Chemical equations

Rumor has it that Mg + O2 —> MgO (not MgO2). Similarly, H2 + O2 —> H20 (not H202, even though H202 DOES exist (Hydrogen Peroxide, right?)). One more example:
Al + O2 —> Al203, I’m told. Go figure.
Obviously, there’s some overarching principle at work here that I’m missing. I like to think I must have understood this stuff in 11th grade, but now (some 20+ years later) that my kid is asking me about it, I’m looking more and more like an embecile every day. Can anyone explain in 25 words or less (or whatever it takes) how to “do” chemical equations.

“embecile” was a JOKE, ok?

OK, consider H2 and O2 getting together to make some H2O. A hydrogen atom has just one electon going around it in an orbit which has room for one more electon. (If this atom had one more proton and one more electron, it would be a helium atom.) An oxygen atom’s outermost orbit has room for two more electrons. So one oxygen atom hooks up with two (not one) hydrogen atoms, and their respective electrons fit in each other’s orbits, and they have a threesome (a.k.a. molecule).

that’s what comes of listening to rumors.
try this 2Mg + O2–> 2MgO
Is free hydrogen H2? been along time. you need a periodic chart and a chart of valences
12Al+9O2–> 6Al2O3 I don’t remember. you can also wind up with reslts such as —> Co2+ 4O2. You’re right Rmat it should be ‘umbilicle’ POWER TO THE PEOPLE!! FREE THE HYDROGEN RADICALS!!

“Pardon me while I have a strange interlude.”-Marx

I don’t mean to seem unappreciative, dlv, but your response reads like an explanation that can only be understood by someone who doesn’t need it. How is that one is to KNOW that hydrogen “has room for one more electron”? Is there some physical law that makes this so? And why, in the case of Oxygen, is there room for two more electrons? Surely there is some consistent principle that explains this phenomenon? You see, I wasn’t kidding when I said I was an ediot.

ok ok ok, “ediot” was ALSO a joke. And Mr John, you’re as bad as dlv! What’s with the “free hydrogen.” Is most hydrogen costly, enslaved. Can someone explain this like you’re talking to 7th grader? Or, to make things a little closer to reality, to someone with a degree in . . . (shall I dare say it?) a social “science”?

It’s spelled ‘idjit’ free hydrogen or free any element is an element all by it self. free Oxygen usually is found as O2. Its cause of the electron stuff. oxygen don’t like to be by itself its got one empty ‘hook’ so it latches on to somethin else that that has an empty ‘hook’ ozone is O3, but lets keep it simple. Carbon, C, has two empty hooks so you get CO2. For 7th grade, you gotta make sure you got the same amount of elements on each side of the equation. I guess you’re doin homework where you gotta balance some equations that are already given, like your examples? I don’t think in 7th you gotta start from scratch. In the back of the textbook should be a chart of valences, that is how many hooks each element has, some have positive hooks some negative some elements wont combine with each other. some won’t combine with anything, He ,Helium, for example. It is one of the Noble or inert gasses. Like I said it has been way long my major was biology.( 1 F [where f is fly] + 1F --. leventyzillion maggots.) but to start from scratch you gotta know how many "hooks’ each element has and if it is a ‘positve’ or ‘negative’ hook. You either memorise those or look um up. I had to memorise um to pass chemI forgot um right after the test. The proff carried a pocket chart. POWER TO THE ESTABLISHMENT!! CAPTURE THE FREE RADICALS!!!

“Pardon me while I have a strange interlude.”-Marx

Please believe me, Mr John, when I say I don’t mind being condescended to; indeed, it’s sort of what I was hoping for. I suspect your “hook” imagery would actually be quite illuminating–IF you had bothered to explain what the hood represents. Can we be concrete here: When oxygen (O2)combines with hydrogen (H2) to form water (H20), what becomes of the other O (down from O2 to O[1], if you will)? And if hydrogen peroxide, H2O2 (???) isn’t formed by combining H2 and O2, just how DOES it occur? Why is it (as another rumor has it) that Al + O2 --> Al2O3? Trust me when I tell you that this is not intuitive. What is it about the structure or properties of each of the various elements that makes it so dang clear what the combination ends up looking like? If providing repeated examples is the best way to explain this, so be it; there must, however, be some overarching principle at play here that will explain ALL chemical equations, right? After all, this is REAL science, not the sort of squishy crap I received an advanced degree in.

The equations are given in whatever least common multiple allows the numbers to come out right. For example:
2 H2 +O2 = 2 H2O (Yes, hydrogen in it’s ordinary form is H2.)
2N + 3 H2 = 2 NH3 (nitrogen plus hydrogen makes ammonia)
2 CH4 + 3 O2 = 2 CO2 + 2 H2O (methane plus oxygen makes carbon dioxide and water).

