Electron orbitals

Electrons in an atom behave very differently. Students will see that when an electron becomes a part of an atom, it is spread around the nucleus like a cloud. They will also study the shapes of s- and p-orbitals. Later, students will cover the Pauli Exclusion Principle, which states that only two electrons can share the same orbital.

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

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In the pre­vi­ous les­son you saw that an atom con­sists of a tiny nu­cle­us sur­round­ed by an elec­tron cloud.

But what is this elec­tron cloud? In­deed, it is one of the most im­por­tant con­cepts in chem­istry.

To un­der­stand elec­tron be­hav­iour is to un­der­stand why iron rusts, but gold does not,

why we can breathe oxy­gen, but not he­li­um.

In the pre­vi­ous les­son, we saw elec­trons in a he­li­um atom. Let's take some­thing with big­ger atoms to­day. Like our pen­cil.

Let's look in­side. Ready to dive?

We have to zoom in a bil­lion times to see in­di­vid­u­al car­bon atoms.

Let's choose one of those atoms and get clos­er.

You can see that in a car­bon atom the nu­cle­us is larg­er than in a he­li­um atom and we see more elec­trons. And they have strange fun­ny shapes.

Let's take apart our car­bon atom to see what this atom is made of.

This car­bon atom con­sists of 6 elec­trons – neg­a­tive­ly charged par­ti­cles; 6 pro­tons – heavy, pos­i­tive­ly charged par­ti­cles; and 6 neu­trons that have al­most the same mass as pro­tons but have no charge.

Now let's re-as­sem­ble it. First, we will make a car­bon atom nu­cle­us with 6 pro­tons and 6 neu­trons.

We chemists write a spe­cial sym­bol C_6_12 to show that this car­bon atom con­tains 6 pro­tons and that to­tal atom­ic mass is 12 which is the num­ber of pro­tons and neu­trons com­bined.

As you al­ready know, elec­trons are so light that we do not take them into ac­count.

And now the most in­ter­est­ing thing. Let's start to add elec­trons one by one and see what hap­pens.

We start with the first elec­tron. A neg­a­tive­ly charged elec­tron is at­tract­ed to the pos­i­tive­ly charged nu­cle­us.

But when the elec­tron be­comes a part of the atom, it is spread around the nu­cle­us like a cloud, and we call it an or­bital.

Does that sound weird? That is how na­ture works at an atom­ic lev­el.

Now let's add the sec­ond elec­tron. It will take the same place as the first elec­tron. The first two elec­trons will share their or­bital.

And here is an­oth­er new fact. Only two elec­trons can share the same or­bital.

Look what hap­pens when we add the third elec­tron. It can­not go into the or­bital where there are al­ready two elec­trons. So it has to go to the next or­bital.

Now just look what hap­pens to the oth­er elec­trons when we add them one by one.

Dur­ing the next few lessons you will see why elec­tron or­bitals are or­ga­nized in this way.

This mi­croscale world is very dif­fer­ent from the one we know.

Let's go back to our lab­o­ra­to­ry.

In this les­son you have seen an atom of car­bon. In the pre­vi­ous les­son you saw an atom of he­li­um con­tain­ing two pro­tons, two neu­trons and two elec­trons.

What will be the to­tal mass of that he­li­um atom?

The to­tal mass will be four atom­ic mass units.

Pro­tons and neu­trons have al­most the same mass, and we count each of them as one atom­ic unit.

An elec­tron is more than a thou­sand times lighter, so we ig­nore the elec­trons' mass.

Teacher's notes


atoms, elec­trons, elec­tron cloud, elec­tron or­bitals, or­bital shape

Com­mon mis­con­cep­tions

From the Bohr-Ruther­ford di­a­gram:

  • Elec­trons ac­tive­ly re­volve around the nu­cle­us
  • Elec­trons are rough­ly the same size as pro­tons and neu­trons
  • The 'dis­tances' from the nu­cle­us to the elec­trons and be­tween elec­trons

Stu­dents will

  • Learn that the nu­cle­us is sur­round­ed by an elec­tron clouds
  • Be­gin ex­plor­ing or­bitals
  • Learn that two elec­trons can share one or­bital
  • Learn that or­bitals have dif­fer­ent shapes and sizes

His­to­ry and sources of knowl­edge

  • His­to­ry from Ruther­ford to Bohr and mod­ern the­o­ry.
  • Quan­tum the­o­ry: de­scrip­tion, com­pu­ta­tion­al mod­els.
  • Sci­en­tif­ic ap­proach: the best sci­en­tif­ic ex­pla­na­tion is based on ev­i­dence (ob­ser­va­tions) and sci­en­tif­ic knowl­edge. A the­o­ry should ex­plain ob­ser­va­tions and make pre­dic­tions.

Top­ics to dis­cuss

  • How and why did atom­ic mod­els de­vel­op? The sci­en­tif­ic ap­proach man­dates cor­rect­ing a hy­poth­e­sis if it is in­suf­fi­cient to ex­plain new find­ings (ob­ser­va­tions, mea­sure­ments).
  • How can we trust the­o­ries about things we can't see?
  • Emis­sions and the ab­sorp­tion spec­tra of el­e­ments as a source of in­for­ma­tion about en­er­gy lev­els.
  • En­er­gy lev­el con­cepts.
  • Lad­der with a fixed po­si­tion anal­o­gy: when climb­ing a lad­der, you can only step on the rungs' fixed po­si­tions, not in be­tween them. Like­wise, elec­trons can only have cer­tain en­er­gy lev­els. In this anal­o­gy, the gaps be­tween the lad­der's rungs would be much small­er, and small­er still at the top.
  • Quan­tum the­o­ry.


  • What’s the dif­fer­ence in shape be­tween s- and p- or­bitals? (sym­me­try de­scrip­tion)?


  • How many elec­trons do the first 3 no­ble gas­es have (with ex­pla­na­tion)?


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.