Atom size

Students will see that electron orbitals look more like a fuzzy cloud of electron distribution without clear boundaries. So how is atom size defined? In this lesson, students 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

Com­mon mis­con­cep­tions

  • Atoms have clear­ly-de­fined size and spher­i­cal shape.

Stu­dents will

  • Re­call how small atoms are
  • See the prob­lem of mea­sur­ing atom­ic size
  • Ex­am­ine the def­i­ni­tion of atom­ic ra­dius

Hands-on ac­tiv­i­ties

Be­fore VR

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

This ex­am­ple asks stu­dents to mea­sure the size of a light spot from a flash­light. Put a flash­light in a stand, so that the light spot shows on the ta­ble sur­face. Switch off the over­head 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 atom with the small­est ra­dius is he­li­um. He­li­um was dis­cov­ered not on Earth, but on the Sun. The atom with the largest ra­dius is cae­sium, near­ly 10 times big­ger than he­li­um.


  • Give an ex­am­ple of some­thing (not an atom or a light spot) where size is not ob­vi­ous. How can we de­fine its size?


Please see be­low for the link to a Google form con­tain­ing a quiz on the ma­te­ri­al above.

This can be as­signed dur­ing class time or as home­work. The quizzes are marked and the sys­tem shows which ques­tions stu­dents get cor­rect and in­cor­rect. Please note that stu­dents should record their scores, as they will not be view­able lat­er.­jjyPqB­n­Jy­dx­c9