History of Atomic Theory

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TABLE OF CONTENTS
NO| TITLE| PAGE|
01| Introduction| 3|
02| Democritus| 4|
03| Plato and Aristotle| 6|
04| John Dalton| 7|
05| J.J. Tomson| 9|
06| Robert Millikan| 11|
07| E. Goldstein| 14|
08| James Chadwick| 16|
09| Ernest Rutherford| 18|
10| References| 21|

INTRODUCTION

It is quite remarkable to know that the work of scientist hundreds years ago lies at the heart of our understanding of the structure of atom. New Zealand scientist Ernerst Rutherford was the first to show that the atom is the building block of all matter that consists of a positively charged nucleus surrounded by tiny negatively charged electrons. The extraordinary achievements of these scientists have been instrumental development of chemistry over the past century. Today, we take the existence of atoms for granted. We can explain many aspects of the structure of the atom and in fact, current technology allow us to “see” and even manipulate individual atoms. However, scientific evidence for the existence of atoms is relatively recent, and chemistry did not progress very far until that evidence was found.

Democritus

Democritus ( 460 a.c.– 370 a.c.) was an Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated an atomic theory for the universe. His early deductions about the composition of the basic component of nature led him to believe in unseen and uncuttable particles , called atomos. This Greek philosopher wondered how he could be sitting in one part of his house and detect that bread was baking elsewhere in the house. Could it be that small particles of the bread were breaking away from the loaf and traveling through the air into his nose? Noticing that wet clothing gradually got drier and lighter led to the explanation that small, invisible pieces of water were gradually leaving the clothing. What was the smallest piece of this matter? In democritus’ culture, these pieces of matter were thought to be indivisible, unable to be broken down further, or uncuttable. In their language, the particles of the basic elements were atomos. This is the origin of the world’s atom. The lack of certain instruments that we use routinely today made it impossible, in Democritus’ time, to perform the experiments that have have led to our current understanding of matter. For instance, the invention of the balance for the measuring mass was one of the most significant significant experimental advances in the history of science. In fact, two important chemical laws were discovered with the use of the balance. These laws helped to shape our current understanding about the ideas put forth in Democritus’ era. His theory, matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut”. To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. Although Democritus’ idea was not accepted by many of his contemporaries (notably Plato and Aristotle), somehow it endured. Experimental evidence from early scientific investigations provided support for the notion “atomism” and gradually gave rise to the modern definition of elements and compounds.

Plato and Aristotle

Plato (424/423 BC – 348/347 BC) was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Aristotle (384 BC – 322 BC) was a Greek philosopher and polymath, a student...
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