Presumably, the reactions include lots of intermediate steps involving breaking the molecules apart into short-lived free radicals. This is why for example hydrogen mixed with oxygen is metastable at room temperature- requires a spark or heat source to cause a reaction.

Hydrogen peroxide cannot be formed by directly adding oxygen to water. It has to be formed by intermediary chemical reactions. My Encyclopedia Britannica mentions two: reaction of peroxydisulfate with water and barium peroxide with sulfuric acid.

RMat, I’m going to flunk the 25 word or less test, but:

Each element is defined as having a fixed number of protons in it’s nucleus. This is it’s atomic number. It is also the total number of electrons orbiting around the nucleus.

These electrons occupy “shells”. You can think of the shells, as they are often depicted, as the orbits of planets around the sun. Each shell can hold more than one electron, however. The number of electrons that each shell can hold is specific for each shell:

shell 1 = 2
shell 2 = 8
shell 3 = 18
shell 4 = 32

Except, sorry, it more complicated than that - each shell can have sub-shells. Each level sub-shell has a set number of electrons it can hold:

shell 1 = 2s
shell 2 = 2s + 6p
shell 3 = 2s + 6p + 10d
shell 4 = 2s + 6p + 10d + 14f

(Ask another doper for why these letters are used)

The shells are filled in a somewhat strange order. It helps to look at a periodic table while thinking about this - here’s a link:

1s - then start new row
2s + 2p - then start new row
3s + 3p - then start new row
4s + 2d + 4p - then start new row
5s + 3d + 5p - then start new row
6s + 3f + 4d + 6p - then start new row
7s + 4f + 5d + 7p & if we knew of anymore elements, we’d start a new row…

Elements are at their most stable when all of their sub-shells with any electrons are full. (= 8 total electrons in the outer shell) All of the elements at the far right of the periodic table, which have 8 electrons in their outer shell, are called inert gasses, because it is so hard to make them react with anything.

Electrons can give, take, or share electrons to fill/empty their shells as needed.

Sodium (Na) has only 1 extra electron - it would like to give it away. Alternatively, it could take 7 extra electrons, but that is not usually how these thing work. Chlorine (Cl) has 7 total electrons in it’s outer shell. If a sodium atom gives up 1 electron to a chlorine atom, both have 8 electrons & both are stable. Both are no ionized (sodium has a +1 charge; Chlorin a -1 charge, because the number of electrons is no longer equal to the number of protons.

Looking again at the periodic table, and ignoring, for the moment, hydrogen, all of the elements on the far left column normally want to get to +1. All in the 2nd column want to get to +2. All in the column under Fluorine (F) want to get to -1, and all under Oxygen (O) want to get to -2. These are called valences. Each element except inert gases have a certain valence, or valences, that they prefer, which is based upon the total number of electrons in the outermost shell. This explains why Al2O3 is the usual salt formed between oxygen & aluminum - each of 2 Al atoms gives up 3 electrons, while each of 3 oxygen atoms takes in 2 electrons - net transfer of 6 electrons.

Sometimes, though, it is advantageous for atoms to share electrons.

If you combine 1 oxygen & 2 hydrogen atoms, for example, each hydrogen contributes 1 free electron, each oxygen contributes 6 free electron (total = 8 electrons)

The end result is H-O-H, with 2 electrons shared between the oxygen atom & each hydrogen atom (hydrogen can only hold 2 electrons in it’s outer shell), and 4 electrons circling the oxygen by itself.

So, each hydrogen atom has 2 electrons circling it, & the oxygen has 8 (2x2 = 4 electrons shared with hydrogen + 4 electrons which are not shared).

This may be more than you asked for, but explaining why certain elements form certain kinds of salts really is dependent on understanding electrons.

Sue from El Paso

Experience is what you get when you didn’t get what you wanted.

Way to go sue. I was typing a long wordy explanation of what you just did. Only I couldn’t remember how many ‘hooks’ there were in each shell. I had a crash before i could send it, good thing, cause i got to see yours. I was working on that Al formula of Rmats as i typed, I was emptying a few aluminum cans. the CH3CH2OH added to the wordiness but didn’t aid my memory any.

“Pardon me while I have a strange interlude.”-Marx

Good job, Sue!!

I just read rmat’s follow-up question to me and thought of explaining it, but you did it so much better than I ever could.

Thank you.

