Periodic Trend: Ionic Radius (Simplified) - Online Tutor, Practice Problems & Exam Prep

Ionic radius refers to the distance between an ion's nucleus and its outer shell. Cations, or positive ions, are smaller than their neutral forms due to the loss of electrons, resulting in a decrease in ionic radius. Conversely, anions, or negative ions, are larger than their neutral counterparts because gaining electrons increases the ionic radius. Thus, the trend shows that more electrons lead to a larger ionic radius, emphasizing the importance of electron count over periodic table position in determining ionic size.

Periodic Trend: Ionic Radius (Simplified) Concept 1

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 going to 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 1s² 2s¹. It has 1 electron in its valence electron shell. Right? Because its second shell only has 1. When it becomes a lithium ion, it loses 1 electron. It loses 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 1s², 2s², 2p⁴. Here these electrons are in the second shell, and there's 6 of them total. When it becomes the oxide ion, it gains 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, as we said earlier, the ionic radius increases as the number of electrons increases. So essentially, more electrons equal a larger ionic radius. So we're going to 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 the ion has. The more electrons it has, the bigger its ionic radius.

Periodic Trend: Ionic Radius (Simplified) Example 1

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, an atom means it's neutral. Calcium is in group 2A, so its charge is 2+. Remember, we said earlier that cations lose electrons, which causes a decrease in 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. The 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 equal a 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+, remember cations are smaller than their neutral form because they have fewer electrons. So this is false in our answer. The bromide ion, I means a negative charge; it's an anion. Anions are larger than their neutral forms because again, more electrons equate to a larger ionic radius. So the only statement here that's false would have to be option C.

Which of the following statements is false?

a) The phosphide ion has a smaller ionic radius than the phosphorus atom.
b) The phosphide ion has a larger ionic radius than the phosphorus atom.
c) The phosphide ion's ionic radius cannot be compared to the phosphorus atom.
d) The phosphide ion has an equal ionic radius than the phosphorus atom.

• A. A and C
• B. B and C
• C. A, C, and D
• D. B, C, and D
• E. All of the statements are false.

Here’s what students ask on this topic:

What is the ionic radius and how does it differ between cations and anions?

The ionic radius is the distance between an ion's nucleus and its outer shell of electrons. For cations, which are positive ions, the ionic radius is smaller than their neutral forms because they lose electrons, resulting in fewer electron shells or a reduced electron cloud. Conversely, anions, which are negative ions, have a larger ionic radius than their neutral forms because they gain electrons, increasing the electron cloud and repulsion among electrons. Thus, cations are smaller, and anions are larger compared to their neutral counterparts.

Why do cations have a smaller ionic radius compared to their neutral atoms?

Cations have a smaller ionic radius compared to their neutral atoms because they lose one or more electrons. This loss reduces the number of electron shells or decreases the electron cloud's size. For example, a neutral lithium atom (Li) has the electron configuration 1s² 2s¹. When it loses one electron to become a Li⁺ ion, it loses its entire second shell, resulting in a smaller ionic radius. The reduced electron-electron repulsion also contributes to the smaller size of cations.

How does the ionic radius change when an atom becomes an anion?

When an atom becomes an anion, its ionic radius increases. This is because the atom gains one or more electrons, which adds to the electron cloud and increases electron-electron repulsion. For instance, a neutral oxygen atom (O) has the electron configuration 1s² 2s² 2p⁴. When it gains two electrons to become an O^2- ion, the added electrons increase the size of the electron cloud, resulting in a larger ionic radius. Thus, gaining electrons leads to a larger ionic radius for anions.

Why is the periodic table not used to determine the trend in ionic radius?

The periodic table is not used to determine the trend in ionic radius because the ionic radius depends on the number of electrons in the ion rather than its position on the periodic table. The trend shows that more electrons lead to a larger ionic radius, and fewer electrons result in a smaller ionic radius. Therefore, to determine the ionic radius, one must consider whether the ion is a cation or anion and the total number of electrons it has, rather than its placement on the periodic table.

How does the number of electrons affect the ionic radius?

The number of electrons directly affects the ionic radius. When an ion gains electrons (becoming an anion), the increased electron-electron repulsion causes the electron cloud to expand, resulting in a larger ionic radius. Conversely, when an ion loses electrons (becoming a cation), the reduced electron-electron repulsion and fewer electron shells cause the electron cloud to contract, resulting in a smaller ionic radius. Thus, more electrons lead to a larger ionic radius, and fewer electrons lead to a smaller ionic radius.