Lesson video

I'm so pleased you've joined me today to learn about properties of group seven.

This is part of the unit groups of the periodic table.

My name's Mrs. Clegg and let's move on.

And so the outcome from today's lesson is that we'll feel much more comfortable about being able to describe the patterns and properties of group seven elements and explain how those are determined by the electron structures that they have.

Here are some of our key words that I want you to listen out for.

So we've got salts, halogen, halides and diatomic.

So listen out for those.

Do you know what they mean? If not, don't worry.

We've put them in a sentence here.

So you might like to pause the video and take some notes for yourself to refer back to.

And you could do that at any point during the video.

So I have split today's lesson into three parts.

The first part will look at the electronic structure of group seven.

Then we'll look at the properties.

And finally, we'll look at the reactivity of group seven non-metals.

So let's start with electronic structure.

Group seven refers to elements in the second to far left column in the periodic table.

So here's a periodic table and look for the numbers across the top there.

Find seven and there is group seven.

Group seven are non-metals.

They include fluorine, chlorine, bromine, iodine, astatine and tennessine.

Make sure you recognise the symbols for group seven elements.

Group seven are called the halogens, and that's from Greek origins and halo in the halogen bit means salt and gen means provider or maker.

So if you put those together, a halogen is a salt provider or a salt maker.

They produce these salts when they react with a metal.

Salts are ionic compounds formed from positive ions, which are usually metals and negative non-metal ions.

And remember, halogens are non-metals.

So which of these elements that you can see is a halogen? That's great if you noticed bromine.

Bromine is a halogen.

So group seven elements all have seven electrons in the outer shell of their atoms. Here we've got fluorine.

You might like to find fluorine on your periodic table.

It has atomic number nine.

So there are nine protons and nine electrons.

And if we look at that in the electronic structure diagram, we've got two electron shells, two in the first shell, so that's a full shell.

And then seven in the outer shell.

We can also represent that with numbers, two, seven representing the shells.

Here's chlorine.

So chlorine has an atomic number of 17, so 17 protons, 17 electrons.

And here's the electronic structure diagram.

So there's three shells of chlorine, 2, 8, 7.

So the outer shell has seven electrons.

So let's have a quick check, identify the halogen or halogens with the correct electronic configurations.

So look at the diagrams and the numbers in brackets.

Which of these is a halogen? So let's have a look at these.

A, if you look quickly, it appears to have seven electrons on the outer shell, but if you look at the numbers in brackets, there should be three shells, 2, 8, 7.

So there is a shell missing, so that's not correct.

B, that has eight outer electrons, so that wouldn't be a halogen.

And it also should have three shells according to the electronic structure, 2, 8, 8.

So that's not correct.

So C is therefore correct, three shells and seven electrons on the outermost shell.

Halogen are diatomic molecules.

As such, each atom has a full outer shell of electrons and is stable.

So here we have fluorine as a diatomic molecule and the molecular formula is F2.

That means two atoms of fluorine.

We've used blue circles to represent the electrons of one of the fluorine atoms and black crosses to represent the others just to make it easy to interpret.

And if you look, each atom now has eight outer shell electrons because they share an electron to form a covalent bond.

The shared negatively charged electrons are attracted to the positively charged nuclei of the bonded atoms. Which of these correctly represents halogen molecules? Great if you'd said iodine and bromine, B and C.

A is incorrect because that would be astatine.

But the two is superscript instead of subscript, that's incorrect.

And D, that's a chlorine atom rather than a chlorine molecule.

Let's have a look at a task.

So what you've got to do is complete the molecular formula and the mass table for each of the halogens.

This is where it might be useful to have your periodic table to help you.

Match each of the halogens then to the correct electronic information once you're done.

Pause the video and come back when you are ready.

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

So we've got molecular formula, we've got the formula mass.

If we look at the periodic table, it's 19.

So there's two atoms, so it's two times 19, which is 38.

And we know that there are nine protons, so it will be two, seven.

Chlorine, 35.

5 is a mass on the periodic table.

So there's two of those, 71.

2, 8, 7, because we know there's 17 protons, so there would be 17 electrons.

