Lesson video

Welcome to today's lesson.

It's called The History of the Periodic Table, and it's part of the unit, atomic structure and the periodic table.

My name is Mrs. Mytum-Smithson and today we're going to learn about the development of the periodic table.

Don't worry if you don't know anything about this 'cause we're going to go through it step-by-step and look at all the different scientists who proposed how to organise elements along the way.

So by the end of this lesson, you should be able to describe the development of the periodic table and explain its current form.

Today's keywords are isotope, atomic weight, relative atomic mass, Dmitri Mendeleev.

On the next slide, there's some sentences that include these keywords, so if you want to read them, then pause the video and then press Play when you're ready to start the lesson.

Today's lesson consists of two learning cycles.

Firstly, we're going to look at scientists who organised elements.

Then we're going to look at one particular scientist called Mendeleev and what's happened beyond Mendeleev's organisation of elements.

So let's get going with organising elements.

Now we understand a lot about elements, but it wasn't always that way.

Before the discovery of protons, neutrons and electrons, scientists attempted to classify elements by arranging them in order of their atomic weights.

We now call these relative atomic masses.

So this scientist is called John Dalton, you can see a picture of him here.

And in John Dalton's time in 1803, there were only 20 known elements.

So Dalton arranged all the known elements at that time in order of atomic weights.

Here's a list of Dalton's elements and their atomic weights.

And you can see at the time there are only 29 elements, some of the names you might recognise.

So we've got sulphur, that's got an atomic weight of 13.

Copper's got an atomic weight of 56, and all he's done is he's organised them from the lowest atomic weight to the highest atomic weight, like a big list.

There were some problems with Dalton's model though, 'cause the properties of the elements did not match up with the arrangement of the elements.

So there were no patterns of similarities between these elements in the list.

A quick check for understanding.

John Dalton arranged his elements by atomic weight, number of electrons, colour, or did he do it alphabetically? Pause the video now, select one of these answers, then press Play, and I'll tell you if you're right or not.

Well done if you said John Dalton arranged his elements by atomic weight.

True or false? John Dalton had 118 elements to arrange by using their number of protons.

Is that true or is that false? Pause the video, decide if it's true or false, then press Play and I'll tell you the answer.

So well done if you said that that was false.

Now I want to justify your answer.

So is it that protons, electrons and neutrons were not known at this time; there were only 20 known elements for him to sort, or was it protons and electrons were known, but neutrons were not; there were only 60 elements to sort.

So pause the video, select your justification, and press Play and we'll go through the answer.

Well done if you said protons, electrons and neutrons were not known about at this time and there were only 20 known elements for him to sort.

So not really very many.

In 1817 there was a German physicist called Johann Wolfgang Dobereiner, and he attempted to classify the elements by their properties.

So that's the way that they behave or the way that they look.

So in 1829, he managed to sort some of the known elements into groups of three and he called these triads.

So here's his triads and the elements in each triad had similar properties.

So he was saying that lithium, sodium and potassium, they all behaved similarly, had similar kind of reactions, looked similar, and that's why he grouped those together.

A quick check for understanding now.

Johann Wolfgang Dobereiner sorted the elements in 1829 into triads and each triad contained three elements that had similar names, three elements that had similar properties, three compounds that had similar properties, or three elements that had similar atomic weights.

Pause the video now, select an answer, then press Play when you're ready and we'll see if you right or not.

So well done if you said that Johann Wolfgang Dobereiner sorted the elements into triads and each triad contained three elements that had similar properties, well done.

In 1860 at Karlsruhe in Germany, the first international conference of chemistry took place and it was really, really important.

It was attended by all the world's leading chemists, including a man called Dmitri Mendeleev.

And they agreed the atomic weights of all the 60 known elements.

So in 1860 there were about 60 elements that were known and they all agreed these atomic weights at this conference.

In 1872, a French geologist called Alexander-Emile Beguyer de Chancourtois, I'm pleased that I managed to pronounce that all right, arranged the elements in the order of their atomic weights on a cylinder along a helix or a spiral.

So if you imagine there's a cylinder, and what he's done is he's put a spiral around that cylinder, and on that he's placed some elements.

