Okay, moving on. Sesquicentennial means 150th anniversary, and this year is the 150th anniversary of the Periodic Table of the Elements. If you went to high school, you've seen one, but that isn't quite the same as appreciating it in all its glory. I'm not sure I fully appreciated it until I had been teaching it for a few years. Ready to nerd out with me a little. Here we go.
Dmitri Mendeleev knew of only about sixty-seven elements and their masses. The proton had not yet been discovered, so the number we now arrange the table by (thanks to Henry Mosley) did not yet exist. Mendeleev wasn't the first to attempt to develop an organizational method for the elements. He was just the first to be successful. Your chemistry teacher may have told you that he dreamed the periodic table, and that's true. People really like to focus on that part. What you may not know was that he been working for three days with insomnia before a snowstorm forced him to stay home, which was when he finally fell asleep his unconscious brain was able to put the pieces together.
If you had a mean chemistry teacher that made you memorize the numbers on the periodic table (I'm sorry they didn't understand the stupidity of that), you may think the two numbers are the only information the table gives you. While the atomic number (number of protons) and the atomic mass (items in the nucleus for individual atoms, the weight of a mole for samples) are important, they still don't tell you how amazing the arrangement of the table is. Let's talk about families and periods.
Families are the vertical columns on the periodic table. Everyone in the same family has similar characteristics. Sodium, lithium, and potassium, all strip hydrogen from water molecules and then ignite the hydrogen. They all bond with chlorine in a dramatic reaction. All members of a representative family give away or take the same number of electrons when making an ionic bond, so you can know what charge it will have just be looking at the family it is in. Helium, neon, and argon don't bond at all. So if you know about one element in a family, you know a little something about all the elements in the family. Periods are the horizontal rows of elements. Every element in a row has the same number of energy levels for the electrons it holds. As you go across a period, certain properties increase or decrease predictably. Then, that property starts over again when you get to the next row. So just by looking at whether an element is on the left or the right of a row, you know something about its size, its attraction for electrons, its metallic quality, even how much energy it would take to take an electron away from it.
If you have ever thought the shape of the periodic table was a bit strange, you might not have learned about the orbital arrangement of electrons. See if this gives you any flashbacks:
The first two columns on the table represent the s orbital in each energy level, which can only hold two electrons. The six columns on the far right represent the p orbitals in each energy level, which can hold six electrons. Those ten short columns in the middle represent d orbitals, which you may have guessed, hold ten electrons. Even those two weird rows on the bottom that we pulled out of position to save space. Have you ever noticed there are fourteen elements in each of those rows? Well, that's because f-orbitals hold 14 electrons. Neither Mendeleev nor Moseley knew about energy levels, and yet it lines up perfectly. Even alternative shapes to the table would reflect this because the periodic nature is what matters, not the specific shape. Imagine these on the wall of your science classroom.
There are a lot more things that I won't bore you with, so let me put it this way. If you had to write out the information you get about an element from the periodic table, you would need a book. You couldn't write it all in one book. You would need several books just to hold it all. As my colleague, Jenny Bomgardner once said, "It's like all the world's knowledge on a sheet of paper." We both know it's not ALL the world's knowledge, but it is an awful lot about every single element.
Mendeleev's greatest contribution was letting us know that we didn't know everything. As I said earlier, the world only had knowledge of about 67 elements. There are 92 elements in nature, so he was playing with only about two-thirds of the cards. And, it wasn't like he knew about numbers 1 through 67. The world had known gold (72) and mercury (80) for a long time, but they had not yet discovered silicon (14). As Mendeleev was arranging the table, he left blanks where things didn't fit the pattern they should in a family. He then predicted that an element would be discovered to fill that blank and predicted what's properties would be. Most of his predictions were spot on. Before you start thinking too highly of him, he was wrong about a lot of things. He also predicted an element lighter than hydrogen (although to be fair that's because he didn't know about protons. His story ends tragically; he lost his mind near the end and stopped believing in atoms altogether. That doesn't change, however, the contribution he made. We are still adding to the table today as we synthesize new elements, and the pattern he established means we know exactly where to place them. The fact that the pattern still works even with elements that don't exist in nature shows just how well designed the table is.
nice
ReplyDeleteClassification of elements in groups provides us a fixed pattern in which the elements change their properties periodically. The periodic table made the study of the physical and chemical properties of elements simple and organised. This science topic is necessary to learn for all school students to score well in their exams. Classification of elements is necessary to make their study easy. By the process of classification of elements, we group together the elements with the same characteristics. As it is impossible to study each element separately and deal with their different properties so they are grouped together in the periodic table.
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