative charge result in a larger ionic radius?

A more negative charge results in a larger ionic radius because it increases electron-electron repulsion within the ion. When an ion gains electrons, the added negative charge causes the electrons to repel each other more strongly, which forces them to spread out further from the nucleus. This increased repulsion leads to an expansion of the electron cloud, thereby increasing the ionic radius.

How do you determine the ionic radius of transition metals like Fe²⁺ and Mn⁺?

To determine the ionic radius of transition metals like Fe²⁺ and Mn⁺, first calculate the total number of electrons for each ion. For Fe²⁺, iron has an atomic number of 26, so Fe²⁺ has 24 electrons. For Mn⁺, manganese has an atomic number of 25, so Mn⁺ also has 24 electrons. Since they are isoelectronic, compare their charges. Mn⁺ has a less positive charge than Fe²⁺, making Mn⁺ larger in ionic radius due to less nuclear attraction on the same number of electrons.

What is the relationship between electron configuration and ionic radius?

The electron configuration of an ion directly affects its ionic radius. Ions with more electrons generally have larger radii due to increased electron-electron repulsion. For isoelectronic species, the ionic radius is influenced by the charge: a more negative charge results in a larger radius, while a more positive charge results in a smaller radius. Understanding the electron configuration helps predict the size of the ion by considering both the number of electrons and the charge.