And he shows that the elements with similar properties occurred at regular intervals and he called this his telluric screw.

So if you have a look here, we've got lithium, sodium and potassium and you can see that they line up vertically and these would be going around and there would be other elements that would be lining up vertically along this helix or spiral, and these had similar properties.

Now there were some issues with de Chancourtois' telluric screw model and it wasn't accepted by other scientists for a number of reasons.

One, when he published his findings, it was poorly presented and it didn't have that diagram in there and that made it very difficult to understand.

So there was a description, but there was no diagram when he published his findings.

The other thing was many chemists were unaware of his efforts and that was because he was a geologist, so he usually studied rocks, and then he published his findings in a geological journal.

So he didn't put his information and he didn't put his diagram in a chemistry journal.

He put it in a geological journal.

So therefore it wasn't accepted by other scientists because they just didn't find the description very easy and a lot of them just didn't find his information.

A quick check for understanding now.

How did de Chancourtois arrange the elements? Did he arrange them in a spiral on a sphere, straight lines on a cuboid, a spiral on a cylinder, or straight lines on a cylinder? Pause the video now, choose your answer and then press Play and we'll go through it together.

So well done if you said that he arranged his elements in a spiral on a cylinder.

So well done if you've got that one right.

So Newlands' law of octaves had some problems with it.

So here it is, it's in a table.

And using his pattern, some of the elements that were placed together had totally different properties.

For example, you can see that iron, sulphur and oxygen are placed together and they've got completely different properties.

So irons are metal that's solid at room temperature.

Then we've got sulphur is a non-metal that's solid at room temperature, but it doesn't look at all like a metal and oxygen is a colourless gas.

And he was trying to say that elements that were placed together had similar properties and these three had totally different properties.

The next issue was that there was also more than one element placed in the same position to try and fit that repeating pattern.

And there's an issue with that too.

So his idea was not accepted by the other scientists and he was actually publicly ridiculed.

So people made fun of him because of his exceptions and the fact that his patterns did not work enough.

A quick check for understanding.

Newlands tried to sort the elements into groups where every what element showed some similar properties.

So was it every third element, every sixth element, every eighth element, or every ninth element? Pause the video now, choose your answer and then press Play and we'll go through it together.

So well done if you said eighth.

Newlands tried to sort the elements into groups where every eighth element showed some similar properties.

What I want you to do in this check for understanding is select all the correct answers to explain why Newlands' arrangement of the elements was not accepted.

Was it because sometimes two elements were in the same position? Was it because elements placed together had totally different properties? Or was it because every eighth element repeated similar properties? Pause the video now and then press Play when you've gotten the answer.

Well done if you said that the arrangement of the elements was not accepted because sometimes two elements were in the same position and sometimes elements placed together had totally different properties.

Well done if you got that right.

Well done for working really hard this lesson, what I want you to do for part one of task A is match up the name of the scientist to the method they used to order the elements.

So here's the list of the elements.

First, the tricky one to pronounce, de Chancourtois.

Then we've got Newlands, Dalton, and Dobereiner.

Then we've got atomic weight, groups of three, grouped every eighth element, or in a spiral/helix.

So match up the scientist to the method that they used to order the elements.

Pause the video whilst you do that and press Play when you've completed the task.

Well done for working hard and completing that task.

Let's go through the answers.

So de Chancourtois, he had them arranged in a spiral or a helix around a cylinder.

And then we've got Newlands.

Newlands grouped every 8th element.

So he had the octave element.

We've got Dalton.

Dalton did them by atomic weight.

Remember, there were only 20 at the time for him to sort, and finally, Dobereiner.

He did his groups of three, did his triads.

So well done if you got those right.

Well done for working hard.

Let's have a look at task A part two.

We're going to order the scientists and their method for arranging the elements from the earliest to the latest, so imagine a timeline.

So we're gonna start with the earliest person and then we're going to go to the latest person.

So we've got de Chancourtois repeating patterns of properties in the telluric screw.

Dalton's organised elements by atomic weights.

Newlands organised the elements by atomic mass, that's the law of octaves.

And Dobereiner organised the elements by properties into triads.

