Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you've decided to join me today for this lesson on determining ionic formula.

It's part of the structure and bonding unit.

We're going to work through this lesson together and I'm here to support you all the way, especially through the tricky parts.

Our learning outcome for today's lesson is I can deduce the formula of ionic compounds, including oxides, hydroxides, halides, nitrates, carbonates and sulphates, given the formula of the constituent ions.

We have some key words for you.

Empirical formula, polyatomic ions, ide and ate and the last two are endings.

So let's have a look at the key words in some sentences now.

And empirical formula is the simplest whole number ratio of atoms of each element in a compound, polyatomic ions are ions made up of more than one type of atom.

The name of a compound that ends in ide, well, that's if there is a metal and a non-metal present for example, potassium iodide, the name of a compound ends in ate.

If there are three or more elements present, one of which is oxygen, for example, potassium sulphate.

So please pause the video now and copy down these sentences with the keywords so that you can refer to them later on in the lesson.

When you've done that, press play and we'll move on to get started with the lesson.

In our lesson today on determining ionic formula, we have two learning cycles.

The first is writing ionic formula, and the second is moving on to writing more complex ionic formula.

So let's get learning and started with our first learning cycle.

Chemical formula are used to represent the different atoms or ions in a chemical compound, it's a bit like a shorthand for chemists.

We don't always want to be writing out all the long words, so we use different symbols and that also tells us lots of information.

So we've got two examples here.

First of all, the water molecule, it contains an oxygen atom, which is represented by the red circle and two hydrogens, which is represented by the white, its formula is H2O.

So that tells us that a water molecule contains an oxygen and two hydrogen atoms. We can also see the sodium chloride crystal lattice containing sodium ions and chloride ions.

And its formula is NaCl.

So if we count the ratio, we will find out that it is NaCl a one to one, and that links to the idea of an empirical formula.

And this shows the simplest whole number ratio of atoms of each element in a compound.

So to keep it simple, we're gonna have a quick look at ethane.

The ethane molecule consists of carbons and hydrogens, and if we count up the carbons and hydrogens, we'll find that it has two carbons and six hydrogen atoms. So its molecular formula is C2H6, but the simplest ratio of carbon to hydrogen is actually one to three, one carbon to three hydrogen atoms. We can divide by two and that's what we end up.

So therefore, the empirical formula is CH3.

Now the formula of an ionic compound is also an empirical formula, that's because there is no definite number of each atom in the structure.

In fact, there will be millions and millions of sodium plus ions and chloride ions in a large crystal lattice, we can't count them.

So what we actually do instead, we say for every sodium plus ion in the crystal lattice there is one chloride ion the ratio of NA plus to CL minus is one to one.

Therefore, the formula is sodium chloride.

So that's important to make that distinction when we are talking about ionic formulary, whenever we're talking about ionic formulary, it is always an empirical formula.

So to determine the empirical formula for an ionic compound, what we need to do is find the ratio of ions in the lattice.

So we've got a lattice here showing magnesium oxide and the magnesium ions have been labelled.

They are shown in green and the oxide lay ions have been labelled and they are shown in red.

If we count them up, we will find that for every Mg2 plus ion in the crystal lattice there is one O2 minus ion.

So the ratio of Mg2 plus to O2 minus is one to one.

Therefore, the formula or the empirical formula of magnesium oxide is MgO.

Okay, let's have a quick check for understanding, which of the following diagrams correctly represent the ratio in sodium iodide.

Is it A, B or C? Well done if you chose C.

In C, we can see the alternate pink and purple in regular lines that shows lattice structure in A, we've got a different number and in B, it's only showing the empirical formula.

So that's not giving the proper particle diagram for sodium iodide.

So really well done if you chose C, the formula of an ionic compound can be determined also from the charges on the ion and this is the way that we do it most of the time.

So let's have a look and see how this works.

Well, positively charged ions, also known as cations are things like lithium plus, sodium plus, potassium plus, negatively charged ions, also known as anions are the fluoride ion F-, the chloride ion CL- or the bromide ion.

And we can work this out quite easily from the group of they are in the periodic table.

