About The Atom

The term atom was originally thought of by the ancient Greeks. They believed that the atom was the smallest thing in the world and that it was used to create bigger things. For a long time this was accepted and an atom was known as the building block of matter. Since everything has matter, the atom was believed to make up everything. (To visualize this, imagine zooming in very close on a picture. When you do so you find that the picture is made up of tiny squares. These squares represent atoms and the picture represents everything in the world around you!)

A single atom is the smallest unit of matter that is recognizable as a chemical element. What all of this is saying is that if you could take some matter (remember this is anything in the universe) and break it down you would see that it is made of compounds and molecules. The compounds and molecules are made up of elements. If you then broke that element down as small as you could then you would have an atom. (To imagine this think of a Lego castle which represents matter. The castle is made up of walls, bridges, rooms, etc. These represent the compounds and molecules. The walls and such are made up of individual Legos which represent elements. If you were to remove all of the Legos in a wall and separate them then each individual Lego would be an atom).

If atoms are so small then how do we know they exist? As we mentioned earlier, the ancient Greeks thought of the idea of the atom. When we say ancient we mean the 5th century B.C. (that is about 2600 years ago!). The Greeks did not have the tools to test their theories this far back in time.

By the 19th century (or the 1800's) technology had advanced greatly and many elements had been discovered to work with. Remember that elements are a group of similar atoms. Using the available elements scientist such as John Dalton and Amedeo Avogadro forced them to interact with one another. From these interactions they were able to prove the existence of the atom.

Towards the end of the 19th century a man by the name of J.J. Thomson discovered that the atom was made up of smaller particles. Through experiments with the gas neon, Thomson discovered a negatively charged particle with a mass much smaller than the atom. This particle is known as the electron and is one of the constituent particles of the atom.

Atoms are typically neutral (they do not have a charge). So when the electron (a negative particle) was discovered, scientist reasoned that there must be a positive charge somewhere in the atom to offset the negative charge. Electrons are also very light compared to the mass of an atom. Therefore, the missing positive particle had to be heavy to account for the missing mass of the atom.

In 1911 the scientist Ernest Rutherford found the missing particle by shooting positively charged rays at the element gold. Rutherford noticed that some of the rays were bent or deflected. The deflection was caused by the positively charged particles within the element (much like two positive magnets repel one another). The way in which the rays bent helped Rutherford to discover that the positive particles were very massive and concentrated in a small region. Today the positive particles are called protons and the region that they are concentrated in is known as the nucleus.

After the discovery of the electron and the proton, a scientist by the name of Niels Bohr constructed a model of what he thought an atom should look like in 1913. Prior to Bohr, scientist drew the atom as a nucleus with electrons floating randomly (see the diagram to the left). In the Bohr Model the nucleus occupies a dense central region and the electrons orbit the nucleus at discrete distances much like planets orbiting the Sun. Within these 'shells' energy of the electrons is restricted to certain discrete values. One says that the energy is quantized. This means that only certain orbits with certain radii are allowed; orbits in between simply don't exist. For instance, the first shell holds exactly two electrons that are always located at a certain distance away from the nucleus. The second shell holds a maximum of eight electrons and the third shell is capable of holding eighteen electrons.

You might be asking yourself how in the world did Niels Bohr ever think of such a thing as energy shells for the electrons to rotate about. Bohr actually made his discovery by perfoming numerous experiments on the element hydrogen. When he shot a lot of energy at the hydrogen it emitted electromagnetic radiation at a particular wavelength. (For those who do not know, each color has a particular wavelength so Bohr was able to determine the wavelength by the color that was emitted from the hydrogen once he charged it with energy). From these observations, Bohr made his hypothesis about the discrete energy shells. He also proposed that an electron can move to a higher shell if enough energy is supplied to move it there. This is known as an excited state. Once the electron moves to a higer state the atom then releases electromagnetic radiation.

Although Bohr's model gives a qualitatively accurate description of atoms, it does not give quantitatively accurate results for atoms more complex than hydrogen. In order to describe such atoms, it is necessary to use quantum mechanics. In quantum mechanics the electron orbits are replaced by probability distributions that only indicate in which regions of space each electron is most likely to be found.

After the Bohr model of the atom, scientists felt they knew all there was to know about the electron so they turned their attention to the nucleus. It had been discovered by J. J. Thomson, and Francis William Aston, that for a given element the nucleus sometimes occurs in several different forms that differ in mass. These chemically similar but physically distinct atoms were called isotopes. This led scientist to believe that there was another particle in the nucleus other than the proton.

The total number of protons in an atom dictates the atom's electric charge. Since the weight of an electron is nearly zero then the mass of an atom should also be dictated by how many protons there are in the nucleus. Upon close inspection, however, scientists realized that in some atoms the number of protons predicted by the electric charge was not the same number of protons that the mass predicted. To account for the difference in mass scientist predicted that another particle was present in the nucleus. In 1932 James Chadwick found this missing particle and named it the neutron. The neutron is slightly heavier than a proton and carries no electric charge. This allows it to account for the missing mass in some atoms without changing the electric charge of the atom.

Using the presence of neutrons we can now better understand what an isotope is. The definition of an isotope is an atom of the same element that has a different mass. This means that the electric charge of the two atoms is identical (so the number of protons does not change) and since the electrons are practically massless the difference in mass must come from the neutrons. A better definition of an isotope is an atom of the same element that differs in mass due to the presence or absence of neutrons in the nucleus. Take a look at the diagram below to see the different isotopes of hydrogen.

So far we have found that an atom can be split into protons, electrons, and neutrons. The next question you might be asking yourself is whether or not these subatomic particles can be split into smaller particles. Well, many scientists have asked the same question and it turns out that the answer is yes. In the 1940's and 50's scientists began using particle accelerators to create new particles that were similar to protons, electrons, and neutrons. The presence of these particles suggested that protons, electrons, and neutrons were not the fundamental particles that make up everything. Instead, another particle had to exist that was used to make up protons, neutrons, electrons, and the newly discovered particles. Today these particles are known as quarks.