The atomic theory has changed much since John Dalton's time. In the
late seventeen hundreds John Dalton first discovered that elements combine in specific proportions because they are made of individual atoms. For
example, hydrogen and chlorine always combine in a specific proportion to
create hydrochloric acid. Dalton thought that atoms were small particles, like a small marble. Dalton's atomic theory consisted of three parts. They were:
*All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed.
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