The World's Greatest Cheat Sheet
In honor of Back to School Week, Common Science is publishing an homage to the world’s greatest cheat sheet, the Periodic Table of the Elements. Somewhere during the first week of high school chemistry (yes, I know some of you never had chemistry in high school but I try not to think about that) your teacher presented a Periodic Table which probably looked like the one at the top of the page. Sometimes when I get into conversations with people about their experience in high school chemistry, I ask them “why is the Periodic Table called the Periodic Table?” I usually don’t get an answer. Periodic means “repeating”. It’s called the Periodic Table because the columns are designed around repeating patterns. The man credited with creating the modern version in 1869 was a Russian Scientist named Dimitri Mendeleev. The Periodic Table of the Elements, is in all relevant aspects, a cheat sheet. And it’s a good one.
Most of us know that atoms are made of protons, neutrons, and electrons. An element is a particular type of atom. Elements have the same number of protons and electrons. Protons and neutrons form the nucleus of the atoms with the electrons floating around nearby. The simplest of all the elements, Hydrogen, has one proton and one electron. The next simplest element is Helium which has 2 protons and two electrons. The atomic number of each element corresponds to the number of protons/electrons in the atom, e.g. Hydrogen and Helium have atomic numbers 1 and 2, respectively. Elements also have simple abbreviations. Hydrogen is H and Helium is He.
The difficult part for your teacher came next. Some elements, depending on their number of electrons, are stable and, therefore, tend not to react with other elements. There is a repeating pattern of the number of electrons in an element which corresponds to stability. The story gets a bit more complicated by introducing the concept of orbitals. Usually orbitals are presented in simplified form in high school as looking like circles around the nucleus. (See the picture below this paragraph.) The challenge for the teacher at this point is that explaining why some orbitals are stable and some are not. This requires a discussion of quantum mechanics which the students are not yet ready for. So the teacher has to just tell the students that they need to accept that some configurations are stable and some are not. The first three stable configurations have 2, 10 and 18 electrons (2 in the first orbital, 8 in the second and 8 in the third). The periodic table is organized around these patterns of stability.
So let’s start a walk across the periodic table. Hydrogen (H) has one proton and one electron. The one electron orbiting the proton is not a stable configuration since the first orbital, in order to be stable, must have two electrons. So the Hydrogen atom needs to find a partner with whom to share electrons in order to be happy. The most common way that hydrogen does this is to combine with another Hydrogen atom to form an H2 molecule (See the cartoon below this paragraph to visualize). Now the two electrons from the two Hydrogen atoms both orbit around the two protons creating a stable first orbital.
If we move over to Helium, it has two protons and two electrons. The two electrons form a stable first orbital without any help. Helium is the first of the “noble gases”, so named because they are too proud to stoop to share their electrons with other elements. Looking down the column from Helium we get a great example of the cheat sheet potential of the Periodic Table. Below Helium are the other noble gases, Neon, Argon, Krypton, Xenon, Radon and Ununoctium (more on it below). All of these atoms have the exact number of electrons required such that outermost orbital has the proper number of electrons to be stable. Therefore, just like helium, the other noble gases tend not to participate in any chemical reactions.
Permit me a brief tangent on Ununoctium, the largest know element with atomic number 118. In preparing to write this blog I got out my chemistry textbook from my freshman year of college (1984) and looked at the section on the periodic table. (In case you are wondering, yes, I still have all of my college textbooks). My 1984 text book states that there are 108 known elements. It’s both fascinating and disturbing to reflect that I am old enough that 10 new elements have been “discovered” since I went to college. I use quotation marks since these additional elements have essentially been synthesized rather than discovered by smashing other atoms together at incredibly high speeds. Ununoctium was first synthesized in a Russian lab (Mr. Mendeleev would be proud) in 2002. Since 2002 scientists think that they have synthesized either 3 or 4 atoms of Ununoctium, each existing for several milliseconds before breaking apart through spontaneous nuclear fission.
OK, so back to the table. Lithium (Li) has 3 electrons and 3 protons. (Don’t worry I am not going to run through the entire periodic table.) Two of the electrons form the first orbital. The next orbital needs 8 electrons to be stable. A Lithium atom only has one in the second orbital (see the picture above), hence it is on a desperate search for 7 electrons other electrons to share in its outer orbital. The simplest way for Lithium to achieve this goal is to hook up with Fluorine (F). Fluorine has 9 electrons, 2 in the first orbital and 7 in the second. Therefore, Lithium and Fluorine are like a marriage made in heaven forming a LiF molecule , with one Lithium atom and one Flourine atom. Now we have another great cheat sheet example. Since we know that one Lithium will connect happily with one Fluorine, we know that the same will also be true for all other pairings in the columns. If you look at the columns you will find a familiar couple Sodium (Na) and Chlorine (Cl) which form up in one-to-one pairs to make NaCl, table salt.
As a last example, let’s start to discuss how you can start to use the Periodic Table to figure out how larger molecules are formed. Oxygen (O) has atomic number 8 so it has 8 electrons. From above we know that 2 of these electrons will form the first orbital around the nucleus of the oxygen atom. This leaves 6 electrons for the second orbital which needs 8 to be stable. Many oxygen atoms gain the two extra electrons that they need by reacting with two hydrogen atoms which contribute one electron each to create the stable and familiar molecule of H2O, water. So here again, we can use just pick up our cheat sheet and start to figure out which molecules are possible to form and which are not.
Over the years, as students found that they could use their Periodic Table cheat sheets during tests, more and more information was crammed on to the paper. You can easily find periodic tables where each square looks like the one below.
You can answer just about any chemistry question with a cheat sheet like that. Happy Back to School week!
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