The structure of the atom is very well understood today, but that hasn't always been the case. The idea of the atom first originated in ancient Greece and went through many changes as we gained more and more knowledge about the atom and what the world is made of.

Ancient Greece

The first model of the atom, really the first idea of an atom, came from a philosopher in ancient Greece named Democritus. He and a few of his friends said that all matter is made up of tiny particles, so small that you couldn't see them. Democritus' atom was simply a small object that had different shapes and that these objects interacted according to their shapes. Things that were strong like iron were made of little balls with hooks that connected together. Things like water were made of slippery balls and air was made of light, whirling shapes. Whatever the shape, Democritus said that these atoms were indivisible, meaning that they couldn't be broken, and fundamental, meaning that they were the most basic and were not made of anything smaller. An important thing to note is that Democritus' atoms were inert and didn't change.


After Democritus, atoms went largely unnoticed for a long time. In the late 1700's and the early 1800's, two scientists working on gases came up with some new ideas about atoms. Dalton proposed that atoms were, like Democritus, inert and indivisible, but they could combine to form new substances. This meant that atoms didn't have to have different shapes, but they did have different weights. Dalton was able to show this by splitting water into hydrogen and oxygen and then measuring the weights and volumes of the gases. Dalton also came up with the Law of Definite Proportions. It says that atoms can only combine in whole-number amounts (you can't have half-an-atom, in other words).

Shortly after Dalton, another scientist came along named Amadeo Avogadro. He did some work with gases and discovered that gases have the same number of particles if they have the same volume and are at the same temperature and pressure. He used this to determine that some chemical compounds were made of different numbers of atoms. Dalton had thought that the simplest compound was a 1:1 ratio. Avogadro showed that atoms can combine in many different ways, though always in definite proportions.

Plum-Pudding Model

The first person to suggest that the atom was not a little ball of stuff was J.J. Thomson. Thomson did an experiment (which we show in class) where he was able to use an electric field to cause a beam of light to change direction. Only things with an electric charge are affected by an electric field so he figured that the beam of light wasn't a beam of light, but of little charged particles. He reasoned that these particles were negatively charged and had to have come from the atom (since there wasn't any other place to come from). These were called electrons and Thomson used them to invent the Plum-Pudding Model. Plum pudding is an old English dessert which is very much like those yogurts that have fruit pieces in them. In the Plum-Pudding Model, the electrons exist inside a ball of pudding. The electrons are negatively charged and the pudding has a positive charge, which means that the atom as a whole is neutral. This model showed that atoms were not inert and not as indivisible as people had thought. It also meant that atoms weren't made of little balls of stuff like everyone thought, they had a structure to them.

Rutherford Model

Shortly after Thomson's idea of the Plum-Pudding Model, one of Thomson's students, Rutherford, came up with a new idea of the atom. His idea said that atoms were made of two parts: the nucleus, which is very small and hard and is at the center, and the electrons, which were orbiting around the atom like the planets around the sun. This meant that most of the mass of the atom was in the nucleus (along with all the positive charge) and that the electrons were way far away from it. It also proved that matter is mostly empty space since each atom is mostly empty space.

Bohr Model

The last historical model of the atom was the Bohr model. It was very similar to Rutherford's model, but it was developed to explain certain behaviors of atoms when they were electrified. When you electrify an atom (or more likely, a bunch of atoms in the gas state) they glow with a certain color. If you take that light and put it through a prism, instead of getting a rainbow you only get a few colors in particular. These colors are called spectral lines and they are called lines because that's exactly what they are: lines of a single color, unlike the rainbow which has thousands of different colors and shades.

In Rutherford's model, the electrons orbited the nucleus liek planets, but were allowed to change their orbits if something knocked into them, getting a little closer or a little farther from the nucleus. Bohr said that instead of orbiting anywhere, electrons were only allowed to orbit in certain paths called energy levels and nowhere else. Each path had a certain amount of energy associated with it, so electrons in orbits farther away from the atom had more energy than electrons closer. Since electrons could only exist on those orbits, it would take a specific amount of energy to get them to change. That amount of energy is associated with a specific color of light, which explained the lines. Bohr's model is pretty close to the modern model of the atom, but it's a little off.

Modern Atomic Model