Prediction of Element Properties Based on Periodic Trends

Atomic Radius Trend Periodic Table

If I were to tell you to describe the physical properties of atoms and molecules, where do you think you'd start? Perhaps you'd tell me that they make up everything, they can be in different states or something of that sort. However, I'd reckon that you'd include in there somewhere that they're small- really, really small. While this is certainly true, not all atoms are created equal. The periodic table can actually be read in terms of atomic radius.

If we want to look for the radius of an atom, we need its size. Intuitively, we can think of this radius as reaching from the center of its nucleus to the edge of the furthest orbital in its electron cloud. However, because atoms are so hard to observe, we can’t measure them directly in this fashion. There is a clever workaround to this, though. If we were to place two atoms directly next to each other, it's more feasible to measure the distance between the two nuclei, and then half it. This will give us a rough idea of what the radius is for an atom, and therefore, its size.

If we did this for every element on the periodic table, we’d find that the radius of atoms increases as we move from the right to the left across each period, and will also increase as we move down each group. We know that this is true because as we go down each group, we’re adding additional orbitals to the electron cloud, and therefore expanding the atomic size.

• So we've established a rule: generally, more electrons means a larger radius.

Ionic Radius Trend

Let's try to apply this rule to ions. If you recall, we get an ion when we take an atom and either remove or add electrons. For example, we know that table salt, NaCl, is just two ions in a complex: Na⁺ and Cl^-. (Remember that a negative charge implies an additional electron, and a positive charge implies a missing electron.) So what happens when we look at these ions under our new rule? As it turns out, it holds true. The atomic radius shrinks as Na is converted into Na⁺, and Cl's radius increases as it is converted from Cl to Cl^-.

• From this, we can derive our second rule: cations decrease the atomic radius, and anions increase the atomic radius.

If you recall, each element has an ion that it gets converted to most often. Therefore, if we apply our two rules together, we find another trend, the ionic radius is proportional to the number of electrons.

To visualize this trend, here it is applied to the periodic table.

Figure 1: Drawn periodic trend for atomic and ionic radius

Ionization Energy Periodic Trend

For the next set of periodic trends, recall that atoms are significantly more stable when their valence electron shells are full. This means that when atoms are extremely close to having a full outer shell, they will more readily give up or accept electrons to get there. Let's define our Ionization Energy and Electron Affinity before we go further.

Now, let's try to think about this practically for a moment. Our atoms don't particularly care how many shells they have. Instead, they care much more about whether or not the valence shell is full. For example, let's consider Oxygen.

We could easily show you a trend for ionization energy and electron affinity. However, this much more intuitive approach will help you during the AP exam to visualize what is going on with each element. We can do one more example with sodium.

Hopefully, you're starting to get a grasp of how this intuitive method makes sense. If we were to do this with every element, we'd see something like this.

Figure 2. Drawn periodic trend for ionization energy, electronegativity and electron affinity.

Electronegativity Periodic Trend

Electronegativity and electron affinity are very closely related. While electron affinity is the actual energy released when we add an electron, electronegativity is the actual characteristic of an atom wanting to have an electron added to it.

Therefore, we can intuitively say that elements that have a high electron affinity will have a high electronegativity. This logically makes sense: if an element wants to have electrons added to it, it's highly electronegative. So when this electron is added, a high level of energy will be released, indicative of a high electron affinity.

The trend for electronegativity will be the same as the trend for electron affinity, as pictured in the graph in the previous section.

Metallic Character Periodic Table

You've probably already covered that there are metals and nonmetals on the periodic table. As you might've gathered by the previous covered periodic trends, metals typically lose an electron when in a reaction. (Recall the sodium example we discussed earlier!) Just as electronegativity describes how readily an element accepts electrons, metallic character describes how readily an element gives up an element.

When you move across a period from the right side to the left side, metallic character increases because of a diminished interaction between the nucleus and the valence shell. It also increases as you move down a group. This is because additional shells are added, further distancing the valence electrons from the nucleus. This makes the nucleus' pull on the valence electron weaker, resulting in a valence electron that is more easily given up.

If we were to visualize this trend, it would look something like this.

Figure 3: Drawn periodic trend for metallic character.

Periodic Table Size Trend

Hopefully, the methods that we've described help to give you a stronger grasp on periodic trends. Being able to predict the qualities of different elements by remembering a few key trends will eventually help you to predict reaction conditions, and will prove to be useful as you advance into higher-level chemistry courses. To put everything together here are all of the periodic trends put together.

Figure 4: Drawn graph of the overall periodic trends.