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

Atomic radius is a measure of the size of an atom, defined as the distance from the nucleus to the valence electron shell. As you move down a group in the periodic table, atomic radius increases due to the addition of more electron shells, which can accommodate an increasing number of electrons. However, moving across a period, the number of electrons in the same shell increases, enhancing the attraction to the positively charged nucleus and causing a decrease in atomic radius. This results in a general trend where atomic radius decreases as you move from left to right across a period and up a group towards the top right corner of the periodic table. Exceptions to this trend can occur, particularly with transition metals and elements synthesized in laboratories, which may have less stable or well-defined atomic radii. Understanding this trend is crucial, rather than memorizing specific atomic radii values, which are typically measured in picometers.

Periodic Trend: Atomic Radius

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 proton and our neutron.

Now we're going to say here, going down a group, we're going to say 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 increase. 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 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 in a decrease 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

So remember we said the general trend is as we head from left to right and up a group, our atomic radius will decrease. So remember, as we head up a group, we say that the number of shells is going to decrease. So this causes a decrease in my atomic radius at the same time as moving from left to right of the periodic table. We're going to say that the shell number stays constant, but the number of electrons within that shell is increasing. This causes greater attraction to the nucleus, which compresses the atomic radius a bit.

So if we take a look here, we can see that typically we have our atomic radius given to us within picometers. We can see that hydrogen is pretty small at 37 picometers and we can see that as we head from hydrogen to helium, there's a small decrease. Now of course, there's going to be exceptions here and there. A lot of them happen within the pit here with our transition metals. Also realize that the last row of the periodic table, those elements, because they're so large and so heavy, because they've been synthesized in laboratories, they're a little bit unstable. So we don't have definitive information on terms of their atomic radius.

So we just leave them out and realize that we have our other transition metals that are below this, our inner transition metals. They have the same issue. So when it comes to atomic radius, these are the ones that are most important. You're not expected to memorize these numbers, so don't worry. You just need to understand the general trend and it is as we head towards the top right corner of the periodic table, atomic radius for the most part will decrease. So keep that in mind when looking at questions dealing with atomic radius.

Periodic Trend: Atomic Radius Example 1

So here it says which of the following atoms has the largest atomic radius. So here we're looking at KRBY senior and California. So to make it a little bit harder, I remove the values. Now you can go and take a look, but just rely on the general trend.

We said as you head towards the top right corner, your atomic radius should decrease. But here we're looking for the largest atomic radius. So think of it in the opposite way. Who's the furthest from the top right corner?

The answer would be RB. RB would be the one here with the largest atomic radius based on the choices given to us. So just remember that general trend, it would be larger than all of the others presented before us.

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

• A. Sr > Zn > Se > Ne
• B. Zn > Sr > Se > Ne
• C. Ne > Sr > Zn > Se
• D. Zn > Se > Sr > Ne
• E. Se > Sr > Zn > Ne

Which alkaline earth metal has the smallest atomic radius? 

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

In moving from top to bottom in the same column on the periodic table, what trend is expected for atomic size? 

• A. Increase because the nucleus is getting stronger.
• B. Increase because electrons are being placed in higher n-values.
• C. Decrease because the nucleus is getting stronger.
• D. Increase because electrons are being added to degenerate orbitals.
• E. Decrease because electrons are being placed in higher n-values.

Here’s what students ask on this topic:

What is atomic radius?

Atomic radius refers to the size of an atom, typically measured from the center of the nucleus to the outer boundary of the surrounding cloud of electrons. Since the electron cloud doesn't have a firm edge, the atomic radius is often determined by how far apart atoms are when they bond together. There are a few different ways to define it, such as the covalent radius, the metallic radius, and the van der Waals radius, each relevant in different contexts.

The atomic radius tends to decrease across a period (left to right) on the periodic table because the number of protons increases, which pulls electrons closer due to the stronger electrostatic attraction. Conversely, the atomic radius generally increases down a group (top to bottom) because additional electron shells are added, making the atom larger. Understanding the concept of atomic radius is crucial in chemistry as it influences the atom's chemical properties, including reactivity and bonding characteristics.

Why does atomic radius decrease across a period?

Atomic radius decreases across a period on the periodic table primarily due to the increase in the number of protons in the nucleus of the atoms as you move from left to right. As the atomic number increases, the positive charge of the nucleus also increases. This greater positive charge attracts the electrons more strongly, pulling them closer to the nucleus.

Additionally, across a period, electrons are being added to the same principal energy level or shell. Since these electrons are not significantly shielded by additional layers of electrons, the increased nuclear charge has a more direct effect on the electron cloud. The effective nuclear charge (the net positive charge experienced by electrons in the valence shell) thus increases, causing the electron cloud to be drawn inwards, which decreases the atomic radius.

This inward pull overcomes the repulsion between electrons in the same energy level, resulting in a smaller atomic size despite the increasing number of electrons.

Why does atomic radius increase down a group?

Atomic radius increases down a group on the periodic table primarily due to two factors: the addition of electron shells and the effect of electron shielding.

As you move down a group, each successive element has an additional electron shell compared to the one above it. This means that the outermost electrons are found in orbitals that are further away from the nucleus, which naturally increases the size of the atom.

Additionally, with each new electron shell, there's an increase in the shielding effect. Inner shells of electrons shield the outer electrons from the attractive force of the nucleus. This shielding reduces the effective nuclear charge that the outermost electrons experience. Because these outer electrons are less strongly attracted to the nucleus, they can orbit at a greater distance, which further increases the atomic radius.

In summary, the atomic radius increases down a group due to the addition of electron shells and the increased electron shielding, which both cause the outermost electrons to be held less tightly and exist further from the nucleus.