High School Chemistry/Early Development of a Theory

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You learned earlier how all matter in the universe is made out of tiny building blocks called atoms. The concept of the atom is accepted by all modern scientists, but when atoms were first proposed about 2500 years ago, ancient philosophers laughed at the idea. It has always been difficult to convince people of the existence of things that are too small to see. There are many observations that are made on atoms, however, that do not involve actually seeing the atom itself and science is about observing and devising a theory to explain why those observations occur. We will spend some time considering the evidence (observations) that convince scientists of the existence of atoms.

Lesson Objectives[edit]

  • Give a short history of the Concept of the atom.
  • State the Law of Definite Proportions.
  • State the Law of Multiple Proportions.
  • State Dalton's Atomic Theory, and explain its historical development.

Democritus and the "Atom"[edit]

Figure 4.1: Democritus was known as "The Laughing Philosopher". It's a good thing he liked to laugh, because most other philosophers were laughing at his theories.

Before we discuss the experiments and evidence which have, over the years, convinced scientists that matter is made up of atoms, it's only fair to give credit to the man who proposed "atoms" in the first place. 2,500 years ago, early Greek philosophers believed the entire universe was a single, huge, entity. In other words, "everything was one". They believed that all objects, all matter, and all substance were connected as a single, big, unchangeable "thing". Now you're probably disturbed by the word unchangeable. Certainly you've seen the world around you change, and those early Greek philosophers must have too. Why, then, would they think that the universe was unchangeable? Well, strange as it may sound to you today, back then the generally accepted theory was that the world didn't change – it just looked like it did. In other words, Greek philosophers believed that all change (and all motion) was an illusion. It was all in your head! Compared to this crazy idea, the atom is looking pretty good, isn't it?

One of the first people to propose "atoms" was a man known as Democritus (Figure 4.1). Democritus didn't like the idea that life was an illusion any more than you probably do. As an alternative, he suggested that the world did change, and he explained this change by proposing atomos or atomon – tiny, indivisible, solid objects making up all matter in the universe.

Democritus then reasoned that changes occur when the many atomos in an object were reconnected or recombined in different ways. Democritus even extended his theory, suggesting that there were different varieties of atomos with different shapes, sizes, and masses. He thought, however, that shape, size and mass were the only properties differentiating the different types of atomos. According to Democritus, other characteristics, like color and taste, did not reflect properties of the atomos themselves, but rather, resulted from the different ways in which the atomos were combined and connected to one another (Figure 4.2).

Emerald
Ruby
Figure 4.2: Democritus believed that properties like color depended on how the atomos were connected to each other, and not on the atomos themselves. Interestingly, Democritus was partially right – the green emerald and the red ruby both contain atoms of aluminum, oxygen, and chromium. The emerald, however, also contains silicon and beryllium atoms.

Even though the idea of the atomos seems much more reasonable than trying to explain experience as an illusion, the early Greek philosophers didn't like it, and they didn't for the following reason. If all matter consists of tiny atomos that float around, bang into each other, and connect together in various ways, these atomos must be floating in something. But what? Well, according to Democritus, the atoms floated around in a void (empty space or "nothingness"). That does seem a bit strange, doesn't it? Certainly, if you pound your fist on the desk in front of you, it doesn't feel like there's any empty space in it. What's more, Greek philosophers thought that empty space was illogical. In order to exist, they argued, nothing must be something, meaning nothing wasn't nothing, but that's a contradiction. Their arguments got quite confusing, but the end result was that Greek philosophers dismissed Democritus' theory entirely. Sadly, it took over two millennia before the theory of atomos (or "atoms", as they’re known today) was fully appreciated.

Greek Philosophers Didn't Experiment[edit]

Figure 4.3: Greek philosophers liked to think – they didn't, however, like to experiment all that much.

Early Greek philosophers disliked Democritus' theory of atomos because they believed a void, or complete "nothingness", was illogical. To ancient thinkers, a theory that went against "logic" was far worse than a theory that went against experience or observation. That's because Greek philosophers truly believed that, above all else, our understanding of the world should rely on "logic". In fact, they argued that the world couldn't be understood using our senses at all, because our senses could deceive us (these were, of course, the same people who argued that all change in the world was an illusion). Therefore, instead of relying on observation, Greek philosophers tried to understand the world using their minds and, more specifically, the power of reason.

