How did we first figure out which substances are elements and which are compounds?

Submitted by sapphics4satan t3_1170s3h in askscience

Imagining a time where one couldn’t simply reference the periodic table, how would scientists have determined that something such as gold, once smelted and purified, was just one element, as opposed to being a compound?

Water was thought of as classical element and of course we know now that it’s made of hydrogen and oxygen. I can imagine how we might figure that out by either combining the elements that make water or breaking water up into its elements, but how could we have been certain back in the day that substances like gold were elements and not just compounds that are really hard to break apart? Could we really have been completely certain of this before we had the technology to directly observe individual atoms?

This might be a bit more of a history question. Google didn’t give me any satisfactory answers, I don’t think it really understood the question I was trying to ask.

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VT_Squire t1_j9c7wmf wrote

a long time ago....

"element" was just a word, nothing like how we use it today. Earth, wind, water, fire... then in the 1600s some clever guy thought about what we know today as chemical elements. That was the advent of "the atom." This meant the irreducible composition of a substance. By that point, you had early chemistry/alchemy actively TRYING to decompose substances into their basic elements and experimenting with different combinations. Like your question of gold... well, could it be reduced further? No? Okay, then it's an element.

It wasn't until the early 1900s that the modern definition of the element rooted in numbers of electrons and protons and such took hold. A few centuries worth of experimentation turned out to have some useful merit. Within a few decades of this, we built the bomb and haven't had a world war ever since.

Thank you for coming to my Ted talk.

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baggier t1_j9c9ai7 wrote

Back in the day the key was weights. If two things combine to give a new substance that is heavier (say iron + oxygen to iron oxide) then it is obviously a compound. If a substance cleanly decomposes to two new substances, then the new substances are simpler and might be elements (say hydrogen peroxide to oxygen and water). After building up lots of these reaction it became clear that some things (e.g carbon) couldnt be decomposed into anything simpler and must be an element.

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uh-okay-I-guess t1_j9cgbga wrote

There was a ton of guessing involved. One of the key guesses, which turned out to be correct, was that all the metals were elements. But people also guessed that "earths" were elements -- we now know they're oxides of the true elements.

Lavoisier made an influential list of elements, and some of them were wrong. He listed lime, magnesia, alumina, baryte, and silica as "earth" elements. He also missed some, like sodium, even though derived substances like lye and salt were well-known.

Lavoisier also guessed, based on analogies with known elements, that there were some elements that had not yet been isolated, like fluorine and chlorine. Interestingly, his reasoning was wrong. He thought that acids were produced by a nonmetal reacting with oxygen. Therefore, the existence of "muriatic acid" meant that there must be some "muriatic" element which combined with oxygen to produce this acid. In reality, muriatic acid did contain an unknown element (chlorine), but oxygen wasn't involved -- it's just HCl.

Most of Lavoisier's mistakes were resolved fairly soon. Davy isolated calcium, barium, and magnesium, which was fairly convincing evidence that the other "earths" were really compounds of unknown metals. (He also made chlorine, sodium, and potassium.)

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Spats_McGee t1_j9ckytw wrote

Replying to VT_Squire (#1,890,286)

Actually if I understand correctly scientists in the 1700's and 1800's understood ideas like "combining ratios" that were antecedents to modern chemistry. So, they'd know that 2 parts hydrogen 1 part oxygen was water, etc, long before they understood the structure of the atom (which was Rutherford in 190x).

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Indemnity4 t1_j9nctl1 wrote

Timeline of the history of elements

Only 15 elements were know before modern times. It's a short list.

Ancient times there were a whole lot of competing theories about what makes up stuff. Classical elements of fire, earth, wind, water + other, were slightly more complicated that just those words. Atomism was the idea that everything could be divided smaller and smaller into discrete but unknown particles; versus substance theory that things when divided were just smaller versions of themselves.

That was just enough philosophy to explain metals. You can take some "earth" and divide it (by smelting, etc) until you get to an undivisible particle. That's how we get gold, copper, iron, tin, etc. It was really obvious that a person could manipulate something to get a pure form of something without needing to invoke ghosts or the void or phlogisten.

The general idea is something is an elementary particle until proven otherwise. Lots of missteps and bad guesses along the way.

There was a really weird short lived theory called tria prima that every substance was composed of three elements: a combustible element, a fluid and changeable element, and a solid, permanent element. That was a weird side tangent for explaining how smelting could make a pure element, by burning off the combustible and mixing with the correct flux to remove the fluid. But we still have the idea that there are unique particles of matter.

It was ~1730 that scientists started to get serious about observing the world. Antoine Laviosier proposed the new term of element to describe the basic undivisible particles we know and love today.

The killer discovery for finding elements was electrochemistry. It allowed someone to very carefully divide elements from each other, so long as you could dissolve the material and separate/capture what come off it.

My favourite is aluminium. It was predicted in 1756 because someone could see it was an oxide of something, but it wasn't isolated until 1824. In this example, a person found a new "earth" very early on, any every new it had to be smelted to isolate the atom inside. It just took a really really really long time to figure out how to dissolve it, such that electrolysis could remove the oxide or "earth" bits.

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