Students will learn that different carbon atoms always contain 6 protons but can contain different numbers of neutrons. These different atoms are called isotopes. This lesson will reinforce understanding of element symbol notation.

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

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We have al­ready learned that an atom con­sists of a small heavy nu­cle­us sur­round­ed by elec­trons spread out in space. To­day, we will learn more about the struc­ture of a nu­cle­us.

Let's take our pen­cil and look in­side. Ready to dive?

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

Now let's choose one of those atoms and get clos­er to it.

Let's dis­as­sem­ble our car­bon atom to see what it is made of.

Please point – where are the pro­tons?

Pro­tons are pos­i­tive­ly charged par­ti­cles from the atom’s nu­cle­us.

Now where are the elec­trons?

Elec­trons are neg­a­tive­ly charged par­ti­cles over a thou­sand times lighter than pro­tons or neu­trons.

Fi­nal­ly, where are the neu­trons?

Neu­trons have al­most the same mass­es as pro­tons, but they are not charged.

We use sym­bol the C_6_12 for this atom to show that it has 6 pro­tons and its to­tal atom­ic mass is 12, which is the num­ber of pro­tons and neu­trons com­bined.

The num­ber of pro­tons, 6, is what makes this atom a car­bon atom. This is called the atom­ic num­ber.

Let's look at an­oth­er car­bon atom.

As you can see this car­bon atom con­tains 6 elec­trons, 6 pro­tons and 8 neu­trons.

Atoms with the same num­ber of pro­tons but a dif­fer­ent num­ber of neu­trons are called iso­topes.

What is the mass num­ber of this car­bon iso­tope?

As you know, pro­tons and neu­trons have al­most the same mass. And elec­trons are much, much lighter. So the atom­ic mass num­ber is the num­ber of pro­tons and neu­trons com­bined. In this case, the atom­ic mass num­ber is 14.

For this iso­tope, we would use the sym­bol C_6_14, where 6 is the num­ber of pro­tons and 14 is the to­tal atom­ic mass.

Now, let's go back to our lab­o­ra­to­ry.

What is the atom­ic mass of a Sodi­um iso­tope with 11 pro­tons and 12 neu­trons?

The atom­ic mass num­ber is the num­ber of pro­tons and neu­trons com­bined. Eleven plus twelve is 23.

Teacher's notes


atoms, elec­trons, nu­cle­us, pro­tons, neu­trons, iso­topes, mass num­ber

Com­mon mis­con­cep­tions

  • Iso­topes are dif­fer­ent el­e­ments be­cause their nu­clei are dif­fer­ent.
  • Con­fu­sion be­tween mass num­ber, atom­ic num­ber, and atom­ic mass.

Stu­dents will

  • Re­call that an atom’s nu­cle­us con­sists of pos­i­tive­ly-charged pro­tons and un­charged neu­trons
  • Learn that pro­tons and neu­trons are much heav­ier than elec­trons
  • Learn to in­ter­pret el­e­ment no­ta­tions, e.g. 126C, 146C
  • See that atoms of the same el­e­ment can con­tain dif­fer­ent num­bers of neu­trons in their nu­clei
  • Learn how to cal­cu­late an atom’s mass num­ber

Hands-on ac­tiv­i­ties

Af­ter VR

Dis­cus­sion of half-life con­cept.

Ex­plore how ra­dioac­tive iso­topes de­crease in quan­ti­ty over time.

Stu­dents are giv­en a cup of small grains (like rice) and are in­struct­ed to re­move half of the re­main­ing amount ev­ery 30 sec­onds.

His­to­ry and sources of knowl­edge

Top­ics to dis­cuss

  • Half-life con­cept.
  • Why the num­ber of neu­trons in an atom does not af­fect its chem­i­cal prop­er­ties.
  • Atom­ic mass as av­er­age mass of dif­fer­ent iso­topes.

Fun facts and quotes

  • You can ac­cu­mu­late heavy wa­ter (wa­ter that con­tains deu­teri­um, a hy­dro­gen iso­tope with one pro­ton and one neu­tron) in your kitchen, if you don't re­place the wa­ter in your ket­tle for a very long time and add more wa­ter as the wa­ter in the ket­tle evap­o­rates.
  • An iso­topic anal­y­sis can not only tell you when or­gan­isms lived, but also what they ate and their mi­gra­tion paths.
  • All syn­thet­ic el­e­ments only have un­sta­ble (ra­dioac­tive) iso­topes.
  • Ra­dio­car­bon dat­ing is used to de­ter­mine the ages of once-liv­ing or­gan­isms (from mil­lions of years old to thou­sands of years old).
  • The ori­gin of wine can be eas­i­ly de­ter­mined by the ra­tio of sta­ble iso­topes of the dif­fer­ent el­e­ments in the wine.


  • What dis­tin­guish­es the atoms of one el­e­ment from the atoms of an­oth­er?
  • Two atoms have the same num­ber of neu­trons but a dif­fer­ent num­ber of pro­tons. Are they iso­topes?
  • Two atoms have the same num­ber of pro­tons but a dif­fer­ent num­ber of neu­trons. Are they iso­topes?
  • What does the num­ber 7 rep­re­sent in the iso­tope 7Li?
  • List the num­ber of pro­tons, neu­trons, and elec­trons in this pair of iso­topes: 4020­Ca/4420­Ca.


  • Cal­cu­late the num­ber of neu­trons in the dif­fer­ent iso­topes of oxy­gen.
  • Cal­cu­late the atom­ic mass from the per­cent­age and mass of two iso­topes (for ex­am­ple, for 35Cl/37Cl).
  • Cal­cu­late the iso­topic ra­tio (in per­cent­age) from the mass-spec­trum (for ex­am­ple, 63Cu/65Cu).


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.­FVv9EBpXZF­FkZD9