From Triads to the Periodic Law

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Tellurium has an atomic mass of one hundred and twenty seven point six. Iodine has an atomic mass of one hundred and twenty six point nine. So tellurium is the heavier one, right? And yet, Mendeleev placed tellurium before iodine in his table, and it still sits before iodine in the periodic table today! So, was that a mistake? Or did he see something that the masses could not show? By the end of this video, you will know the answer. Along the way, we will travel from small groups of three elements all the way to the modern periodic law. Questions on these milestones do come up in JEE, so let's begin.

0:44Döbereiner's triads

So, where does the story begin? With Döbereiner, and his triads. Triad simply means a group of three. Döbereiner took three similar elements, and wrote them in order of atomic mass. Okay? And he noticed something neat. The atomic mass of the middle element is about the mean of the other two. The wavy equals sign on screen means approximately equal. So, about the mean, not exactly the mean. Keep that word, about, in mind. Right? Now let me ask you this. Suppose I give you the atomic masses of three similar elements. How would you test Döbereiner's rule on them? Yes, exactly, we add the masses of the first and the third elements, and halve the sum. That gives their mean. Then compare it with the mass of the middle element. We will do exactly this in example one. Clear?

1:37Newlands' octaves

Next comes Newlands. He arranged the elements in order of atomic mass, and found that every eighth element resembles the first. Groups of eight, and so the name: Newlands' octaves. Got it? Now, think about it. Does this pattern of eight work for every element, all the way through the list? No, it does not, and that is its weak point. Newlands' octaves work only up to calcium. After calcium, the pattern fails. So, a good start, but not the full story. With me?

2:12Mendeleev (1869): periodic in atomic mass

Now the big one: Mendeleev, in eighteen sixty nine. His law says that the properties of the elements are periodic functions of their atomic masses. Periodic means they repeat, again and again, as the atomic mass increases. Okay? And here is what made Mendeleev special. He left gaps in his table, and predicted the elements that would fill them. Two famous ones: eka-aluminium, which is gallium, G a on screen, and eka-silicon, which is germanium, G e on screen. Got it? Now, back to our puzzle. On screen, T e is tellurium, with mass one hundred and twenty seven point six, and capital I is iodine, with mass one hundred and twenty six point nine. If we go strictly by atomic mass, which one should come first? Yes, iodine should come first, because it is the lighter one. The numbers in brackets are the atomic masses, and by mass order, iodine comes before tellurium. Right? But Mendeleev did the opposite! He put tellurium before iodine, out of mass order, to keep similar elements together. He trusted the properties over the masses. Brave, but his own law, based on mass, could not explain it. Hold on to this for one more minute.

3:31Moseley (1913) and the modern periodic law

Clear? Then came Moseley, in nineteen thirteen, with X-rays. On screen, nu, the Greek letter, is the frequency of an element's characteristic X-rays, and the tick shaped sign over it is the square root. Z is the atomic number, and small a and small b are constants. So, root nu equals small a, times the bracket Z minus small b, with the bracket worked out first. In words, the square root of the X-ray frequency is linear in the atomic number. Okay? So, tell me. Which quantity really decides the order of the elements: the atomic mass, or the atomic number? Yes, exactly, it is the atomic number! And that gives us the modern periodic law. The physical and chemical properties of the elements are periodic functions of their atomic numbers. So the words are atomic numbers, not atomic masses. Write this line down. Right? And now, look at our puzzle again. Arrange the elements by atomic number, and tellurium does come before iodine. Tellurium before iodine is in increasing Z, so there is no anomaly at all. Mendeleev's placement was right, and the atomic number explains it. Got it? And there is a bonus. Isotopes are atoms of one element with different masses. By mass, they would each need a separate place. But isotopes all have the same Z, so they need no separate places. With me? Now, here is a common trap. Mendeleev's law uses atomic mass; the modern law uses atomic number. Statement questions sometimes swap these two words, so read each word carefully. Clear?

5:16Example 1: test Dobereiner triad rule on Cl, Br and I

Now, example one. Test Döbereiner's rule on chlorine, bromine and iodine. On screen, C l is chlorine, B r is bromine, and capital I is iodine, and the numbers in brackets are atomic masses. Chlorine is thirty five point five, and iodine is one hundred and twenty six point nine. The actual atomic mass of bromine is seventy nine point nine. Pause the video and try it first. Done? Okay, let's check. Bromine is the middle element, so we need the mean of the outer two. Thirty five point five plus one hundred and twenty six point nine gives one hundred and sixty two point four. Divide that by two, and we get eighty one point two. With me? Now let us compare the two values. The rule predicts eighty one point two, and the actual mass of bromine is seventy nine point nine. So, does the rule hold here? It holds roughly, yes! Eighty one point two minus seventy nine point nine is just one point three. Close, but not exact, and that is all the rule ever promised: about the mean. Nice and simple, right?

6:28Example 2: eka-silicon is germanium; its properties matched

Now, example two, a slightly deeper one. Mendeleev predicted eka-silicon. Which element is it, and what property made the prediction convincing? Pause the video and try it first. Done? Okay, let's check. The first part we have already seen: eka-silicon is germanium, G e on screen. It filled one of the gaps that Mendeleev left in his table. Okay? Now for the deeper part of the question. Anyone can say that some element is missing, right? So what would make his prediction truly convincing? Yes, exactly, it is the properties that convince us! Look at germanium. Its atomic mass is seventy two point six. Then its density, and the formula of its oxide, G e O two. The small two written low, just after the O, means two oxygen atoms for each germanium atom. With me? And all of these matched Mendeleev's predicted values closely. That is what made the prediction convincing. Not a lucky guess, but real properties that matched the values he had predicted. Clear? And here is one small trap to avoid. Eka-aluminium is gallium, G a, and eka-silicon is germanium. It is easy to swap these two, so learn them as a pair. Got it?

7:50Quick revision: triads, octaves, Mendeleev and Moseley

So, let us do a quick revision. First, the early patterns: in a triad, the middle mass is about the mean of the other two, and in Newlands' octaves, every eighth element is like the first, but only up to calcium. Second, Mendeleev: properties periodic in atomic mass, with gaps for new elements, and tellurium placed before iodine. And third, Moseley gave us the modern law: properties are periodic in atomic number, and that settles our tellurium puzzle. Now go and try the practice problems on this page. There are just two, and you will find them easy. All the best!