You can find a full list of my A to Z challenge posts here. :)
My favorite class in high school was Chemistry. Really. Was freaking awesome. I loved it so much I started college as a Pre-Vet Med/Chemistry DOUBLE major. Love, love LOVE chemistry. I've probably taken more collegiate Chem classes than most anyone else who ended up with an art degree. Organic, inorganic, biochem, pharmochem, applied chem... Ah, just thinking about it makes me giddy.
My high school chemistry teacher, Mr. Larry Dean, was incredible. He was a tough old bastard, but scary smart, sweet, ornery, and a quirky old coot, all at the same time. When we started Chemistry as high school juniors (I think there were about 25 of us, maybe more) and the first thing he made us do was learn our elements and valences. On the first day of class he gave us a list of about thirty elements with their symbols each with some weird numbers he said were valences. He told us to memorize it, there'd be a test the next day, then he went on to begin explaining the periodic table, which was in our book.
The next day we did have a test. He'd say the name of an element and we had to write its symbol and all of its valences. In the order he spoke them. Any mistakes, and I mean ANY mistakes, it was counted wrong. Forget a plus or minus? The whole entry's wrong. Forget to capitalize the first letter of the symbol? Wrong. Forget a valence number? Wrong. We handed our papers to the kid beside us for grading and we were to tally up the ones they'd missed, and the ones they got right. Then we took the right ones minus the wrong ones, and that was our score for the day's valence test. So, out of 30 items, if you miss 8 you got 22 right, so it's 22-8 which becomes 14 out of a possible 30 points. That's less than 50%!
Right Minus Wrong grading is a bitch kitty to endure. You either learn the stuff fast and KNOW it, or you're going to fail.
Before the first month or so was over (and we'd moved from Valences to doing the same right minus wrong thing with a blank periodic table, filling in symbols, atomic numbers, weights, and ionic numbers), we'd whittled down to eight of us, three girls, five boys. We were together for two years, through basic and advanced chemistry. I can't recall any of us screwing up an experiment because we knew our stuff, by god. It was branded into our brains. Balancing equations became incredibly easy, like breathing. So many on this side equals the same number on that side. Gotta count those electrons, baby!
What a valence shows is how many electrons a given atom has to either give up or gain to combine with another atom (and make a compound or molecule). For example, in one water compound (H2O) there are two Hydrogens (H) and one Oxygen (O) - the teensy 2 says there are 2 Hydrogens and no number beside the Oxygen means there's only one. The H's each have a valence of +1 (it has an extra electron out there all alone it wants to lose), and the Oxygen has a valence of -2 (it has a space for two more electrons to gain). The problem is how to make the plusses and minuses add up to zero.
So, H(+1) plus H(+1) plus O(-2) = Zero. Compounds that add up to zero are stable and tend to occur naturally, and they don't get much simpler than water.
Since many elements (including Hydrogen and Oxygen) don't like to be alone, they tend to pair up with themselves (or have molecules of more than 2 of that element, but Hydrogen and Oxygen make pairs). As they come together to make the compound, it still has to add up to zero, on both sides. And, since there's 2 Hydrogens for every 1 Oxygen, you need twice as much H as O.
So, for water, it's
Four Hydrogens plus two Oxygens make two waters.
Mathematically, that's 2(2(+1)) + 2(-2)=2(2(+1)-2) Both sides equal zero, so it's a balanced equation.
Yeah, there's a LOT of math in Chemistry and it's not possible to do the math correctly if you don't know your valences. I first took Chemistry more than thirty years ago and I STILL know them, they were burned so brightly into my brain. :)
Fwiw, Inorganic Chemistry (what lots of kids take in high school) is comparatively simple, it's just straight equation balancing with simple, straightforward bonds. For example, Iron Oxide (common rust) is Fe2O3 and looks like this. Organic Chem is a LOT harder and more complicated, because there are so, so many different kinds of bonds between the atoms and they make very convoluted shapes. As an example, simple Glucose (table sugar) is C6H12O6 and looks like this. Still pretty simple. Really. How about insulin? Its formula is C256H381N64O79S6 and it looks like this. Imagine balancing those equations!!
All of the chemicals, compounds, molecules and structures in the universe exist because of valences. If atoms weren't wanting to get rid of or add on electrons, nothing would bind together. So we owe those little loose electrons a lot. :)