The identification of chemical substances is both simple and complex. Every chemical, whether it is an element or a compound, has unique characteristics. It’s kind of a fun puzzle to identify what you might have when given a set of ‘mystery’ substances in a chemical lab or lab classroom.
The neat thing about the elements on the periodic table is what happens to them when they combine with other elements. Take chlorine, for example. It’s an extremely reactive non-metal element, meaning it wants to be in a combined state at all times. Imagine two single chlorine atoms, milling about in space. Once one of the chlorine atoms gets close to the other, they will snap together quite rapidly, each one sharing an outer electron with the other sort of like a handshake (where the other person won’t let go!) and forming a compound. You and I would never see a chlorine atom by itself. Its’ need to combine is so strong that we always find this element in combination with something else in nature.
The cool thing about this compound is that it makes the new chlorine molecule (a word you can only use when things SHARE electrons, though we love to call all compounds molecules…) really stable and allows it to mimic, at least on it’s exterior, the look of a noble gas. Noble gases are things you are familiar with such as Helium and Argon that don’t do much but sit around on the periodic table feeling superior to most of the other elements, unless you give them a nasty jolt of electricity. Which may serve them right, with that attitude.
So here you have two of the ways a chemical is identified; its reactivity and its ability to share some outer electrons with another non-metal atom. This sharing is called ‘covalent bonding’; co- as in co-worker, valent representing the valence, or outer electrons.
Our molecule of chlorine is generally in the company of billions and billions of other chlorine molecules. Once there are enough molecules collected for humans to see, we can determine that this is in fact chlorine gas using some other characteristics. This compound will be yellowish-green in color, in the gas form at room temperature, and deadly to humans should they breathe it (this was well known to our European friends in World War I, who had it used on them as a poison gas from time to time).
We have now added color, the state the compound is in at room temperature, and it’s effects on humans to our list of characteristics that identify our chemical. Do yourselves a favor and don’t test things on people or living things though, it’s really not nice at all.
Chlorine is not really fussy about with whom it combines. You and I see chlorine in a number of places all the time. I think you all know about table salt, which is sodium chloride. Have you ever seen a piece of salt up close, or seen rock salt? The smallest crystal of sodium chloride looks really similar to the larger rock salt crystal. This is pretty interesting because now the chlorine in the compound is looking and acting quite differently. In fact, it is layered into a lattice with the sodium. This lattice looks a bit like a ‘connect the dots’ game of some sort where each green dot (representing chlorine) is surrounded by 4 white dots (representing sodium) and each white dot (sodium) is surrounded by 4 green dots (well, you know). Kind of a chemical jungle gym, if you will.
At this point chlorine is employing an entirely new set of strategies to do the combining. Sodium is a metal and refuses to share or do the “handshake” thing with chlorine, even though chlorine is perfectly willing. Instead, sodium and chlorine rely on the fact that each one has an uneven number of outer electrons. Both elements would be much happier if sodium just GAVE chlorine an electron, because it would take care of the lopsided feeling both elements are experiencing. (Elements like to have eight outer electrons…imagine how bumpy a car ride would be on three tires! It’s sort of like that and happens any time you have less than eight outer electrons, even or uneven.) When this happens, the sodium becomes somewhat positive electrically, and the chlorine becomes somewhat negative electrically. Voila! Attraction! Just like the poles on a magnet. This attraction is called “ionic bonding”, since a positively or negatively charged atom is called an ion.
Now that we have sodium chloride, or table salt, we have created a compound that is in the solid state at room temperature (it won’t even melt at temperatures you can reach at home in your kitchen, though it will dissolve, which is entirely different), is a crystal, is white-transparent, and is safe to put on your food. Unless you over-do it, of course.
The fun thing about crystals is that they allow you to wander over into the realm of geology. Now we have an entirely new set of characteristics by which we can classify our sodium chloride, which is still a chemical even though we might have mined it from the earth. Add luster, hardness, and a host of other characteristics to the identification process for this everyday chemical.
Of course, the MOST fun is when you are given a group of unknowns and allowed to use known chemicals to test them. They may fizz, turn color, smoke, or just sit there mournfully without doing anything you can see whatsoever. How they react..or don’t react…gives you clues to what you might have there. If you like to solve mysteries, classifying chemicals simply cannot be beat, and it makes a nifty career to boot.