So order them from earliest to latest.

Pause the video whilst you do that, then press Play when you've completed this part of task A.

Well done for working hard and completing part two of task A.

Let's have a look at the order of the scientists from the earliest to the latest.

So we've got Dalton, remember he only had 20 elements to sort and he organised the elements by atomic weights.

Then Dobereiner, he then organised them into.

This time by properties into triads.

Followed by de Chancourtois repeating patterns of properties in the telluric screw.

Remember that's in a spiral around a cylinder.

And finally, the latest one was Newlands and he organised the elements by atomic weights.

And he came up with a law of octaves.

So well done if you've got those four in the correct order.

Well done for working really hard during the first learning cycle of organising elements.

Now we're going to have a look at our next learning cycle, Mendeleev and beyond.

Now we're going to have a look at somebody who is really, really important.

So this is Dmitri Mendeleev, and by 1869 he'd spent 10 years trying to do the order of the elements.

So imagine working on the same thing, he's trying to order these elements for 10 years.

And at that time there was 63 known elements.

So legend has it that he created his periodic table by writing each element name and atomic weight on a card.

And he then played a game of chemical solitaire.

And finally, so he tried to sort these elements so he put them on cards and he played this game of chemical solitaire, so he was trying to organise them into groups and into some kind of pattern.

And finally, after three days and three nights trying to do this, he fell asleep and during that sleep he dreamed the order of the elements.

So these are Mendeleev's notes for sorting the elements and it came to him in a dream.

Imagine that, 10 years and then it all comes to you in a dream, great dream.

So Mendeleev proposed the most widely accepted model for the arrangement of the elements.

And he did this by; he placed these elements in a table and he used their properties and their atomic weights.

Previously, scientists had either sorted them by their properties or by their atomic weights, but he combined the two and he sorted them into seven groups.

Now this was the real genius thing.

He actually left gaps for the undiscovered elements.

So he knew that there were more elements to discover than the 63 that he already had.

And in doing this, he also predicted the properties of the undiscovered elements.

So the undiscovered elements were named with the prefix eka- and that means one in Sanskrit.

So these were things like this one.

So here he left this gap, he put a dash there, that was his genius.

He left this gap and he said this is eka-aluminium 'cause it's one away from aluminium.

And then he predicted the properties of these missing elements.

Mendeleev predicted the properties of eka-aluminium.

When it was discovered and renamed gallium, the predictions were very accurate.

So here's a list of them.

So if we have a look at the predicted property of eka-aluminum.

So we take relative atomic mass, that's the first one.

He predicted that it would be about 68.

When it actually was discovered it was 70.

Then he looked at the density.

He predicted that the density would be 6.

0 grammes per centimetre cubed and it was actually 5.

9 centimetre cubed.

So his predictions were pretty accurate.

He also said that it would have a very low melting point.

Now that's really unusual for a metal, but he was confident in his predictions and he predicted that it was lower than 40 degrees C.

When they discovered it was actually 29.

9 degrees C.

So that is indeed lower than 40.

Then he looked at the formula of the oxides and he predicted that the formula would be E203, E being the symbol for eka-aluminium.

When it was discovered, the formula was GA203, so that's exactly the same.

So we've got two gallium atoms and three oxygen atoms. Then we have a look at the formula for the chloride.

And again, he was correct.

So for every one gallium atom, there were three chlorine atoms. So you can see here that his predictions were very accurate indeed.

Mendeleev was such a genius that he also predicted the properties of some of the other gaps.

So these elements weren't known and some of these weren't discovered until after Mendeleev had died.

So we've got eka-boron, which is now called scandium, so he predicted those properties, eka-silicon which is now called germanium, eka-manganese which is now called technetium.

And the discovery of these elements confirmed Mendeleev's predictions.

So he predicted the properties, he left gaps for them, and when they were discovered, the properties matched up his predictions, what a genius.

Now he did this because he swapped the order of the elements by atomic weights to fit them into groups of similar properties.

And he also swapped some of the elements around.

And then at the time, isotopes were not known about, but later on they were discovered and this explained why he had to swap over some of the elements to adjust their atomic weights.

