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

Atomic radius is the distance from an atom's nucleus to its outer electron shell. As you move down a group in the periodic table, atomic radius increases due to more electron shells. Conversely, moving across a period from left to right decreases atomic radius because of increased nuclear attraction from added electrons in the same shell. The general trend shows that atomic radius decreases towards the top right corner of the periodic table, with exceptions primarily in transition metals, where electron arrangements complicate predictions.

Periodic Trend: Atomic Radius (Simplified) Concept 1

So atomic radius is the distance between an atom's nucleus and its outer electron shell, otherwise known as its valence shell. So here, if we take a look, we have our nucleus in the middle, and here we have our outer shell. The distance between the nucleus and the outer shell is our atomic radius r. Now remember, within the nucleus we have our protons and our neutrons; our protons are positively charged particles, neutrons are neutral, and the nucleus itself contains our protons and our neutrons.

Now, we're going to say here, going down a group, that the number of electrons increases because our shells get larger and larger, and they can hold more and more electrons. And we're going to say that the number of electron shells also increases. But, we're going to say moving across a period though, we're going to say that the number of electrons within the same shell also will increase. We're going to say increasing the number of shell electrons in the same shell causes a greater attraction with the nucleus. And what this does is it causes a slight decrease in our atomic radius.

So, we have these two forces at work; we're adding more electrons, and as a result our atom gets larger and larger with more and more shells. But as we add more and more shells, there’s going to be more electrons found within each of those shells. This is going to cause some issues with our atomic radius. So, there's an increase and a decrease, a type of phenomenon happening here with the increase of a number of electrons. The overall periodic trend is that as we move from left to right, so remember we're always heading towards the top right corner of the periodic table, our atomic radius will decrease.

So, click on to the next video and let's take a look at what this periodic table would look like in terms of atomic radius.

Periodic Trend: Atomic Radius (Simplified) Concept 2

As I stated before, as I move from left to right across a period, and as we move up a group, our atomic radius will decrease. Here I've reimagined the periodic table to show how the atomic radius changes as I move across it. Here we have hydrogen and its average atomic radius is 37 picometers, and we can see as we go from hydrogen to helium, the number drops. If we go to row 2, we have lithium, which is 152 picometers, and as we go from left to right, we can also see for the most part the atomic radius will decrease. Now, of course, with chemistry, there are exceptions. A lot of these exceptions happen with the transition metals. We're going to say the array the electron arrangement for transition metals makes their patterns less predictable, so we just don't include them. Now you don't have to memorize these numbers at all for each of the elements. All you need to remember is the general trend, and the general trend is as we head towards the top right corner of the periodic table, we expect the atomic radius overall to decrease. So just keep that in mind when you're looking at atomic radius.

Periodic Trend: Atomic Radius (Simplified) Example 1

Here for this example it asks, which of the following atoms has the largest atomic radius? So we have potassium, rubidium, yttrium. Now remember, we said that the transition metals are hard to predict, so we can ignore this one. Calcium and strontium. Alright. So I've highlighted the four that we're looking at. And remember, we have the numbers here so it's a little bit of cheating, but what's the general trend? As we start heading towards the top right corner, we get larger, and we get smaller in terms of the atomic radius. We get smaller in terms of atomic radius as we head towards the top right, but you are not looking for the smallest atomic radius, we're looking for the largest. So what's the opposite trend of that? Well, as we head towards the bottom left, we expect our atomic radius to get bigger. So, of course, we have the numbers here, but remembering the general trends if you don't have the numbers is important. And we know that rubidium is most to the left and it's furthest down, so that's why it has the highest atomic radius out of the four. So here, this would make option b the correct choice.

Which alkaline earth metal has the smallest atomic radius? 

• A. Ca
• B. Rb
• C. Na
• D. Ra
• E. Fr

Which alkaline earth metal has the largest atomic radius?

• A. Na
• B. Ba
• C. Cl
• D. Mg
• E. Li

Arrange the following atoms in order of decreasing atomic radius:Sr, Se, Ne, Ga

• A. Ne > Se > Ga > Sr
• B. Ga > Se> Ne > Sr
• C. Sr > Ga > Se > Ne
• D. Se > Ne > Ga > Se

Here’s what students ask on this topic:

What is atomic radius and how is it measured?

Atomic radius is the distance from an atom's nucleus to its outer electron shell, also known as its valence shell. It is typically measured in picometers (pm) or angstroms (Å). The atomic radius can be determined using various methods, such as X-ray diffraction or by calculating the distance between the nuclei of two bonded atoms and dividing by two. Understanding atomic radius is crucial for predicting and explaining the chemical behavior of elements.

How does atomic radius change as you move down a group in the periodic table?

As you move down a group in the periodic table, the atomic radius increases. This is because each successive element has an additional electron shell, making the atom larger. The increased distance between the nucleus and the outermost electrons reduces the effective nuclear charge experienced by these electrons, allowing the atomic radius to expand.

Why does atomic radius decrease as you move across a period from left to right?

As you move across a period from left to right, the atomic radius decreases. This occurs because the number of protons in the nucleus increases, which enhances the nuclear charge. The added electrons are in the same shell and experience a stronger attraction to the nucleus, pulling them closer and resulting in a smaller atomic radius.

What are the exceptions to the general trend of atomic radius in the periodic table?

The general trend of atomic radius decreasing from left to right and increasing down a group has exceptions, primarily among the transition metals. The electron arrangements in transition metals are more complex due to the filling of d-orbitals, which can lead to irregularities in atomic radius trends. These exceptions make the patterns less predictable for transition metals compared to main group elements.

How does the atomic radius of hydrogen compare to that of helium?

Hydrogen has an atomic radius of approximately 37 picometers, while helium has a smaller atomic radius of about 31 picometers. Despite both elements being in the same period, helium's greater nuclear charge pulls its electrons closer to the nucleus, resulting in a smaller atomic radius compared to hydrogen.