History of Atom


Thousands of years ago no one had the scientific equipment to perform actual chemistry experiments as we do today. It was only through intellectual thought that “discoveries” of our world could be made. Two famous Greek philosophers came up with some very interesting theories.

Democritus (460-370 B.C.) invented a surprising correct theory of matter.

i) Matter was made up of tiny particles he called “atomos”.

ii) Atomos were solid, homogeneous, and indivisible.

iii) Different matter is made up of atomos that have different shape and size. For example, honey would be made up of atomos that are smooth and round because it is pleasing and vinegar would be made up of atomos that were pointy because it is bitter.

iv) Changes in matter is caused by a changing of the grouping of atomos, not in a changing of the atomos themselves.

 Unfortunately, Democritus had no way of proving his theory with everyday observations, so when a very famous philosopher named Aristotle (384-322 B.C.) invented a (incorrect) theory that seemed to explain everyday life, most people believed him.

*All matter is comprised of one of the four elements: earth, fire, wind, and water.

*When given the opportunity, the matter will split apart into their elements. For example, wood must be made of earth and wind because when fire is added part of the wood rises to become wind again (smoke) and part falls to become part of the earth again (ash).

*Even though we now know Aristotle’s theory to be completely wrong, it could be used to explain most of the things people experienced in their lives. Plus he was famous for other theories that were correct, so his theory was believed for many hundreds of years.

Around a thousand years after Aristotle, the mystery of what made up matter again became important to some people. These people were called Alchemists, and were looking for the Philosopher’s Stone, magical matter that could turn lead into gold, and give eternal life.

*The alchemists were the first to use the term element in reference to real chemicals, but thought there were only three - mercury, salt, and sulfur (and salt we now know is not an element).

*The alchemists did try analyzing matter with special equipment and invented techniques like distillation and crystallization that we still use today. The alchemists didn’t want just anyone to benefit from their work, so they often used riddles and words with secret meanings to talk about their experiments. This meant real chemistry progressed slowly until hundreds of years later chemists decided to openly share their experimental results with anyone who would listen.

The theory of the atom had changed during the time of the alchemists, but a huge advance was made by John Dalton (1766-1844). 

Dalton's Atomic Theory
Dalton's atomic theory (1808) is based on the following two laws:

·        law of conservation of mass

·        law of definite proportions.

He also proposed law of multiple proportions, as a logical consequence of his theory.

·        Every element is composed of extremely small indestructible particles called atoms .

·        Atoms of any one element are all similar but they differ from atoms of another element.

·        Atoms of each element are fundamental particles, have a characteristic mass but do not have any structure.

·        Atoms of various elements take part in a chemical reaction to form compound (which is called molecule).

·        In any compound, the relative number and kinds of atoms are constant.