The overall charge on a compound is always neutral.

So it's really important that the charges on the ions balance each other out and that is the most important takeaway from this slide.

The overall charges must be neutral in a compound.

So let's see how it works.

Potassium bromide contains potassium plus ions and bromide minus ions.

There is one positive and one negative charge.

Okay, so if we go back to look at that table, we would see potassium forms a one plus ion and bromide forms a one minus ion.

So the charge is balance out for every plus there is one minus, which means that potassium bromide has an overall charge of zero, it is neutral.

So the formula is KBr.

So let's have a look at another example, this time we'll look at magnesium oxide.

Magnesium contains Mg2 plus ions and the oxide ions are O2 minus ions.

So this time there are two positives and two negative chargers.

The Mg2 plus ion has two chargers.

The O2 minus ion has two negative chargers.

The charges is balance out, so the formula is NgO really important, we can see we've got that one to one ratio.

Okay, let's have a quick check of understanding the chemical formula for lithium fluoride.

Is LiF true or false? Well done if you pick true that is the correct answer.

Now, why A or B lithium forms Li2+ ions and fluorine forms F2 minus ions, so the charges cancel out or B lithium forms Li+ ions and fluorine forms F minus ions.

So the charges cancel out, which is correct.

Well done if you picked B.

Now in this answer, both of them had charges cancelling out, so they were both neutral, but what we need to remember here is lithium is in group one of the periodic table.

It forms one plus ions, not two plus ions.

Fluorine is group seven of the periodic table, it forms seven minus ions.

So that's really important that we get the charge correct, we can't just jump straight to the formulary, so well done if you got that right.

Okay, let's move on to another example.

This time we're gonna look at calcium chloride.

Calcium chloride contains Ca2+ ions and Cl- ions.

So this time we can see there is a different charge on the ion.

So what actually happens, well let's think about it for a moment.

We have two positive charges from our calcium two plus and we have one negative charge.

So what we need to do is we need to make these charges balance, so that they cancel each other out.

And the way we do that is by adding a second chloride ion.

So the overall charge is neutral and the formula is CaCl2, we just add the extra ion to make it balance.

Let's look at another example this time potassium oxide.

We've got K plus ions and O2- ions.

So we go through the same process as we did for the previous example.

First of all, this time there is only one positive charge on the K plus and there are two negative charges on the O2-.

So what do we need to do to balance it out? Well, we need 2K+ ions for each O2- ions.

So we add in another one, it balances out.

The overall charge is neutral, so the formula is K2O.

So if you draw it in a systematic way like we've just done here, then we'll get it right every time.

So here are some common ions formed by elements and you will recognise most of them.

We've got group one metals, which form lithium sodium and potassium one plus ions.

Hydrogen is the only non-metal ion that forms a positive ion.

That's something you will have learned in previous lessons.

We've got a couple of examples of two plus ions, calcium and magnesium, and they're in group two of the periodic table and aluminium three plus, which is in group three.

For our negatively charged ions we've got the halogens, they always form a minus ion fluorine goes to fluoride chloride and iodide and bromide.

Then we've got some two minus ions, the oxide and the sulphide ions.

So you've got to get pretty familiar with these as you use them, but remember you can always go back to the periodic table and work them out.

Another very useful tool for modelling ionic formula and to help work them out is to use what we call chemical jigsaws.

And what you can see here is a couple of chemical jigsaws.

Now the plus ion has lost an electron, so it's got a big hole in it and that's shown by the red piece of the jigsaw.

And the minus ion has gained an electron, that's what we're representing here.

So it's got a big bit sticking out of the side and you can get one plus pieces and one minus pieces and two plus pieces and two minus pieces and three plus pieces and three minus pieces, but we're not showing all of them here.

So if we wanted to use a chemical jigsaw to work out the formulate for potassium chloride, we would take a plus a red piece with one plus a blue piece with one minus, put them together.

And that would tell us that the formula for potassium chloride is KCl, because there's one of each.

If on the other hand we had magnesium iodide, we know that has magnesium two plus ions and I minus ions.

