Atom size

You will see that electron orbitals look more like a fuzzy cloud of electron distribution without a clear boundary. So how do we define atom size? In this lesson, you will be introduced to several ways to define atom size.

This lesson is a part of MEL Chemistry VR. Learn more →

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You know, that an atom is small, very small. How small is it?

Look at a sheet of pa­per. The thick­ness of it is one mil­lion atoms. What is one mil­lion?

Let's take one mil­lion sug­ar cubes and stack them on top of each oth­er.

We get a tow­er above 30 000 feet, that is high­er than com­mer­cial air­lines fly.

Com­pare: one mil­lion sug­ar cubes and one mil­lion atoms.

So atoms are very small.

Let`s look at the hy­dro­gen atom. Here it is.

Be­fore, we showed you atoms as balls.

But in fact, atoms looks like a spher­i­cal cloud. Like this.

So how can we mea­sure the size of an atom? Let`s try.

Maybe this is the size of an atom?

Or that?

It is a tricky ques­tion.

It`s like try­ing to mea­sure the size of a light spot. What is the size of it?

This? Or that?

Sci­en­tists agree on how to de­ter­mine the ra­dius of an atom. We need two sim­i­lar atoms com­bined in one mol­e­cule. For a hy­dro­gen atom, it would be H2. Here it is.

We mea­sure the dis­tance be­tween the nu­clei of two atoms.

Then, di­vide it by 2.

Fi­nal­ly, we would call this - ra­dius of hy­dro­gen atom.

Dif­fer­ent atoms have dif­fer­ent atom­ic ra­dius. The small­est atom is he­li­um. And one of the largest atoms is ce­sium.

Now we go back to the lab. Some atoms ex­ist sep­a­rate­ly and do not form com­pounds. They are called no­ble gas­es. For ex­am­ple, he­li­um.

He­li­um in a bal­loon is a gas, con­sist­ing of in­di­vid­u­al atoms. For such atoms, the size can only be cal­cu­lat­ed the­o­ret­i­cal­ly.

Bromine two is a real mol­e­cule. It con­sists of two bromine atoms. The dis­tance be­tween the nu­clei of bromine atoms is 228 pi­come­ters.

Cal­cu­late the atom­ic ra­dius of bromine atom.

Yes, you are right.

Teacher's notes


atoms, atom­ic struc­ture, elec­trons, atom size, atom­ic ra­dius

Stu­dents will

  • Be re­mind­ed how small atoms are
  • See the prob­lem of mea­sur­ing the atom size
  • Be in­tro­duced to the def­i­ni­tion of atom­ic ra­dius

Hands-on ac­tiv­i­ties

Be­fore VR

The aim is to give stu­dents a real-life ex­am­ple de­pict­ing that there are ob­jects that are hard to mea­sure be­cause you first need to de­fine what the size would be for them.

This ex­am­ple asks stu­dents to mea­sure the size of a light spot from a torch. Put a torch in a stand rod, so that the light spot shows on the ta­ble sur­face. Switch off the light and ask the stu­dents to mea­sure the light spot. Com­pare the re­sults from dif­fer­ent stu­dents.

His­to­ry and sources of knowl­edge

Top­ics to dis­cuss

  • Not ev­ery­thing can be pre­cise­ly mea­sured.
  • Im­por­tance of a def­i­ni­tion.
  • Dif­fer­ent ways to de­fine atom­ic size.
  • Atom­ic size of met­als.

Fun facts and quotes

  • The small­est atom is he­li­um. He­li­um was dis­cov­ered not on Earth but on the Sun.
  • The largest atom is cae­sium, near­ly 10 times big­ger than he­li­um.


  • Give an ex­am­ple of some things (not atom or light spot) that size is not ob­vi­ous. How can we de­fine their size?


  • Cal­cu­late the ra­dius of hy­dro­gen if, in its mol­e­cule (H2), the dis­tance be­tween two hy­dro­gen nu­clei is 0.074 NM.
  • Cal­cu­late the phos­pho­rus atom ra­dius if a chain of 1,000 side-by-side phos­pho­rus atoms is 200 NM.