And here's the rest of the answers.

You might like to pause the video to check yours.

Great if you've got all those correct.

Well done.

Let's move on to the second part of the lesson there, which is about the properties of group seven.

So group seven elements are pretty reactive and actually highly toxic, poisonous.

For this reason, they're mainly used in quite small amounts to make many useful compounds.

Some of the examples of the applicational uses include toothpaste, fluoride in toothpaste, chlorine to sterilise water and iodine to treat wounds.

Astatine and tennessine are radioactive elements and let's have a look at some of the facts about astatine 'cause we don't often look at it.

It occurs naturally, but it's very, very, very rare.

One of the rarest elements on earth and its isotope is used to treat cancer and it has a melting and boiling point which is higher than iodine.

Let's look at tennessine now and it is synthesised and it decays after one second.

So it doesn't occur naturally and its melting and boiling points because it decays so quickly have had to be predicted.

What could be the main reason why chlorine gas was used during fighting in World War I? It's very reactive and it is highly toxic or poisonous.

Well done.

Group seven elements exist as diatomic molecules and their physical states at room temperature and pressure changes as we go down the group.

Looking at that word diatomic, remember di means two.

So diatomic, two atoms. Here's the group as we go down the group then, fluorine and chlorine are gases and chlorine is a yellowy green gas bromine is a liquid, a sort of red-brown liquid and iodine and astatine are solids and iodine is a sort of silvery grey solid.

So which of these is correct? Halogens exist as monoatomic elements.

Is that true or is that false? And once you've decided, justify your answer.

So you should have said false.

Halogens do not exist as monoatomic elements because they exist as diatomic molecular substances.

The justification in A there, diatomic molecular gases.

Remember iodine, for example, is a solid and bromine is a liquid.

Diatomic elements are often gases.

Which diatomic halogens is or are not gaseous at room temperature? Well done if you said bromine, great for astatine and also iodine.

Well done.

Group seven elements show trends in some of the properties as we go down the group.

So let's have a look at some of those.

Properties that will increase as we go down the group, the size of the atoms, molecules, will increase, so the mass of the atoms and their molecular mass will increase, their density will increase and then melting and boiling points will increase.

You might like to note these down.

So when we go down the group, what happens to the size and mass of group seven elements? They increase.

The size of the atoms and molecular mass of the group seven elements increase down the group.

So here we've got a bar chart.

Along the bottom we've got the group seven elements, and along the Y axis we've got molecular mass.

And as you can see there's a clear trend with fluorine at the top of the group and astatine towards the bottom of the group that the molecular mass increases.

Atoms get heavier because they have more protons and more neutrons and they get larger because they have more electrons.

And so more electron shells.

The melting boiling points of group seven elements increase down the group.

And here we've got group seven elements along the X axis and we've got temperature on the Y axis.

There are two bars for each element.

The first bar is for boiling point and the second bar for melting point.

And if you look at both of those with fluorine at the top of the group, astatine towards the bottom of the group, you can see that there is a clear trend.

Boiling point and melting point increase down the group.

And the increase in melting and boiling point is due to the increase in size of the molecules down the group.

If we look at this little graphic, we've got some molecules and then we've got forces of attraction between the larger molecules represented by the lines there.

And you can see they're stronger between the larger molecules, iodine's much larger than fluorine.

And so where you've got these stronger forces of attraction, you need more energy to overcome them.

Which group seven elements have higher melting points than chlorine? So again, it might be helpful to have a periodic table so you can see the order of the elements.

Great if you noticed bromine and astatine and of course they are below chlorine in the periodic table.

So is this statement true or false? Group seven elements have electrostatic attraction between their molecules.

Is that true or is that false? It's indeed false.

And the justification for that answer, why is it false? Which one of those would you choose? And the answer is A.

Another trend here, group seven also shows an increase in density as we go down the group.

So have a look at this.

You can see the trend as we go down.

As the size of the molecule increases, the forces between them get stronger.

So this allows the molecules to become much more tightly packed together.

And so density will increase.

Fluorine and chlorine are the smallest halogens at the top of the group and they're the least dense and so they exist as gases at room temperature.