In this case, we would start off with a two plus positively charged jigsaw piece and would need two minuses and the formula for that is MI2.

So again, it's a really useful tool to help understand and get the formulae correct.

Right we've got another example here.

Lithium oxide contains Li+ and O2- ions.

We know that there is one positive and two negative charges they need to balance out.

So we need a second positive to balance out the negative charges which we can add and so formula is Li2O.

Okay, your turn now magnesium bromide contains Mg2+ ions and Br- ions.

Pause the video and see if you can work out the formulae.

Okay, let's have a look at the answer.

Well there are two positive charges from the Mg2+ and one negative charge.

To balance we need one Mg+ and two Br- ions, so the formula is MgBr2.

Excellent work if you got that correct, well done.

So we're going to move on to our first task now on writing ionic formula.

So for question one, we'd like you to first of all define the term empirical formula and then explain why an ionic compounds formula is an empirical formula.

So for question two, we'd like you to complete the table, you have been given a load of formulas.

You need to work out what the positively charged ion is or the cation, what the negatively charged ion is.

And then write down the formula, pause the video, have a go at the questions and then when you're ready press play I will go through the answers together.

So let's define an empirical formula.

Well, it's the simplest whole number ratio of atoms of each element in a compound.

For question B, explain why ionic compounds formula is its empirical formula.

Well it should have written something like this.

There's no definite quantity of each atom, so we must find the simplest ratio of the different ions present, which is the empirical formula.

And for the table, well potassium chloride has K+ ions and Cl- ions.

So it's KCl magnesium oxide has Mg2+ and O2- ions, so it's MgO.

Those first two are quite simple, because the charge is the same.

Sodium sulphide is a little harder because we have Na+ and S2-, so we must have Na2s.

We need two sodiums for every sulphur.

Beryllium fluoride is Be2+ with F-.

And this time we need two fluorides to negative charges to balance out the positive.

So it's BeF2 aluminium oxide, which is La3+ and O2-, well this one's even harder.

It's a Al2O3, so we have to get it up to six chargers to answer that.

So if you've got those right, you're doing really well done excellent work, really good.

Now we're gonna move on to question three.

Some pupil have determined the formula for the compound magnesium sulphide, which is MgS from a particle diagram.

So we've got this particle diagram that shows the sulphide ions and the magnesium ions in a layer of the lattice, I want you to identify which pupil is correct and explain why the others are incorrect.

So they're trying to work out the formula from that diagram.

So Andeep says there are six Mg2+ ions and six S2- ions, so the formula is Mg6S6.

Lucas says there is one S2- ion for each Mg2+ ion and six of each ions in total, so the formula is six MgS.

Sophia says there is one S2- ion for every Mg2+ ion, so the is MgS.

So study the diagram carefully have a look at these answers and then try and work out which pupil is correct and explain why the others are incorrect, so pause the video while you have a go at this question.

Okay, let's have a look at the answer.

Who is correct? Well if you picked Sophia, well done.

There is one S2- ion for every Mg2+ ion, so the formula is MgS.

Well what about Andeep and Lucas? So unfortunately Andeep has not got it right.

The simplest ratio of magnesium to sulphur ions is one to one, 'cause that's how we work out the empirical formula, which is the formula we use for ionic compounds.

And what about Lucas? Where has he gone wrong? 'Cause he says it's six MgS.

Yeah, he has got it wrong, so well done If you spotted that.

And again, there are many ions in a giant ionic lattice, the formula gives the simplest ratio of Mg2+ to S2- ions.

So excellent work if you first of all got the the pupils right or wrong.

And if you got those reasons, that's really, really good.

It's an important to be able to determine the ionic formula from an image such as this, well done.

Now we've just completed our first learning cycle on writing ionic formula and now we're gonna move on to look at some more complex ionic formulae.

Before we really get into that, what we're gonna do first is we're just gonna think a little bit about naming compounds.

So if a compound contains two elements, a metal and a non-metal, the name of the metal always comes first and the non-metal name changes its end to ide.

And most of the examples we looked at in our first learning cycle fell into this category.

So we have an example here if we have lithium and fluorine as our metal and non-metal, the compound is called lithium fluoride.

