2-1: Atomic Models Through History

What is a Model?

In science, a model could mean something that you physically build, or something that you calculate or draw, to represent something else. You can also use it to try to test out an idea that you have about how something might work; then you do experiments or gather evidence to see if your model could be right.

A great example of this is how scientists have made models of the Solar System to try to explain how things move across the sky. For a long time, scientists had a model of the Solar System that put Earth at the centre:

At first glance this seems right, because you can stand on any sidewalk and, if you're patcient enough, watch the Moon, Sun, planets and stars go around you. But this model did not work to explain why Mars appears to dance back-and-forth in the sky a bit over a few weeks, why Venus looks the way it does, or how Jupiter can apparently have things orbiting it.

Eventually, scientists started to accept that a different model of the Solar System was correct, with the Sun in the centre instead.

Since observations and calculations agreed more with this model of the Solar System, it eventually became what the scientific community accepted as its real structure.

What are Atoms?

An atom is the smallest piece of an element that you can have, which still has the same chemical and physical properties as that element. A good analogy for this is building blocks:

There are many different types of building blocks that you can assemble into anything you want. The analogy here is that there are different types of building blocks, and there are different types of atoms. In chemistry we have discovered about 120 different types of atoms, about 90 of which appear in nature.

But... how do we know that the world is made of atoms? In recent decades, we have taken pictures of individual atoms; in the picture below, each dot you see is an atom of either molybdenum, sulphur or selenium (which one is which doesn't really matter here).

Decades before pictures of atoms became possible, though, scientists had gathered evidence to suggest that, indeed, atoms were the way that matter was put together. But how did they arrive at that conclusion? The answer is: a lot of models, a lot of experiments, and a lot of revisions.

Atomic Models

1. Ancient Greece, 400 BCE

The idea of having all matter being made up of small particles seems to go back 2400 years to ancient Greece, and a philosopher named Democritus of Abdera.

In ancient Greece, "science" was not done the way it is done today, with experiments and evidence and calculations. Instead, "natural philosophers" such as Democritus would merely think and debate about what things should be like, and why... which is why Democritus is not seen as a scientist (by our modern definition).

He called the smallest particles of a substance "atoms," from a word meaning "indivisible."

In this example, water atoms could easily move past each other, which resulted in water being able to flow. Meanwhile, atoms of iron were shaped in a way that made them lock together, which meant that a large piece of iron could be very hard and solid.

Is this right? No (water isn't even an element). Is this supported by evidence? No (they didn't do experiments). But at the time it's the best that anyone could do.

2. The Islamic Golden Age, 700s - 1200s

Scientists and mathematicians in what is now Saudi Arabia, Iran, Iraq, Egypt and surrounding areas furthered the study of science and mathematics in a variety of areas. Initially they only wanted to translate ancient Greek and Roman books into their own languages, but eventually they started challenging those ideas by doing their own experiments and coming up with new materials, techniques and models.

Islamic scientists developed an early form of chemistry called alchemy, which allowed scientists to put new ideas into practice. The applications of these new ideas were wide-ranging, from medicine to the study of metals, and these new discoveries eventually made their way around the world.

For an excellent article about Arabic alchemy, visit the Science History Institute here.

3. John Dalton, early 1800s

In Europe in the 1600s, scientists there began doing more rigorous experiments, which gave results which built upon ideas and discoveries from Islamic scientists. During the 1700s there were many discoveries about how different gases worked, and new elements were found.

Dalton used his knowledge of how elements combined to form compounds, and borrowed from the ancient Greeks, to put together a basic atomic theory.

Above are a few of Dalton's original drawings, showing some elements at the top and how these atoms would combine to form various kinds of compounds.

It is said that Dalton's model compares atoms to billiard balls, which are solid and hard:

However, as the 1800s progressed, new evidence was coming to light which showed that Dalton's model was missing some vital parts.

4. J.J. Thomson, 1890s - 1900s

Throughout the 19th century, physicists made many discoveries about electricity and magnetism, and how they are connected. With this growing body of knowledge, scientists applied these new ideas to Dalton's atomic model.

Thomson discovered particles smaller than atoms in 1897, and called them electrons. Devices called cathode ray tubes (or Crookes tubes, after their inventor) had been used to show that so-called "cathode rays," which were mysterious rays emitted from the cathode (negative terminal) of the tube, were not just a form of light. They were actually made up of tiny, negatively charged particles which travelled in straight lines (electrons).

The glass of this tube is specially made so it will glow green when a cathode ray hits it. (As you can see, this "Maltese Cross" shape leaves a sharp shadow on the glass.) A magnet close to this stream of particles will bend it; you will learn later in physics that this means the particles must have an electric charge on it (in this case, negative).

So, where do these particles come from?

Thomson published his model in 1904, and it is often called the "raisin bun" model, since a raisin bun has raisins distributed throughout the bread much in the way electrons were distributed within atoms.

It's also called the "plum pudding" model, and it is said that the electrons "are like plums in a plum pudding." However, this can be a little confusing, since "plum" is a very old word for a raisin, and "pudding" in the English sense (i.e., from England) is not what Canadians would call pudding. So, plum pudding does not contain plums, nor is it what we'd call pudding... so just stick with "raisin bun" here.

Within a few years, though, further experiments determined that, while atoms contain positive and negative charges, the structure of the atom could not be what Thomson's model suggested.

5. The Bohr-Rutherford Model, 1910s

In 1911, Ernest Rutherford conducted experiments with a very thin piece of gold foil. He had a source of positively-charted particles (called alpha particles after alpha, "α," the first letter of the Greek alphabet). These positively-charged particles were fired at the gold foil, which was roughly 400 nm (nanometres) thick. Most of the particles sailed right through the foil and were detected on the other side, but a few of them were deflected slightly to the side, and a tiny fraction of them bounced almost straight backwards.

From this experiment, Rutherford made some conclusions.

So, Rutherford had figured out the central part of the atom, which we now call the nucleus. What about the outside?

Shortly afterwards, in 1913, Niels Bohr put forward his idea about how the outer part of the atom, which must contain the electrons, is structured. Using new ideas in quantum physics, he recognized that light given off or absorbed by different elements could be related to electrons jumping back and forth between levels, called shells or orbits, in the outer layers of the atom.

When you put both of those parts together, you get the Bohr-Rutherord Model, which is what we are going to be working with in this course.

This model will be the basis of our studies on atomic structure, and can explain a lot about why elements have the chemical properties that they do.

Practice

The Basics

1. Place the following atomic models in chronological order, earliest to latest: Bohr-Rutherford, Dalton, Democritus, Thomson.

2. In twelve words or less, describe the defining feature of each of the four atomic models listed in question #1.

Extensions

1. The ancient Greek philosophers had what they called the "four elements." Find out what they are, and how philosophers used this idea to describe how matter is put together.