Unfortunately, when Greek philosophers applied reason to Democritus' theory, their arguments were inconsistent. Democritus' void had to be "something" to exist, but at the same time it had to be "nothing" to be a void. Greek philosophers were not willing to accept the idea that "nothing" could be "something" – that seemed illogical.

Today, we call these contradictions (such as "nothing" is "something") paradoxes. Science is full of paradoxes. Sometimes these paradoxes result when our scientific theories are wrong or incomplete, and sometimes they result because we make bad assumptions about what's "logical" and what isn't. In the case of the void, it turns out that "nothing" really can exist, so in a way, "nothing" is "something".

So how could the Greek philosophers have known that Democritus had a good idea with his theory of "atomos"? It would have taken is some careful observation and a few simple experiments. Now you might wonder why Greek philosophers didn't perform any experiments to actually test Democritus' theory. The problem, of course, was that Greek philosophers didn't believe in experiments at all. Remember, Greek philosophers didn't trust their senses, they only trusted the reasoning power of the mind (Figure 4.3).

Alchemists Experimented But Didn't Seek Explanation[edit]

Figure 4.4: Yet another alchemist searching for the philosopher's stone. Notice how the alchemists used a lot of experimental techniques. It's too bad they were only interested in making gold!

As you learned in the last section, the early Greek philosophers tried to understand the nature of the world through reason and logic, but not through experiment and observation. As a result, they had some very interesting ideas, but they felt no need to justify their ideas based on life experiences. In a lot of ways, you can think of the Greek philosophers as being "all thought and no action". It's truly amazing how much they achieved using their minds, but because they never performed any experiments, they missed or rejected a lot of discoveries that they could have made otherwise.

Some of the earliest experimental work was done by the alchemists (Figure 4.4). Remember that the alchemists were extremely interested in discovering the "philosopher's stone", which could turn common metals into gold. Of course, they also dabbled in medicines and cures, hoping to find "the elixir of life", and other such miraculous potions. On the other hand, alchemists were not overly concerned with any deep questions about the nature of the world. In contrast to the Greek philosophers, you can think of alchemists as being "all action and no thought". Alchemists experimented freely with everything that they could find. In general, though, they didn't think too much about their results and what their results might tell them about the world. Instead, they were only interested in whether or not they had made gold.

To be fair, there were some alchemists who tried to use results from past experiments to help suggest future experiments. Nevertheless, alchemy always had very materialistic goals in mind – goals like producing gold and living forever. Alchemists were not troubled by philosophical questions like "what is the universe made of?" - they didn’t really care unless they thought it would somehow help them find the "philosopher's stone" or the "elixir of life".

What you've probably noticed by reading about the Greek philosophers and the alchemists is that the history of science is ironic. Greek philosophers asked deep questions about the universe but didn't believe in any of the experiments that might have led them to the answers. In contrast, alchemists believed in experimentation but weren't interested in what the experiments might tell them in terms of the nature of the world. Unbelievably, it took over 2,000 years to put the questions asked by the Greek philosophers together with the experimental tools developed by the alchemists. The result was significant progress in our understanding of nature and the universe, and that's what we'll learn about next.

Dalton's Atomic Theory[edit]

Let's begin our discussion of Dalton's atomic theory by considering a simple, but important experiment that suggested matter might be made up of atoms. In the late 1700s and early 1800s, scientists began noticing that when certain substances, like hydrogen and oxygen, were combined to produce a new substance, like water, the reactants (hydrogen and oxygen) always reacted in the same proportions by mass. In other words, if 1 gram of hydrogen reacted with 8 grams of oxygen, then 2 grams of hydrogen would react with 16 grams of oxygen, and 3 grams of hydrogen would react with 24 grams of oxygen. Strangely, the observation that hydrogen and oxygen always reacted in the "same proportions by mass" wasn't special. In fact, it turned out that the reactants in every chemical reaction reacted in the same proportions by mass. Take, for example, nitrogen and hydrogen, which react to produce ammonia (a chemical you’ve probably used to clean your house). 1 gram of hydrogen will react with 4.7 grams of nitrogen, and 2 grams of hydrogen will react with 9.4 grams of nitrogen. Can you guess how much nitrogen would react with 3 grams of hydrogen?

Scientists studied reaction after reaction, but every time the result was the same. The reactants always reacted in the "same proportions by mass" or in what we call "definite proportions". As a result, scientists proposed the Law of Definite Proportions (Figure 4.5). This law states that in a given type of chemical substance, the elements are always combined in the same proportions by mass.