So the metal stays the same and the non-metal changes to ide.

If the compound contains three elements, a metal, a non-metal and oxygen, the non-metal name changes the end with ate.

An example.

So if we have a compound that contains zinc and oxygen and sulphur, we end up with zinc, which is the metal and sulphate, which is the non-metal, because it's the sulphate has got the oxygen and the sulphur part in it.

So that's really important to remember when we are naming some of these more complex ionic formulae.

So the name of a compound ends in ide if there are only two elements present, example, sodium sulphide, it has the sodium element and the sulphur element.

The name of the compound ends in ate.

If there are three or more elements present and one of them is the oxygen, so our example sodium sulphate.

This time it has the sodium, the sulphur and oxygen and that makes all the difference.

So let's see if you can have a go at naming these compounds.

We have two ionic compounds, MgCl2 and mg CO3, have a go and see if you can write down the names of these compounds.

Well done if you put magnesium chloride for the first one, the chlorine changes to chloride and excellent work, well done if you put magnesium carbonate for the second one, we have the carbon with the ate bit, which includes the oxygen.

Great, let's carry on to the next part of the lesson.

A polyatomic ion is an ion form from a group of atoms. And we can break down the word poly means many, so it is an ion made up of many atoms. So an example, the hydroxide ion is an example of a polyatomic ion, because it has an oxygen and a hydrogen atom and the formula is written OH-.

Another an oxide ion is not a polyatomic ion, why? Because it only has one oxygen and its formula is written O2 minus.

So poly means many, atomic means atoms. So if we've got more than one, if we've got lots of atoms in an ion, it is an polyatomic ion.

Calcium carbonate contains Ca2+ ions and CO32- ions.

So how do we write the formulae? Well, there are two positive charges and two negative charges.

The positive charges come from calcium, the negative charges come from the carbonate group.

And so the formula is CaCO3.

Let's have a look at another example.

Sodium hydroxide contains Na+ ions and OH- ions.

So this time we have one positive and one negative.

So the formula is NaOH.

So it's just like the ones we did in the first learning cycle, the overall compound must be neutral, it's always neutral.

So the charges have to balance and cancel each other out.

Some common polyatomic ions are listed in the table.

Now there's really only one positively charged ion that you really need to learn and that's the ammonium ion NH4+ the negatively charged ions.

You'll be familiar with most of them, because of work on acids and alkalis lower down the school.

But we have hydroxide, which is the OH- ion.

We have carbonate, which is the CO32- ion nitrate, which is the NO3 minus ion and sulphate is the SO42- ion.

And to be honest, these are formulas that you really need to learn and be able to recall because at this stage in chemistry learning, it's quite difficult to actually work them out.

Okay, so we're going to look at the example of ammonium oxide.

It contains the ammonium ion, which has a plus charge and the oxide ion, which has a two minus charge.

So there's one positive charge and two negative charges.

So we just do it exactly the same way with the as we did with the more simple compounds to balance, we need two NH pluses and 102 minus ion.

Okay, if we add those up, that's what we get, it becomes neutral.

So the formula is NH4 bracket two O.

Now when we're using polyatomic ions, we have to have brackets in to show that it's every atom in that ion that is multiplied in the formulea.

So that's a little complication that we've got to add.

So the ammonium oxide crystal lattice looks a little bit like this.

We have one oxide ion for every two NH4+ ions or ammonium ions.

So let's check our understanding the formula of calcium nitrate.

You've got the ions there, CA2+ and NO3- is A, B, C or D.

So two things, we need to look at the charges and how it's written, which answer is correct.

Well done if you chose C, C is the correct answer.

We have two nitrates for every calcium.

And when it's an polyatomic ion, we have to put the brackets around it with a subscript two.

So excellent work if you've got that right, well done.

Now transition metals can form one or more positively charged ions.

Again, this is an added complication and for the moment we'll just have to learn these different ions.

But to help us Roman numerals are often written after the name.

And this does make life easier for us, 'cause it's hard to work out.

So for example, ion two nitrate, which forms these beautiful green crystals, is often written ion two.