In the 1890s, William Ramsey discovered the noble gases.

They took a long time to discover because they were so unreactive.

You can see a list of them here.

And the existence of the noble gases confirmed that there was a further group of elements and Group 0, the eighth group was added onto Mendeleev's table.

Mendeleev really did contribute lots to science and chemistry by organising the periodic table and leaving those gaps for those undiscovered elements.

It didn't get him a prize, but he does have an element named after him.

So element number 101 is called Mendelevium.

So there are only 15 elements that are named after scientists.

So here's one of them.

So this is Lisa Meitner, contributed lots to science.

We've got some other scientists there listed who have also got elements named after them.

At the time when Mendeleev organised his periodic table, the structure of the atom was not known.

However, once the structure of the atom had been discovered, links were made to the organisation of the modern periodic table.

So elements were ordered by the number of protons forming groups and periods.

So if you go from left to right on the periodic table, you can see that the atomic number increases by one each time.

And the number of shells occupied with electrons was the same as the period number, so that's the same as the horizontal rows.

And the number of electrons in the outer shell was the same as the group number.

The modern periodic table has rows called periods and columns called groups.

So the periods go from left to right and they are the number of shells occupied with electrons.

So if you've got an element that's in Group 2, it has two shells that are occupied with electrons.

An element in Group 3, that's got three shells with electrons in it.

Then if we have a look at the groups, the number of electrons in the outer shell is the same as the group number.

So everything in Group 3 has got three electrons in the outer shell.

Everything in Group 5 has got five electrons in the outer shell.

Everything in Group 0 has got a full outer shell.

So that's two for the first shell and eight beyond that.

What I'd like you to do now for this check for understanding is you're going to select two correct answers to complete the sentence.

The groups on a periodic table are vertical columns, horizontal rows, linked to the number of electrons in the outer shell of an element, or linked to the number of shells that are occupied by electrons.

Pause the video, select your two correct answers, and then press Play and we'll go through them together.

Well done if you said the groups on a periodic table are the vertical columns.

Remember, horizontal rows are the periods and they are linked to the number of electrons in the outer shell of an element.

So everything in Group 1 has got one electron in its outer shell.

Everything in Group 3 has got three electrons in its outer shell.

Remember, those horizontal rows are called periods and they are linked to the number of shells that are occupied by electrons.

So well done if you've got those two answers correct.

You're probably used to seeing the periodic table on things like mugs and T-shirts.

Europe's largest Mendeleev periodic table of elements is in Moscow, and if you have a look, this was made in 2021 and it's a more familiar version of the periodic table.

You can see it's got all 118 elements that are known.

The periodic table has evolved over time and it's changed as new evidence, for example, subatomic particles was collected and the ideas have been developed.

So in 2016, so that's relatively recently, the newest elements on the periodic table were added.

So these were element 113, 115, 117, and 118.

So we've now got a total of 118 elements on the periodic table.

They're all the known elements to date.

Although the modern periodic table has been around for a long time, scientists are still looking at different ways to organise the known elements into different style of periodic tables.

So you might have seen this one, this circular periodic table was suggested in 1975.

However, it's likely that the Mendeleev style periodic table will be around for a good while yet.

Here's part one of task B for you.

What I'd like you to do for this is choose the correct answer from the brackets to explain how Mendeleev organised the elements in his periodic table.

So Mendeleev's periodic table left gaps, did not leave gaps, and he could not predict properties, or predict properties of missing elements or compounds that were later discovered.

Mendeleev swapped or did not swap the order of atomic weights in order to fit the elements better into groups with different or similar properties.

Mendeleev did not know about compounds or isotopes, so was unable to explain why ordering by atomic weights or properties were not always correct.

The alkali metals or noble gases were discovered later and Group 0 was added.

So pause the video, choose the correct words, and then press Play and we'll go through the answers together.

Well done for working really hard and completing that task.

What I wanted you to do was choose a correct answer from the brackets.

So Mendeleev's periodic table left gaps and he predicted properties of missing elements that were later discovered.

Mendeleev swapped the order of atomic weights in order to fit elements better into groups of similar properties.

Mendeleev did not know about isotopes, so was unable to explain why the ordering by atomic weights were not always correct.

