Atomic and Ionic Radii

Radii refers to the radius of a circular or spherical object. An atomic radii then is the radius of a particular atom and an ionic radii the radius of an ion. In the context of atoms or ions, a radii gives us a good way of comparing the sizes of various elements. While understanding the size of various atoms might not seem terribly useful it is. In the biochemical sciences, it allows us to predict what ion or ions fit through specialized channels. Later we will see how size impacts the stability of leaving groups as well as how an atom’s size impacts the strength of the bonds it can form.

Who Takes Up Space

Before applying our analogy to this trend we need to set the record straight on who takes up space in an atom. Even though the nucleus contains more particles it is far more concentrated than the electrons that surround it. As a result, the electrons of an atom define its overall size.

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Therefore atoms with a greater number of shells are bigger. So as we go from the top of the periodic table to the bottom the atoms get bigger and bigger.

The nucleus does matter though! The bigger and beefer the nucleus the more tightly it can pull in the shells that surround it. So nuclei with more protons will end be smaller. Thus size increases from right to left. Ultimately, the number of shells is the bigger determining factor in an atom’s size.

As a result, if a question asks you to compare two atoms on opposite sides of the periodic table such as H (Hydrogen) versus Ne (Neon) the element with more shells will always be bigger. In this case, Ne ends up being 1 picometer (10-¹²) larger not a big difference, but still bigger than H despite having a lot more protons.

This trend is exceptional in that unlike all the others we have discussed thus far atomic radii increase as we go down and to the left.

Size of Ions

Ionic radii follow the exact same rules as atomic radii. Except here we will usually be comparing atoms with the same number of shells. As a result, the only factor that ends up mattering is the number of protons in the nucleus.

For example, O^2-, F^–, Ne, Na⁺, and Mg²⁺ all have the same electron configuration of 1s²2s²2p⁶. However, magnesium has the largest number of protons of the bunch with an atomic number of 12. In this case, it ends up being the smallest since it is able to muscle its n=2 shell closer than the other weaker nuclei.

When solving questions that ask for either the largest or smallest ionic radii it is often easiest to start first by eliminating answers by charge. If a question asks for the largest radii you want to pick out the ion with the most negative charge. If the question asks for the smallest radii you want to pick out the ion with the most positive charge. If you have two of the same charges remaining in your answer choices then you can go ahead and consult the periodic table and base it on the number of protons.

I remember this by telling myself that electrons take up space and if you add more electrons you take up more space. On the other hand, if you remove electrons you get smaller. Since anions are ions with more electrons they are bigger and likewise since cations are ions with fewer electrons they are smaller.

Ion Channels

Ion channels in our cells use these differences in ionic radii to select certain ions while blocking others. By obeying the Goldilocks Principle*, only the right size ions can get through. Too small and you don’t fit into the channel. Too big and it becomes energetically unfavorable to pass through. As an ion, you have to be just the right size to use the channel. This allows channels to be selective for particular molecules while blocking the passage of other molecules.

This is super important in the nervous system where the resting potential of a cell is maintained by leaky K⁺ channels. The channels are always open so if other ions could freely pass through the whole system would become non-functional.

*I totally made this up but it helps us understand how ion selectivity works