Chemistry

An Overview of the Periodic Table of the Elements



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If you have had the opportunity to study chemistry at all, chances are that you've met the Periodic Table of the Elements. No, it doesn't have to be capitalized, but it sure looks impressive that way, and well it should. This is a powerful reference tool, and unlike the dictionary or encyclopedia, you don't even have to turn pages. The only trouble is, the darn thing is in code. Here's your guide to cracking that code, as presented by your friendly (and only moderately crazy) chemistry teacher.

Elements are listed using their chemical symbols. Some tables are nice and will write out the names as well, but the symbols are the big, bold signposts you'll go hunting for. Often, those symbols are pretty easy to guess. Oxygen, unsurprisingly, is "O". Helium (our favorite of course) is an obvious "He". But then, skimming along the list, you come to element number eleven - sodium. "So what's the problem," you ask? The problem is - it isn't "So". Instead, you get the familiar "Na", to which the careless person might reply: "I don't know, it's Greek to me." And they'd be right. The Na comes from the old Greek name for sodium - Natrium. You might recall that early scientists were obsessed with Greek and Latin, so those were the names they stuck with. There will be a few you'll come across on the table, so here's the list to watch for:

Sodium - Na - Natrium
Copper - Cu - Cuprum
Tungsten - W - Wolfrum (still used in Germany)
Iron - Fe - Ferrum
Lead - Pb - Plumbum (Lead pipes - Plumbers - here's the connection)
Tin - Sn - Stannum
Gold - Au - Aurum
Potassium - K - Kalium
Silver - Ag - Argentium (In French, money is "argent")
Antimony - Sb - Stibium
Mercury - Hg - Hydrargyrum

Once you can find the element you need, it's time to gather some information about it. The easiest place to start is with those nice "atomic numbers". You have certainly noticed them; they seem to count off the numbers in order. We even used "number eleven" to refer to sodium earlier. Aside from being a useful reference number, the atomic also tells you how many protons are found in the nucleus of one atom of that element. Each element is defined based on this number, so any atom with eleven protons has to be sodium. If you took one away, you'd have Neon instead (ten), or if you added a proton, you'd have Magnesium (twelve). Please don't think that's any easy matter though - that'd be a nuclear reaction, with great amounts of energy involved. Remember how many centuries the alchemists spent trying to turn Lead (Pb - atomic number 82) into Gold (Au - atomic number 79)? If only they'd had a periodic table, they'd have known that they had to take away three protons to make that happen, instead of trying so many noxious brews. Now you'll never have to make that mistake! On a side note, in a neutral atom, the number of electrons is equal to the number of protons, so the atomic number is useful there too. Electrons are tricky though - they can be gained or lost, making ions, so stick to the protons for now.

The other number you're pretty much assured to see on a periodic table is the "mass number". While the atomic number is usually above the chemical symbol, the mass number is normally found at the bottom. You can recognize it because it will be roughly double the atomic number on most cases, and it will have a trail of numbers beyond the decimal. Depending on the quality of the table, the number could be carried out anywhere from one decimal place to five (or more!). It's nice to have those extra digits if you're doing some serious calculations, but the average student is well-served by two or three places. The mass number is exactly what it claims to be - the mass of that element, measure in "atomic mass units", or amu. The tricky thing here is that no single atom has that mass. The number is an average (determined by experiment) of the masses of "all" the atoms of that element in a particular area. (The variability happens because there are different isotopes of each element - forms with different numbers of neutrons - but that's a lesson for another day.) What's fun is that the average is different in different places. You can see a difference in the atomic mass if you compare elements mined in, say, Canada and China, and look at the fourth or fifth decimal place. The difference is even more pronounced when comparing atomic masses from Earth to those on comets or stars.

There are sometimes other data included on fancier periodic tables. They tend to make the table look cluttered, and you'll have to refer to the legend to see what they are, but they may include: atomic radius, ionization energy, electronic configuration, common oxidation states, and any other information the author decides to cram in. My personal preference is to keep this information elsewhere, and leave the table uncluttered and easy to use. Obviously others have different preferences. You can choose from thousands of tables, so there's certain to be a perfect one for you.

Some information that is to be found on the periodic table is unwritten. That's right, the very way the table is laid out gives you a wealth of information. For example, elements in the same column (referred to as a group or family) have similar electron configurations, and similar chemical properties. For example, the far right column of most periodic tables (He, Ne, Ar, Kr...) contains the noble gasses. They all have a full set of electrons (research electron configuration to learn more), and as a result tend to be unreactive (or stable). At the opposite end of the periodic table are the "alkali metals" (Li, Na, K...) which all start off with one outer electron, giving them a very high reactivity. Students are often impressed with how violently these elements will react with water.

There are trends to be found in the periodic table as well. In any given row, the atomic radius (or the size of the atom) will generally shrink as you move to the right. As a result, you can easily predict that an atom of Potassium (K) will be larger than an atom of Bromine (Br). Similarly, atoms get bigger as you go down a column, so Bromine, in turn, is much bigger than Fluorine (F). Putting the two together, you can figure that the biggest atoms are to be found at the bottom left of the table, and the smallest at the top right.

Other trends include first ionization energies, electron affinities, and ionic radii. The first two of these deal with the energy involved in gaining or losing an electron, and are biggest near the top right of the table. The ionic radius deals with the size of each atom's ion, and is a little tougher to explain in brief, but generally speaking, you'll have the largest ions towards the bottom right of the table.

One last feature that is common to most periodic tables is "the stair-step". If you don't immediately see it, look for a dark zig-zag that runs along the borders between Al, Si, Ge, As, and so on. Elements are categorized as metals, non-metals, and metalloids, based on some of their general properties. Most metals are ductile (can be drawn into wires), malleable (can be beaten into shape), conduct heat and electricity well, and can be shiny. You find the metals to the Left of the stair-step. Over on the right side are the non-metals, which are brittle, dull, and non-conductive. Right along the stair-step are the metalloids - semi-conductive elements whose properties are generally in between those of metals and non-metals. (If you like computers and solar power, then you should appreciate metalloids.)

Now - with all that wealth of information, all that's left is to go try it out. After all, if you don't use it, how will you remember it? That means it is QUIZ TIME.

Quiz:

1. Find the chemical symbols for Calcium, Strontium, and Iodine.

2. What elements have the chemical symbols Ba, Cr, and P?

3. How many more protons does Uranium have than Thorium?

4. Which two elements have an atomic mass of almost 59? What's surprising about this?

5. Which is bigger, atomic Oxygen or atomic Cesium?

6. S, Sc, and Si are all elements with similar symbols, but very different properties. Categorize them as metal, non-metal, and metalloid - if you can!

End of Quiz

If you want to check your work, feel free to send me your answers and I will get back to you with your score and any missed answers within a couple days.

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