Way to go Sue, IS RIGHT! This is actually approaching comprehensibility. Two more questions:
(1) Do they actually expect 7th graders to understand this stuff? My jr high chemistry class, as best I can recall, entailed alot of playing with Bunsen burners and making colored fluid stir clear. I am virtually certain that I never saw a periodic chart til I was in 11th grade.
(2) Can anybody out there recommend a good chemistry “study guide” or “study outline” suitable for this age group. (Btw, there is no textbook being used in this unit by my kid’s teacher; apparently, its his own frolic and detour. Whether his purpose is to drive my kid mad, me mad, or everyone mad, remains unclear.

I don’t know that they want a 7th grdr to know all the details but knowing that both sides equal is important. 7th grade ah yes, teacher generating hydrogen and filling a balloon, letting it float up to the ceiling, setting it off with a spark, SETTING THE CEILING N FIRE! coolest thing till college qual.analysis final. over boiling my unknown, BLOOP! right out of the test tube past my buddies nose, out the 2nd stry window.hitting the most officious security cop on campus.
Don’t know of any recent books, but try the library for some of Azimov’s non fiction. Don’t remember the title but he wrote one that simplifies a lot of this. Periodic table is easier than it looks once it is explained. its just it’s been so long since i really used one.

“Pardon me while I have a strange interlude.”-Marx

Rmat -

As to whether this is normal 7th grade fare, no I don’t think so. I got a smattering of it in 9th grade physical science, but was in 11th grade before really getting into the stuff I explained above.

However, I know that a lot of teachers are trying to challenge kids with stuff this IS hard for them to grasp at the time. I guess that is where this all fits in.

I suspect the teacher may be trying to sidestep the issue of how many electrons each atom wants to give up, take in, or share, and just let the kids play with the equations. To my way of thinking, that is like trying to teach addition before kids can count, but that’s just my opinion…

Unfortunately, I don’t know of a basic text that would be good for beginners - the high school library might be a good place to check. I did skim thru Amazon’s offerings, hoping for a Chemistry for Dummies listing, but no such luck. (Nothing personal there; I’ve just found the series to be well-written & very comprehensible ;)) There did seem to be some books that aim to give the basics in a user-friendly way; how well they accomplish this I don’t know know.

Chemistry can be a lot of fun or a major obstacle - it all depends on how it is presented.

Sue from El Paso

Experience is what you get when you didn’t get what you wanted.

Thank you, Sue from El Paso, for your very thoughtful assistance

Try this site:
They have links to many science education oriented sites.

Many thanks, doc, as well. I see a similar question was indeed posted there by another:

The answer was helpful, albeit I think still a tad too advanced for my modest purposes (i.e., a twelve year old).

Yeah, Sue did quite a trip there. I’d forgotten most of that stuff, because I’ve never messed with chemistry since undergraduate college. I don’t think they even got into the 's’s, 'p’s, 'd’s & 'f’s even at that level then ('49-'50), in Chem 101-2.

I do remember, though, from 8th grade, the 2, 8, 18, 32 stuff. That was in a San Jose, CA-US public school in 1944. I remember that because I had a rather punk chemistry teacher who was also the kind who sort of invited friction with the kids. I was a good student, though not really an extra-credit type; but I got curious as to why that numerical sequence, 2-8-18-32, and this teacher couldn’t explain it. So I went to the next room, to the physics teacher, and got some kind of answer. I don’t recall now what it was, but the chem teacher found out I had gone to the other teacher and really got jealous, so then I was in trouble with her for the rest of the term. (CA-US lately talks about decreased quality of its K-12 school system, but it was always lousy!)

I seem to recall later hearing that those full-shell electron-quantity numbers require quantum-mechanical considerations in order to determine them. Of course, that high-school physics teacher’s answer didn’t get into that. I never got much into QM. Maybe Sue can give us the full triple-strength dosage for this here. :wink: (Might also solve the fingernail quandary. :wink: )

Ray (not into shell games)

Each electron in an atom is described by 4 quantum numbers. Also, they don’t like to share, so no two electrons will have the same set of four numbers.

The relationships between these are:

Call “n” the “shell number”. Start at 1 and count up.

Call “l” the “shell type”. Start at 0 and count up. There are n of these in shell number n.

Call “m” the “orientation”. Start at -l and count up to +l, so there are 2l+1 of these for each shell type l.

Call “s” the “spin”. It’s either +1/2 or -1/2.

If you work it out, you get 2 x n^2 combinations for each shell n.

n=1: 2
n=2: 8
n=3: 18
n=4: 32
n=5: 50
n=6: 72
n=7: 98

It gets complicated starting in the middle of shell 3, because it turns out that the energy of each electron depends on both n and l, and the n=4,l=0 electrons are at a slightly LOWER energy level than the n=3,l=2 electrons.