Tjis essay explains the evolution of the atomic theory and how it has changed over the last cenutry or so.

Essay by ZackmannJunior High, 8th grade June 2002

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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


*All substances are made of atoms. Atoms are small particles that cannot

be created, divided, or destroyed.

*Atoms of the same substance are exactly alike, atoms of different atoms

are different.

*Atoms join with other atoms of other substances to make new substances.

Dalton's theory was proved wrong when new information was discovered

and Dalton's theory could not explain it. The atomic theory has evolved.

In 1897, a scientist named J.J. Thompson found and identified a

problem in Dalton's theory. He discovered that there were particles inside

the atom.

So, Thompson said, atoms can be divided into even smaller parts.

The particles that were found inside the atom had a negative force. The

negatively charged particles were called electrons. Scientists had already

found out that atoms had no overall charge. Thompson didn't know the

location of the positively charged material so he made a theory that the

negatively charged electrons were embedded in the atom which was made

of overall positive matter. Thompson's model was called the plum-pudding

model. The electrons looked like plums that were in the pudding .

In 1909, a man named Ernest Rutheford, a former student of

Thompson wanted to investigate his former teacher's theory. He aimed a

small beam of radium particles at a sheet of thin gold foil. He thought if

atoms were only soft blobs embedded with electrons like Thompson had

suggested then the beam...