What that means is we have an FE2+ ion.

So if it's an FE2+ ion, the ions presence are FE2+ and NO3- compared to the ion three nitrate compound.

Now ion three compounds are a brown rust colour and this time they contain the FE3+ ion and it's written as ION three nitrate.

The ion's presence are FE3+ and NO3-.

So they've got different ions which give different properties and this is clearly shown by the different colours of the compounds.

So how do we go about writing the formula of ion nitrate? Well first of all, we need to make sure we know which ion nitrate we're talking about.

So if it's ion two nitrate, it's got two positive charges and one minus charge.

If it's ion three nitrate, it's got three positive charges and one minus charge.

So what do we need to do for ion two to balance? We need two minus charges.

So a second NO3- ion is needed.

If we add that in, the charge is balance.

The formula is FE brackets NO3 brackets two.

So those brackets mean that the nitrate ion is basically being multiplied by two.

So for ion three we go through the same routine.

To balance this time, we need three charges that are negative.

So a total of three NO3- ions are needed.

Can add those in, we can see that they all balance now.

And the formula is FE bracket NO3, bracket three telling us that there are three nitrates for every ion three.

So having those Roman numerals are really important.

They're not just there for show.

They tell us which ion is present, which helps us work out the correct formula.

So the formula of copper two sulphate is A, B, C or D.

Well done if you chose D, it's CU.

SO four, because copper two is a CU2+.

And we are told in the question that sulphate is SO42- excellent work.

So we now come to our second task, task B, we've got a couple of questions for you.

First of all, we'd like you to write down the definition of polyatomic ion and give an example and secondly, match each of the foreign compounds to its formulae.

So potassium nitride, you need to draw a line across from the name to the formulae.

So pause the video and have a go at that question.

And then when you're ready, press play and we will look at the answers together.

Okay, so let's have a look at the answers.

First of all, what is a polyatomic ion? And give an example.

Well, a polyatomic ion is an ion formed from a group of atoms. And we've got a few examples here.

You just needed to pick one.

A nitrate ion NO3- a sulphate ion.

So four two minus a carbonate ion CO33- hydroxide ion OH- or the ammonium ion NH4+.

So matching the following compound.

So its formulate for, first of all, potassium nitride is K3N.

Potassium nitrate is KO3 sodium iodate gives it away, it's got an oxygen is NaIO3.

Sodium iodide is NaI, magnesium sulphide is MgS.

And magnesium sulphate is MgSO4.

So well done if you've got all those examples right? Great work.

Right, for the next part of the task, first of all, question three, which of the following ions are polyatomic? Just draw a circle around the ones that you think are polyatomic ions.

And then for question four, write down the formula of the following ionic compounds, so just write it next to it.

Iron two hydroxide, sodium carbonate, ammonium nitrate iron three sulphate and ammonium hydroxide.

So pause the video, answer the question, and then we'll have a look at the answers together.

Okay, so let's have a look at the answers.

So for question three, if you circled the sulphate, the carbonate, and the ammonium ion, you've got the correct answer, well done.

We know there are polyatomic because they have more than one element in their formulae, excellent work.

For the next question ion N2 hydroxide is written FE(OH)2.

Really important that the brackets are included.

Sodium carbonate, well that's Na2CO3.

Ammonium nitrate NH4N, NO3, ion three sulphate.

Well this is another one with brackets FE2(SO4)3 and aluminium hydroxide Al(OH)3.

Absolutely well done if you've got all of those correct, because some of those formula are actually quite difficult to write, 'cause you've got to remember the brackets and to get the subscripts in the right places, well done.

So let's summarise our learning from this lesson.

The key learning points, an empirical formula is the simplest whole number ratio of atoms of each element in a compound.

The formula of an ionic compound is an imperial formula as there is no definite quantity of each ion.

Ionic compounds ending with ide only contain a metal iron and a non-metal ion polyatomic ions are ions that are made up of more than one type of atom.

Ionic compounds ending with ate always contain at least three elements, one of which is oxygen.

I hope that you've enjoyed today's lesson and I look forward to learning with you again very soon.