Ions

We will zoom inside sodium chloride crystal and explore its "atoms". You will see that sodium atoms are missing one electron, while chlorine atoms have one additional electron. In this way, you will be introduced to the concept of ions.

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

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Transcript

A long time ago, sci­en­tists no­ticed that wa­ter that was free of im­pu­ri­ties did not con­duct an elec­tric cur­rent at all.

But if salt is poured into the wa­ter, the so­lu­tion will be­come a con­duc­tor. Look, the light is on!

But why does an aque­ous so­lu­tion of sodi­um chlo­ride con­duct elec­tric­i­ty?

To an­swer this ques­tion, let's take a small grain of ta­ble salt and look in­side it.

So right now you are as small as an atom and you are fly­ing in­side a crys­tal. You have prob­a­bly al­ready no­ticed that there are par­ti­cles of two kinds here - small­er…… and big­ger ones.

These par­ti­cles look like atoms. But let's take a clos­er look at these par­ti­cles.

We start with one that is small­er in size.

It looks like a nor­mal atom.

It has a nu­cle­us and an elec­tron shell.

And it con­sists of 10 elec­trons……. 12 neu­trons…….. and 11 pro­tons.

So, as it con­tains 11 pro­tons it seems to be a sodi­um atom.

But look, there are only 10 elec­trons in this par­ti­cle!

You might say: Well, what's wrong with that? Let it be ten elec­trons, what's the prob­lem?

Let's try to build a sodi­um atom. Pro­tons and neu­trons form the nu­cle­us.

And as we have 11 pro­tons, so we need 11 elec­trons to form a sodi­um atom. But we have only ten! In our par­ti­cle, there is one elec­tron less than in the sodi­um atom and the par­ti­cle as a whole will have a pos­i­tive charge plus one.

Let's add one more elec­tron.

Now it is a sodi­um atom.

It is much big­ger than our par­ti­cle.

Re­move the elec­tron from the sodi­um atom and look again at our par­ti­cle. Our par­ti­cle is much small­er than the sodi­um atom. And it has a pos­i­tive charge.

We call this par­ti­cle an ion.

And we have a spe­cial sym­bol for this pos­i­tive sodi­um ion: sodi­um plus. But who could take an elec­tron from a sodi­um atom? Let's find this thief.

We go back to our salt crys­tal.

Now let's look care­ful­ly at the big­ger par­ti­cles.

Choose this one, for ex­am­ple.

It also has a nu­cle­us and an elec­tron shell. It con­sists of 18 elec­trons…… 18 neu­trons……. and 17 pro­tons.

So, it seems to be chlo­rine.

But if we build a chlo­rine atom from these par­ti­cles, then one ex­tra elec­tron will re­main.

We at­tach this elec­tron to the chlo­rine atom and get our par­ti­cle.

In this par­ti­cle, there is one elec­tron more than in chlo­rine atom and the par­ti­cle as a whole has a neg­a­tive charge - mi­nus one.

Now we go back to the lab.

Now you know that in­side ta­ble salt we have ions - sodi­um plus and chlo­ride mi­nus.

These ions are at­tract­ed to each oth­er. And to re­turn to our ques­tion - why does the so­lu­tion of sodi­um chlo­ride con­duct an elec­tric cur­rent?

Re­mem­ber: wa­ter con­sists of un­charged par­ti­cles, H2O mol­e­cules, but ta­ble salt con­sists of charged ions.

Charged par­ti­cles help elec­trons pass through the so­lu­tion, so an aque­ous so­lu­tion of salt con­ducts elec­tric­i­ty, but pure wa­ter does not.

What is flu­o­rine mi­nus?

The flu­o­rine atom, which gains an elec­tron, be­comes an ion with a mi­nus one charge.

Teacher's notes

Key­words

atoms, ions, charged par­ti­cles, elec­trons, an­ions, cations

Stu­dents will

  • Learn that there are sub­stances that con­sist not of atoms but of ions
  • Find out how ions are formed
  • See that the size of an atom and its ion are dif­fer­ent
  • Ions can be pos­i­tive and neg­a­tive

Hands-on ac­tiv­i­ties

Be­fore VR

The aim is to show stu­dents that ta­ble salt be­ing dis­solved in wa­ter pro­duces charged par­ti­cles able to con­nect an elec­tri­cal cir­cuit.

Con­duct the ex­per­i­ment de­scribed. Check that pure wa­ter doesn't con­duct elec­tric­i­ty. Put a pinch of ta­ble salt and see that the so­lu­tion starts to con­duct elec­tric­i­ty. (Make a 10:1 con­se­quen­tial di­lu­tion to see when the so­lu­tion stops con­duct­ing elec­tric­i­ty).

His­to­ry and sources of knowl­edge

  • Fara­day dis­cov­ery of un­known “species” that trav­elled from one elec­trode to the oth­er in aque­ous me­dia. Though he thought that these species were born un­der elec­tri­cal cur­rent.
  • Dis­so­ci­a­tion the­o­ry by Svante Ar­rhe­nius (No­bel Prize 1903). Ions are formed from crys­talline sub­stances when they are dis­solved in wa­ter.

Top­ics to dis­cuss

  • Why the sodi­um atom is much big­ger than the sodi­um ion?
  • Why a chlo­ride atom is small­er than a chlo­ride ion?
  • Sub­stances are elec­tri­cal­ly neu­tral. There are no charged sub­stances but there are charged par­ti­cles form­ing elec­tri­cal­ly neu­tral sub­stances.

Fun facts and quotes

  • Cal­ci­um, sodi­um, and potas­si­um ions are vi­tal for the prop­er func­tion­ing of your body. Be­sides oth­er im­por­tant func­tions, they great­ly af­fect how your heart works and your think­ing process.
  • Neon lights glow be­cause of the for­ma­tion of neon ions in­side the lamp. Oth­er no­ble gas­es can be used for dif­fer­ent colours (he­li­um: white to or­ange, neon: or­ange-red, ar­gon: vi­o­let to laven­der blue, kryp­ton: off-white to green, xenon: dim blue-grey or in­tense green-blue).
  • Ion en­gine to move a space­ship in space (xenon ions).
  • The small­est ion is the hy­dro­gen ion, which ac­tu­al­ly is just a pro­ton. It's about 2,000 times small­er than a hy­dro­gen atom.

Ques­tions

  • How can you form an an­ion (a neg­a­tive­ly charged ion) from an atom?
  • How can you form a cation (a pos­i­tive­ly charged ion) from an atom?
  • What is greater for a neg­a­tive­ly charged ion, the num­ber of elec­trons or the num­ber of pro­tons?
  • What is greater for a pos­i­tive­ly charged ion, the num­ber of elec­trons or the num­ber of pro­tons?

Cal­cu­lat­ing

  • How many pro­tons, elec­trons, and neu­trons does Li+ have?
  • How many pro­tons, elec­trons, and neu­trons does F- have?