Figure 4.5: If 1 gram of A reacts with 8 grams of B, then by the Law of Definite Proportions, 2 grams of A must react with 16 grams of B. If 1 gram of A reacts with 8 grams of B, then by the Law of Conservation of Mass, they must produce 9 grams of C. Similarly, when 2 grams of A react with 16 grams of B, they must produce 18 grams of C.

Earlier, you learned that an "element" is a grouping in which there is only one type of atom – of course, when the Law of Definite Proportions was first discovered, scientists didn't know about atoms or elements, so the law was stated slightly differently. We'll stick with this modern version, though, since it's easiest to understand.

The Law of Definite Proportions applies when elements are reacted together to form the same product. Therefore, while the Law of Definite Proportions can be used to compare two experiments in which hydrogen and oxygen react to form water, the Law of Definite Proportions can not be used to compare one experiment in which hydrogen and oxygen react to form water, and another experiment in which hydrogen and oxygen react to form hydrogen peroxide (peroxide is another material that can be made from hydrogen and oxygen).

Figure 4.6: John Dalton was a thinker, but he was also an experimenter.

While scientists around the turn of the 18th century weren't making a lot of peroxide, a man named John Dalton was experimenting with several reactions in which the reactant elements formed more than one type of product, depending on the experimental conditions he used (Figure 4.6). One common reaction that he studied was the reaction between carbon and oxygen. When carbon and oxygen react, they produce two different substances – we'll call these substances A and B. It turned out that, given the same amount of carbon, forming B always required exactly twice as much oxygen as forming A. In other words, if you can make A with 3 grams of carbon and 4 grams of oxygen, B can be made with the same 3 grams of carbon, but with 8 grams oxygen. Dalton asked himself – why does B require 2 times as much oxygen as A? Why not 1.21 times as much oxygen, or 0.95 times as much oxygen? Why a whole number like 2?

The situation became even stranger when Dalton tried similar experiments with different substances. For example, when he reacted nitrogen and oxygen, Dalton discovered that he could make three different substances – we'll call them C, D, and E. As it turned out, for the same amount of nitrogen, D always required twice as much oxygen as C. Similarly, E always required exactly four times as much oxygen as C. Once again, Dalton noticed that small whole numbers (2 and 4) seemed to be the rule.

Dalton used his experimental results to propose the Law of Multiple Proportions: when two elements react to form more than one substance, the different masses of one element (like oxygen) that are combined with the same mass of the other element (like nitrogen) are in a ratio of small whole numbers.

Dalton thought about his Law of Multiple Proportions and tried to find some theory that would explain it. Dalton also knew about the Law of Definite Proportions and the Law of Conservation of Mass (remember that the Law of Conservation of Mass states that mass is neither created nor destroyed), so what he really wanted was a theory that would explain all three of these laws using a simple, plausible model. One way to explain the relationships that Dalton and others had observed was to suggest that materials like nitrogen, carbon and oxygen were composed of small, indivisible quantities which Dalton called "atoms" (in reference to Democritus' original idea). Dalton used this idea to generate what is now known as Dalton's Atomic Theory*:

  1. Matter is made of tiny particles called atoms.
  2. Atoms are indivisible. During a chemical reaction, atoms are rearranged, but they do not break apart, nor are they created or destroyed.
  3. All atoms of a given element are identical in mass and other properties.
  4. The atoms of different elements differ in mass and other properties.
  5. Atoms of one element can combine with atoms of another element to form "compounds" – new, complex particles. In a given compound, however, the different types of atoms are always present in the same relative numbers.

∗Some people think that Dalton developed his Atomic Theory before stating the Law of Multiple Proportions, while others argue that the Law of Multiple Proportions, though not formally stated, was actually discovered first. In reality, Dalton was probably contemplating both concepts at the same time, although it is hard to tell from his laboratory notes.

Lesson Summary[edit]

