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The Development Of The Atomic Theory

The Greek concept of atomos: the atom
Around 440 BC leucippus of Miletus originated the atom concept. He and his pupil, Democritus of abdera refined it for future use. Their atomic idea has five major points. All original writings of leucippus and Democritus are lost. The only sources we have for there atomistic ideas are inquotations from other writers. Democritus was known as the laughing philosopher because he enjoyed life so much. At this time Greek philosophy was about 150 years old, emerging in the sixth century bc, centered in the city of miletus on the ionian coast in Asia minor, which is now turkey. The work of leucippus and Democritus was further developed by epicures (341-270 BC) of Samos. He made ideas more generally known. Aristotle also quotes both of them in arguing against their ideas. Most of what we know about leucippus and Democritus was found in a poem entitled de rerum natura (on the nature of things) written by Lucretius (95-55 BC). This poem was lost for over a thousand years and was discovered in 1417.
These are the basic points of their theory.

#1 – all matter is composed of atoms, which are bits of matter to small to be seen. These cannot be split any smaller. The atomists hold that splitting stops when it reaches indivisible particles and goes on no more
Which means there is a limit to division of matter that we cannot go. Atoms are very hard so they cannot be divided. In Greek a means not and tomos means cut. So our word comes from atomos, meaning uncuttable. He reasoned that if matter could be infinitely divided, it could also completely disintegrate and cannot be put back together, however matter can regenerate. Even though matter can be destroyed by splitting, new things can be made by joining other matter together. This process is reversible. The idea of reversibility means there must be a limit to splitting. If it could be split forever, there is nothing to stop it from destroying itself. Epicures insisted on an upper limit also, that atoms are always invisible, it seems obvious; all matter that can be seen is still divisible, so they can’t be atoms.
#2- there is an empty space between atoms. Unless there is a void with a separate being of its own ‘what is’ cannot be moved-nor again can it be ‘many’ since there is nothing to keep things apart.

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So there is an empty space between atoms, or a vacuum. Given that all matter is composed of atoms, then all changes must be a result of movement of atoms. So the movement within the atoms is allowed by a space so atoms can move from place to place.

#3- atoms are completely solid. If there is a space outside there cannot be a space inside, which would cause to disintegrate. But we knowthis is wrong, in 1919 Ernest Rutherford discovered the nucleus, demonstrating that there is an empty space.

#4- atoms are homogeneous (no internal structure)
The solidarity of atoms means that atoms are the same all over, or has no internal structure. There was speculation about sub-atomic structure in the 1800’s introduced it on solid scientific basis, not until 1897, J.J. Thomson’s discovery of the electron that it had internal structure.

#5- atoms are different in?
1- there size.
2- their shapes.

Democritus and leuccippus say thatthere are indivisible bodies, infinite number and shape Aristotle
They have all sorts of shapes and appearances and sizes Democritus
Aristotle and others opposed almost all of the ideas of the atom, so most of the information was lost. There is a pattern of atomic thought but only a few scholars gave it real thought. It wasn’t until 1803 that john Dalton (1766-1844) a schoolteacher put the atom on a solid scientific base. Dalton’s gift for analyzing data allowed him to recognize the connection between atomic weight and weight relations in chemical gases. He was the first to put the idea of atoms and stoichiometry together.

Dalton’s atomic laws are in the following points.
#1- all matter consists of tiny particles called atoms.

The existence of atoms first came up 2000 years ago. Though they remained pure speculation for most of this time.

#2- atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, broken onto smaller parts or be changed into another element. Dalton based this on the law of conservation of mass and experimental evidence.

With the discovery of subatomic particles after Dalton. Atoms could be broken into smaller pieces. It was also discovered that atoms could be changed into different elements. Most don’t consider this a chemical process because the nucleus is unaffected.

#3- elements are characterized by the mass of there atoms.

With the discovery of isotopes, it was found that elements are characterized by their atomic number, or the number of protons.

#4- when elements react, their atoms combine in simple whole number ratios.

This suggests a practical strategy for determining the atomic weights of atoms. Atomic weights could then be used to explain the fixed mass percentages of elements in all compounds.

This explained the law of definite proportion and multiple proportions.

Some of daltons original atomic theories were wrong but the basic concepts (like chemical reactions can be explained by the union and separation of atoms and these characteristic properties.) these ideas are still the basics of modern physical science.

While these beliefs were the mainstay of atomic theory. The belief that they were strutureless and indestructible was demolished by J.J. Thomson’s (1856-1940) discovery of the electron in 1897. It was soon realized that the mass of an atom is from the positively charged electron. Thomson used cathode rays and passed them through a glass bulb. He changed the direction of the rays with an electric field and concluded they were of negative charge. He knew he could move them with a magnetic field. Sending each beam in opposite directions, he concluded that the particles (or corpuscles as he called them) were a hundred times smaller. He found the first sub-atomic particles.

Through Thomson’s discovery of the electron, Robert milikan (1868-1953) discovered how to determine the charge of an electron. He sprayed tiny drops onto the space above two metal plates. A drop of oil would fall through now and then through a tiny hole in the plate. The rate of fall was determined by observation through a telescope. Then the tope plate was connected to a positively charged battery, and the bottom was connected to a negatively charged battery. A beam of x-rays was passed between the plates. The rate of fall of the oil would change in sudden jumps. He determined it was because of a gain or loss of electrons. He created an equation for this fall.

Electric force = E * e * n
Then in 1909 a man named Ernest rutherford (1871-1937) used alpha particles to bombard a piece of thin gold foil. He noticed almost all went through the gold but 1/8000 would bounce back in a wild direction. He quoted as if you fired a 15 inch naval shell at piece of tissue and the shell came right back and hit you from this he concluded that there was a highly concentrated, very small positively charged nucleus, while electrons inhabit the furthest from the atom. His experiment was similar to this.

He also discovered that the number of protons makes up the atomic number. And that the number of protons and neutrons makes up the atomic mass. Some years later Neil’s Bohr (1885-1962) came up with his model of the atom. Bohr’s model described how electrons performed within an atom.

In his model he stated that electrons that raced around the nucleus would make standing wavelengths. He imagined the orbit as a pilot wave, continuous.
It was like a vibrating string
He also came up with the following terms.

Energy level- the specific number of energy that an electron can have.

Principal quantum number- the integer used to identify each energy level.

Ground state- the lowest level of energy
Excited state- when the electrical causes it to move to a higher energy level.

Though the information and discoveries about atomic theory has changed over the years, the atom plays a very important role in our existence.

On internet, Atomic Theory 1996. Dalton’s Atomic Theory The Greek Concept Of Atomos John L. Park 1996
Parry/ Dietz/ Tellefsen/ Steiner Chemistry experimental foundations pretence hall 1983
G. Raymer-Cantham, Chemistry addison-wesley publishers
On internet, Atomic Theory 1996. Dalton’s Atomic Theory The Greek Concept Of Atomos John L. Park 1996
Parry/ Dietz/ Tellefsen/ Steiner Chemistry experimental foundations pretence hall 1983
G. Raymer-Cantham, Chemistry addison-wesley publishers


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