In many ways, college was a simpler time. Every part of the day and week had its allotted activity. Thursday nights were for frantically completing problem sets, Saturday mornings were for sleeping, Sunday afternoons were for doing all the work that hadn’t been done over the weekend. In this accounting, Tuesday night was the time to flunk a chemistry exam - correspondingly, Wednesday morning was for balefully eyeing your Chem professor. In honor of it being Wednesday, I’m going to dip into general chemistry. Don’t worry! The exam will have a very generous curve.
General chemistry gets a somewhat undeserved bad rap. It is a science defined in part by its many comprehensive tables. Tables lack the friendly approachability of species distribution maps, the mysterious elegance of quantum mechanics formulae, or the romance of the double helix. To worse matters, chemical tables cram a lot of information into not a lot of space by using certain shorthands and abbreviations, which can appear nigh on indecipherable to the uninitiated. And on some level, there is no table more incomprehensible or misunderstood than the periodic table.
Just look at that thing! Here’s a more interactive version (http://www.ptable.com). So, what exactly does all of this mean? Time for a quick review. The periodic table lists all known elements, in order of how many protons are in the nucleus of each atom. Hydrogen, as element 1, has 1 proton, osmium, as element 76, has 76 protons, etc. The number of protons in the nucleus is what makes an element. Let’s take a closer look at one of those little boxes on the table.
Sodium (to be referred to afterwards as Na) has eleven protons in its nucleus, and is correspondingly element 11. Hence the 11 in the top left corner. In most elements, the nucleus has a corresponding number of protons, giving each atom an atomic mass of (roughly) twice the atomic number. Naturally, not every atom wants to follow this rule, and there are always atoms kicking around with more neutrons than protons, or less neutrons than protons. These slightly different configurations are referred to as isotopes. Typically, the most common isotope of a given atom has an equal number of protons and neutrons (there are, of course, exceptions to this rule). You can do all kinds of fun and wacky things based on different naturally occurring isotopes, but that is a topic for another day. The atomic mass listed for each element on the periodic table is actually an average - while most Na isotopes have an atomic mass of 22, a certain percentage have an atomic mass of 23 or 21. Hence, the average atomic mass isn’t quite 22. By the way, that mass is not given in any commonly used unit - these are atoms we’re talking about here. “Small” doesn’t quite do it justice.
What about the other numbers on that image? Well, the 883 and 98.0 refer to the boiling and melting points of Na, respectively, the 1.0 has to do with a quality called electronegativity (a topic for another day), and the .971 is a measure of density. By the way, since Na has a density of less than 1, it floats. Stay tuned next week for an explanation of exactly why you should not test that quality. Not every version of the periodic table includes these particular bits of information about an element, useful though they may be. The element number, the mass, and that weird Ne thing at the bottom, however, will always be included in some form or another.
Now, what’s the deal with the [Ne]3s? Here’s where it gets fun. This whole time we’ve been talking about protons as if they were the real stars of the atom. In reality, protons are a bit like male lions. They’re big, they’re flashy, everything is identified in relation to them, they get the starring roles in Disney movies, etc. But in truth, they mostly just sit there. It’s the lionesses (or in this case, electrons) who do all the real work. With the notable exception of radioactivity, almost all chemical behavior fundamentally comes down to electrons. Atoms bond one to another and form molecules by sharing electrons in various ways. Everything is composed, at its most basic level, of various configurations of shared electrons.
Electron behavior is where we start to get into quantum mechanics, so I’ll try to keep this simple. In an atom, you have the nucleus, and then surrounding it, you have the electrons. If you thought the nucleus was small, the electrons are almost unimaginably smaller, but they’re there, and they’re negatively charged. Commonly, there will be an equal number of protons and electrons, giving the atom a neutral charge. Now, the electrons aren’t just randomly zinging around the nucleus. Well, they are, but there’s an order to it. Think of a nesting doll. The doll in the middle is the nucleus. Each of the dolls nested around it represents an area called a valence shell that can hold a certain number of electrons. Elements low in the periodic table, like helium and lithium, only have one or two possible shells, while elements high in the periodic table, like cesium or radon, have many possible shells. Shells fill in order - it’s only possible for the outermost shell to have less than its full complement of electrons.
When an atom has anything other than a full outermost shell, it’s going to react, alone or with other atoms, in an attempt to fill that shell. An atom with only one or two atoms in a shell than can hold eight atoms is likely to shed its outermost electrons, leaving it with a full shell (remember, the second-outermost shell is, by definition, full), while an atom with an outermost shell only missing one or two atoms is likely to acquire the necessary extra electrons. This leaves us with an atom with fewer electrons than protons (positive charge), and an atom with more electrons than protons (negative charge). These two atoms are then likely to bond as a result of charge attraction - an ionic bond. Covalent bonds form when atoms quite literally share electrons - the outermost shell of each atom merges together to form a new, full shell. Pretty cool, huh?
The one thing missing is a nice, simple way to write how many atoms are in the outermost shell. You can’t just write the number, because as more shells accumulate, the shape of the outermost shell, and the number of electrons it can hold, changes. There are a few ways you can write it out. You could write out all of the shells individually, but that takes up a lot of space. All of the shells theoretically possible are named by a letter-number combination. So, you can name the outermost shell, and then list the number of electrons contained. This might looks something like “3d7". Looks weird, but all it means is that the 3d shell is the outermost, and it contains seven electrons. The 3d shell can contain up to 10 electrons, so an atom with that electron configuration is going to bond in a way that gives it those 3 remaining electrons. Still, just writing down the last few shells to fill looks messy, and it’s possible to make a mistake. At this point, we cleverly take advantage of the so-called “Noble Gases”. This refers to a group of elements luck enough to exist with a full outermost shell. Elements from this group are notably unreactive because, well, they don’t need to. One such element is neon (Ne).
And that is where the Ne in [Ne]3s comes from. Na has only one proton and one electron more than Ne. By writing down “Ne”, this shorthand uses the known electron configuration of Ne (full) to indicate that behind that 3s shell in Na is a set of full shells. Oh, and by the way, if there’s only 1 electron in a shell, we don’t bother with the superscript letter. That one, I’ll admit, could be clearer. As you go from left to right across each row in the table, the number of electrons steadily increases by one each time, and the shells fill in order. Based on this order, simply looking at the place of an element in the table already tells you quite a bit about how that element is going to behave. Far to the left? Mostly empty outermost shell, likely to lose those one or two electrons and become a positively charged atom (or cation). Far to the right? Mostly full outermost shell, likely to gain one or two electrons and become a negatively charged atom (or anion). As far right as possible? Nobel gas, generally unreactive. Right smack in the middle of the table? Half-full outermost shell, likely to wind up sharing electrons. At the top of a column? Small atomic mass. At the bottom? Large atomic mass. To be even more helpful, groups of elements that tend to form the same kinds of bonds are shown via color coding. Now, I’ll let you in on a little secret. The rows are called groups. Each group is marked by having the same number of electrons in its outermost shell, so all the elements in a group share certain characteristics (i.e., going straight down from helium, a Nobel gas, you encounter neon, argon, krypton, xenon and radon, the rest of the Nobel gases). And those lovely straight-across rows that tell you how the shells fill with electrons? Those are the eponymous periods.
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