  • 2,500 years ago, Democritus suggested that all matter in the universe was made up of tiny, indivisible, solid objects he called "atomos".
  • Democritus believed that there were different types of "atomos" which differed in shape, size, and mass.
  • Other Greek philosophers disliked Democritus' "atomos" theory because they felt it was illogical. Since they didn't believe in experiments, though, they had no way to test the "atomos" theory.
  • Alchemists experimented and developed experimental techniques, but they were more interested in making gold than they were in understanding the nature of matter and the universe.
  • The Law of Definite Proportions states that in a given chemical substance, the elements are always combined in the same proportions by mass.
  • The Law of Multiple Proportions states that when two elements react to form more than one substance, the different masses of one element that are combined with the same mass of the other element are in a ratio of small whole numbers.
  • Dalton used the Law of Definite Proportions, the Law of Multiple Proportions, and The Law of Conservation of Mass to propose his Atomic Theory.
  • Dalton's Atomic Theory states:
    1. Matter is made of tiny particles called atoms.
    2. Atoms are indivisible. During a chemical reaction, atoms are rearranged, but they do not break apart, nor are they created or destroyed.
    3. All atoms of a given element are identical in mass and other properties.
    4. The atoms of different elements differ in mass and other properties.
    5. Atoms of one element can combine with atoms of another element to form "compounds" – new complex particles. In a given compound, however, the different types of atoms are always present in the same relative numbers.

Review Questions[edit]

  1. It turns out that a few of the ideas in Dalton's Atomic Theory aren't entirely correct. Are inaccurate theories an indication that science is a waste of time?
  2. Suppose you are trying to decide whether to wear a sweater or a T-shirt. To make your decision, you phone two friends. The first friend says, "Wear a sweater, because I've already been outside today, and it's cold". The second friend, however, says, "Wear a T-shirt. It isn't logical to wear a sweater in July". Would you decide to go with your first friend, and wear a sweater, or with your second friend, and wear a T-shirt? Why?
  3. Decide whether each of the following statements is true or false.
    (a) Democritus believed that all matter was made of "atomos".
    (b) Democritus also believed that there was only one kind of "atomos".
    (c) Most early Greek scholars thought that the world was "ever-changing".
    (d) If the early Greek philosophers hadn't been so interested in making gold, they probably would have liked the idea of the "atomos".
  4. Match the person, or group of people, with their role in the development of chemistry.
    (a) Early Greek philosophers a. suggested that all matter was made up of tiny, indivisible objects
    (b) alchemists b. tried to apply logic to the world around them
    (c) John Dalton c. suggested that all matter was made up of tiny, indivisible objects
    (d) Democritus d. were primarily concerned with finding ways to turn common metals into gold
  5. Early Greek philosophers felt that Democritus "atomos" theory was illogical because:
    (a) no matter how hard they tried, they could never break matter into smaller pieces.
    (b) it didn't help them to make gold.
    (c) sulfur is yellow and carbon is black, so clearly "atomos" must be colored.
    (d) empty space is illogical because it implies that nothing is actually something.
  6. Which Law explains the following observation: carbon monoxide can be formed by reacting 12 grams of carbon with 16 grams of oxygen? To form carbon dioxide, however, 12 grams of carbon must react with 32 grams of oxygen.
  7. Which Law explains the following observation: carbon monoxide can be formed by reacting 12 grams of carbon with 16 grams of oxygen? It can also be formed by reacting 24 grams of carbon with 32 grams of oxygen.
  8. Which Law explains the following observation: 28 grams of carbon monoxide are formed when 12 grams of carbon reacts with 16 grams of oxygen?
  9. Which Law explains the following observations: when 12 grams of carbon react with 4 grams of hydrogen, they produce methane, and there is no carbon or hydrogen left over at the end of the reaction? If, however, 11 grams of carbon react with 4 grams of hydrogen, there is hydrogen left over at the end of the reaction.
  10. Which of the following is not part of Dalton's Atomic Theory?
    (a) matter is made of tiny particles called atoms.
    (b) during a chemical reaction, atoms are rearranged.
    (c) during a nuclear reaction, atoms are split apart.
    (d) all atoms of a specific element are the same.
  11. Consider the following data: 3.6 grams of boron react with 1.0 grams of hydrogen to give 4.6 grams of BH3. How many grams of boron would react with 2.0 grams of hydrogen?
  12. Consider the following data: 12 grams of carbon and 4 grams of hydrogen react to give 16 grams of "compound A". 24 grams of carbon and 6 grams of hydrogen react to give 30 grams of "compound B". Are compound A and compound B the same? Why or why not?

Vocabulary[edit]

atomos (atomon)
Democritus' word for the tiny, indivisible, solid objects that he believed made up all matter in the universe.
Law of Definite Proportions
In a given chemical substance, the elements are always combined in the same proportions by mass.
Law of Multiple Proportions
When two elements react to form more than one substance, the different masses of one element that are combined with the same mass of the other element are in a ratio of small whole numbers.
paradox
Two statements that seem to be true, but contradict each other.
void
Another word for empty space.


The Atomic Theory · Further Understanding of the Atom