A Tour of the Periodic Table (Part 3)
In the third and final part of our series about the periodic table of elements, Ask Science explains just what is periodic about the periodic table.
Lee Falin, PhD
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A Tour of the Periodic Table (Part 3)
In Part 1 of this series on the periodic table, we looked at the information contained in each block of the periodic table of the elements. These included things like the atomic number, chemical symbol, and atomic weight of each element. I also mentioned that these blocks of elemental information are arranged in columns called groups and rows called periods.
In Part 2 of the series, we looked at how elements in each column or group share some common properties. Today, in our third and final part of this series we’re going to look at the rows each element is arranged into in order to determine just what’s so periodic about the periodic table.
Before we really get into the details of the chemistry that we’re going to look at today, I need to get something off my chest. The truth is that the periodic properties of the periodic table aren’t really all that periodic. You see, normally in math and science, something that is periodic repeats its values at regular intervals. And just like the exceptions to the properties of elements in certain column groups that I mentioned in Part 2, things aren’t quite as perfect as textbook table artists would like you to believe.
Don’t Be So Negative
The first periodic trend in the periodic table that we’ll look at is called electronegativity. I mentioned electronegativity briefly in my episode on atomic bonds, but in case you’re the kind of person who doesn’t like to dwell on negative things, electronegativity can be thought of as the measurement of how good an atom is at hoarding electrons. For example, chlorine is more electronegative than sodium, so when they get together, chlorine grabs sodium’s lone valence electron in order to complete its collection.
Starting at any given point in the periodic table, electronegativity increases as you move to the right and decreases if you move down. Except when it doesn’t…
The exceptions mainly occur around the noble gasses, a few slackers in the middle of the table, and the handful of elements at the bottom of most charts called the Lanthanoids and Actinoids.
Suck in that Nucleus
Another periodic trend is the atomic radius, which is the average size from the center of the nucleus to the edge of the area where the electrons hang out. As you move across the table from left to right, the atomic radius gets smaller. That’s because as you move from left to right, you are adding one proton each time. Each proton acts like an overprotective vampire, pulling the electrons in a little closer.
In contrast, as you move down the periodic table, the atomic radius increases. This is because moving down the table results in a relatively big increase in electrons. Since electrons don’t like to stay in large groups, they tend to spread out into separate areas called energy levels. Electrons closer to the nucleus also tend to shield the electrons on the outside levels from the effects of the protons, allowing them to drift further away.
Just like electronegativity, however, the periodic trend of atomic radii isn’t as periodic as your periodic table zealots would have you believe. Once again, the noble gasses and a handful of elements in the middle areas of the table tend to be exceptions, along with those pesky Lanthanoids and Actinoids.
Conclusion
There are many more allegedly periodic trends than the ones we’ve mentioned here today, but now you know some of the reasons the periodic table is called the periodic table. It’s a table of elements sorted out in a special way so that some of their properties follow general trends. Except for the ones that don’t.
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