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Atoms are the smallest particles of matter that have distinct physical and chemical properties. Each different type of atom makes up an element which is characterized by an atomic weight and an atomic symbol. Since the atomic theory was first proposed in the early nineteenth century, scientists have discovered a number of subatomic particles.

The development of the atomic theory traces its history to early human civilizations. To these people, change was a concept to ponder. Ancient Greek philosophers tried to explain the causes of changes in their environment, typically, chemical changes. This led them to propose a variety to ideas about the nature of matter. By 400 B.C., it was believed that all matter was made up of four elements including earth, fire, air and water. At around this time, Democritus proposed the idea of matter being made up of small indivisible particles. He called these particles atomos, or atoms. While Democritus may have suggested the theory of atoms, the Greeks had no experimental method for testing his theory.

This experimental method was suggested by Robert Boyle in the seventeenth century. At this time, he advanced the idea that matter existed as elements which could not be broken down further. Scientists built on Boyle's ideas, and in the early nineteenth century, John Dalton proposed the atomic theory.

Dalton's theory had four primary postulates. First, he suggested that all elements are made up of tiny particles called atoms. Second, all atoms of the same element are identical. Atoms of different elements are different in some fundamental way. Third, chemical compounds are formed when atoms from different elements combine with each other. Finally, chemical reactions involve the reorganization of the way atoms are bound. Atoms themselves do not change.

Using Dalton's theory, scientists investigated the atom more closely. They wanted to determine the structure of these atoms. The first subatomic particle was discovered by J. J. Thomson (1856-1940) in the late nineteenth century. Using a cathode ray tube he discovered negatively charged particles called electrons. Around this same time, scientists began to find that certain atoms produced radioactivity. In 1911, Ernest Rutherford (1871-1937) proposed the idea that atoms had a nucleus which the electrons orbited around. This led to the discovery of positively charged protons and neutral particles called neutrons.

Over time, scientists developed a chart known as the periodic table of elements to list all known elements. Atoms on this chart are symbolized by abbreviations called the atomic symbol. For example, oxygen atoms are denoted by the letter O. Each atom also has a unique mass denoted by its atomic weight. The atomic number is also distinct to each type of atom denoting the number of protons in their nucleus.

While atoms generally contain the same number of protons as neutrons, this is not always the case. Atoms which have more or less neutrons than protons are known as isotopes. For example, carbon atoms can have 12, 13 or 14 neutrons. When a nucleus has too many neutrons, as in the case of carbon-14, it is unstable and gives off radiation which can be measured. Radioactive isotopes have found many useful applications in biology. Scientists have used them in radioactive dating to determine the age of fossils. They have also used them as tracer atoms to follow a chemical as it goes through metabolic processes in an organism. This has made them an important tool in medicine.

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