Neils Bohr

In ancient Greek the word atom meant the smallest indivisible particle that could be conceived. The atom was thought of as indestructible; in fact, the Greek word for atom means “not divisible.” Knowledge about the size and make up of the atom grew very slowly as scientific theory progressed. What we know/theorize about the atom now began with a core theory devised by Democrotus, a Greek philosopher who proposed that matter consisted of various types of tiny discrete particles and that the properties of matter were determined by the properties of these particles. This core theory was then modified and altered over years by Dalton, Thompson, Rutherford, Bhor, and Chadwick. The atoms original structure was simple, but as more and more research was done the atom became more complex and puzzling

The five atomic theories of the past two centuries represent the sudden advancement of science in modern times. Beginning with a basic theory on the behavior of atoms to the current model, some changes have been made, and some ideas are still the same. Ancient Greek philosophers believed that everything was made up of invisible particles called atoms. Since then the theory of atoms did not progress until 1803.

John Dalton was the first scientist to compose a theory of matter based on atoms. Dalton’s atomic theory is based on four concepts. He stated:

“1. All elements are composed of atoms, which are indivisible and

2. All atoms of the same element are exactly alike; in particular, they

3. Atoms of different elements are different; in particular, they have

4. Compounds are formed by the joining of atoms of two or more

All of Dalton’s ideas account for the laws of definite and multiple proportions and the law of conservation of mass. Some of Dalton’s points are still thought to be true, but over time this original theory has been modified.

The first of these modifications came in 1897 when J.J. Thomson discovered the electron. Based on the work of William Crookes and his “Crookes tube” (Cathode-ray tube), Thomson discovered a negative charged particle was the cause of the light produced by the cathode-ray tube. He also discovered that these particles are present in all elements. These cathode-ray particles are now known as electrons. Soon after the discovery of electrons the proton was discovered. This led Thomson to conclude that there were an equal number of both particles present in the atom.

Twelve years later Lord Ernest Rutherford was experimenting with alpha particles. He shot a stream of them at a piece of gold foil surrounded by zinc-sulfide. When an alpha particle strikes ZnS it produces a flash of light. The particles mostly stayed in a constant stream through the foil, but a few were deflected. This led Rutherford to believe that there must be a small, dense cluster of protons in the middle of the atoms to deflect the small number of particles.

With all of these alterations to the theory of an atom a few, five to be exact, problems still arose. One of the major problems was the size of an atom. If each electron had its own orbital and the atom had 23 electrons then the atom would be enormous. Another problem with the orbital of an electron was that no energy could be observed by the electron orbit decay. Next, if the center of an atom was composed of protons (+) and the electrons (-) orbited this positive core why didn’t the electrons crash into the protons, causing an ultra violet catastrophe. Also, if the core was composed of just positive protons and opposite charges repel then how did the protons stay together. And the final problem, the atom didn’t weigh enough. When scientists added the weight of the electrons and the weight of the protons and subtracted that from the overall weight of the atom there was a remainder. Something had to be missing from the model of an atom to make up for the weight difference.

The answer to these questions came along with the work of Neils Bohr. Danish physicist Neils Bohr used new knowledge about the radiation emitted from atoms to develop a model of the atom significantly different from Rutherford’s model. Neils Bohr developed a theory known as the Bohr theory of the atom. He assumed that electrons are arranged in definite energy levels, or quantum levels, at a specific distance from the nucleus. The arrangement of these electrons is called the electron configuration. It is much like that of our planetary system.

Using Rutherford’s model of the atom as a miniature solar system, Bohr developed a theory by which he could predict the same wavelengths scientists had measured radiating from atoms with a single electron. However, when conceiving this theory, Bohr was forced to make some startling conclusions. He concluded that because atoms emit light only at discrete wavelengths, electrons could only orbit at certain designated radii, and light could be emitted only when an electron jumped from one of these designated orbits to another. Both of these conclusions were in disagreement with classical physics, which imposed no strict rules on the size of orbits. To make his theory work, Bohr had to propose special rules that violated the rules of classical physics. He concluded that, on the atomic scale, certain preferred states of motion were especially stable. In these states of motion an orbiting electron (contrary to the laws of electromagnetism) would not radiate energy.

There are seven levels, which were derived from the seven colors he saw, each of which has a specific number of electrons that it has capacity for. The first level can only accommodate two electrons, the second can hold up to eight electrons, the third can hold up to eight-teen, and so on. If an atom had four electrons you wouldn’t find two in the first, one in the second, and one in the sixth. Electrons always occupy the lowest energy levels first. Electrons in a “ground state” are in their regular energy level and give off no energy; however, if an electron is in an “excited state” it sends energy in quantum packets (photons) and light is observes. When excited electrons jump up a level they give off light energy: however, they can never go down a level, energy can never be lost only gained.

At the same time that Bohr and Rutherford were developing the nuclear model of the atom, other experiments indicated similar failures of classical physics. These experiments included the emission of radiation from hot, glowing objects (called thermal radiation) and the release of electrons from metal surfaces illuminated with ultraviolet light (the Photoelectric Effect). Classical physics could not account for these observations, and scientists began to realize that they needed to take a new approach. They called this new approach quantum mechanics (Quantum Theory), and they developed a mathematical basis for it in the 1920s. The laws of classical physics work perfectly well on the scale of everyday objects, but on the tiny atomic scale, the laws of quantum mechanics apply.

The completed model that they came up with is the model that students now learn about in school. These scientists did exactly what scientists are supposed to do: test, experiment, and answer questions. Because of the years of study they did we now have a strong idea of what an atom is and what its components are. A theory never becomes fact until all of the bugs are wiped out, if this is true then this atomic theory is well on its way to becoming the facts about atoms.

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