Task B, we've gotta sort the following statements into a table to explain why the size, mass, melting point, and boiling point of the halogen increase down the group.

So you've got three headings there and a number of statements.

So sort them out and put them under each one of those headings.

Pause the video and join us when you're ready.

Let's look at the answers.

So here we've got the statements sorted into the correct column.

Pause the video and check your answer.

Let's look at the last part of our lesson today, reactivity.

Group seven elements are very reactive.

The halogen atoms can gain one transferred electron from another element to increase their stability as they end up with a full outer shell then.

Here we've got a fluorine atom, it's got seven outer electrons and then we've got a fluoride iron because it's gained an electron.

You can see the black cross there and you can see it's now negatively charged because it has gained a negative electron.

They form stable, negatively charged ions called the halide ions.

So select any halide ions that you can see below.

Well done if you say chloride and well done if you selected B, which is the iodide iron.

C is astatine the element, so that's not an ion.

And D has the bromine atom has gained two electrons there, which is not what happens.

So that's incorrect.

Group seven non-metals or halogens readily react with group one metals to form ionically bonded halide salts.

So each group one atom transfers their outer electron to a group seven atom.

And this forms stable oppositely charged ions that electrostatically attract each other.

So here we've got sodium and you can see the one outer electron it has in its outer shell.

And here we've got fluorine with seven outer electrons.

They react together and form sodium fluoride.

Sodium is now positively charged because it has lost a negatively charged electron and you can see its outer shell now contains eight electrons.

The fluoride ion is negatively charged because it has gained one negatively charged electron, but it now has eight outer electrons.

The halide salts are giant ionic lattices with strong uniform ionic bonds in all directions.

So reactivity decreases down group seven.

We've got fluorine at the top there, most reactive.

Astatine at the bottom, least reactive.

And this is because it's much more difficult to gain an electron.

So the size of the atoms increases.

You can see the difference there between fluorine and chlorine.

The distance from the outer electron from the nucleus increases and there is a much weaker attraction from the positive nucleus towards the negative outer shell electrons.

And so it's much more difficult to gain an electron as you go down the group.

Reactivity of the group seven elements decreases down the group.

Is that true or false? And when you've made a decision, will you be able to justify your answer? So the answer is true.

And why is this? So reactivity of group seven elements decreases down the group because.

Which one of those would be correct? Great if you've chosen option A.

It becomes much more difficult to gain an electron.

So we've got a gap fill activity to do here, which is looking at the description and explanation of the trend and reactivity as we go down group seven.

So pause the video and see if you can complete that gap fill and rejoin us when you are ready.

Let's look at the answers.

So reactivity decreases down group seven because the size of the atoms increases and the distance from the nucleus to outer electron shell increases and there is a weaker attraction from the positive nucleus towards the negative outer shell electrons making it more difficult to gain an electron.

Brilliant if you've got all those correct.

Well done.

Here we've got another task to do.

We've got four people talking about the reactivity of group seven elements.

Identify the one person outta the four here who is correct and then rewrite the other three speech bubbles so that they're correct.

Pause the video and join us when you're ready.

So Alex was correct, the reactivity of group seven decreases down the group.

So we've rewritten this one to say that group seven are very reactive and they are diatomic molecules and we've rewritten this one to say they've got seven electrons in their outer shell and so do need to gain an extra electron to have a full shell.

And then we've rewritten this one to say they readily gain electrons to form negative halide ions.

Well done if you've got all those correct.

Well done.

So we've come to the end of our lesson now.

So let's have a look at the summary.

These are the things we've learned about today.

So group seven elements are called the halogens, which we know means salt maker.

They're very reactive and their reactivity decreases as we go down the group.

Properties such as molecular mass, density, melting and boiling points of these elements increases as we go down the group.

And in chemical reactions with metals, they form salts and the group seven atoms gain an outer shell electron to form a halide iron, and then they have a full outer shell of electrons, for example the chloride ion.

And the force attracting the extra electron becomes much weaker as we go down the group because the distance between the positively charged nucleus and the outer electron shell increases.

Well done today.

Lots of learning, lots going on today.

So well done and I'll see you next time.