The noble gases were discovered later, and Group 0 was added.

So well done if you've got all of those correct.

Here's part two of task B.

How did Mendeleev predict the properties of eka-silicon, now called germanium, when it had not been discovered? So pause the video, write your answer and then press Play and we'll go through it together.

Well done for working hard on completing that task.

So your answers should include the following ideas about Mendeleev predicting the properties of eka-silicon.

So you should say that he left a gap, that was really important, when he was organising his periodic table.

And he used the properties of the surrounding elements to predict those of eka-silicon.

So well done if you got that correct.

Part three of task B.

What I'd like you to do now is complete the table to compare Mendeleev's periodic table to Newlands'.

So you've got two columns there.

The first column you're going to write about Newlands, the second one, you're going to write about Mendeleev.

So you're going to say, how were they sorted? Were there gaps or no gaps left for undiscovered elements? Were there repeating patterns? Did they include the noble gases? Did all elements fit the pattern? And were they accepted by the scientific community? So pause the video, complete the table, press Play and we'll go through the answers together.

Well done for working really hard and completing this table.

So remember we're comparing Newlands to Mendeleev and how they were sorted.

So Newlands sorted his elements by atomic weight.

Mendeleev sorted his elements by properties and atomic weight.

Newlands didn't leave any gaps.

Mendeleev left gaps and that was his genius.

Newlands did have repeating patterns and so did Mendeleev.

Did they include the gases, the noble gases? No, neither of them included the noble gases.

Did all elements fit the patterns? Not all of Newlands' elements fitted the patterns, but Mendeleev made sure that his did.

Were they accepted by the scientific community? No, Newlands' was not accepted by the scientific community, but Mendeleev's was.

Well done if you got all of those correct.

Well done for working really hard this lesson.

What I'd like you to do for this task is use a Venn diagram to compare Mendeleev's periodic table to the modern-day periodic table.

So if it's just Mendeleev, you put it in that circle, in the left-hand circle.

If it's just the modern periodic table, you put it in the right-hand circle.

If it's both Mendeleev and the modern periodic table, you put it in that overlap.

And if it's neither, you're going to put it outside of those circles.

So let's have a look at the statements.

Arranged in repeating patterns.

Arranged only by atomic weight.

Did not initially have noble gases.

Had gaps left for undiscovered elements.

Has noble gases.

Arranged by properties and atomic weight.

Contained all known elements at the time.

Vertical groups explained by the number of electrons in outer shell.

Arranged by number of protons.

Arranged only by properties.

Pause the video while you sort those statements, then press Play and we'll go through the answers together.

Well done for working really hard this lesson.

Let's have a look at the statements.

So, arranged only by properties.

That's neither Mendeleev nor the modern periodic table.

So that sits outside.

Arranged in repeating patterns.

That's both.

Vertical groups explained by number of electrons in outer shell.

That's on the modern periodic table.

Remember, they didn't know about the structure of the atom back then.

Arranged by number of protons.

Again, that's only in the modern periodic table because Mendeleev did not know about the structure of the atom.

Did not initially have noble gases, so they were added later on.

Had gaps left for undiscovered elements.

Remember, that's the genius that is Mendeleev.

He left those gaps for undiscovered elements.

They both contained all of the known elements at the time.

Arranged by properties and atomic weight.

That's Mendeleev.

Arranged only by atomic weight.

That's neither.

They both include the properties.

And has noble gases.

That's the modern periodic table.

Remember, Mendeleev's did not initially have noble gases in them.

So well done if you've got all that correct, you've done really well.

Here's a summary for today's lesson.

Dalton, in 1803, arranged known elements in order of atomic weights.

Newlands, in 1865, noticed that every eighth element in order of atomic weight repeated similar properties.

Mendeleev swapped the order of atomic weights in order to fit elements better into groups of similar properties.

Mendeleev's periodic table had gaps and he predicted missing elements that were later found.

Mendeleev was not aware of isotopes, so was unable to explain why the ordering by atomic weight was not always correct.

Well done for working really hard this lesson.

I hope that you've learned a lot and are now more confident about the history and development of the periodic table.