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The History of Atomic Theory Homework
Due Friday, September 13
Since the ancient Greek civilization, people have wondered about the nature of the atom. The atom is
the basic building block of matter. The word “atom” comes from the Greek language. “A…” is a prefix
meaning “not.” The word “tomos” means “cut.” Therefore, “atomos” or “atoms” means “cannot be
cut or divided.”
Many people helped develop the theory of atoms. The first was a Greek named Democritus in the 4th
century BC. He proposed that all matter was made of tiny particles. Democritus thought that empty
space filled the area between the solid atoms. That concept of the atom was unchanged for
centuries. Then an English scientist, John Dalton, proposed an “atomic theory” in 1803. Atomic
theory says that atoms are solid with an atomic mass. They can react because of the forces of
attraction and repulsion. In 1873, James Clerk Maxwell proposed that electric and magnetic fields
filled the empty space in atoms.
In 1897, J. J. Thomson discovered the first subatomic particles which he called electrons. His
experiments found that these electrons have a negative electric charge. He then calculated they had
a much smaller mass than even the smallest atom. Thomas proposed the plum pudding model of the
atom. This model says that negatively-charged electrons are mixed inside a cloud of positive charge.
He thought they were like chunks of plum inside pudding.
Ernest Rutherford did an experiment in 1909 to test Thomson’s model. His results did not agree with
the plum pudding model. Instead, Rutherford was the first to show the nuclear model of the atom.
This model shows that the center of an atom contains positively-charged protons (p+) surrounded by
the negatively-charged electrons (e-). In 1932, Sir James Chadwick found the neutrally charged
neutron (n0). The atomic model now seemed complete after all these discoveries.
Many people had a hand in developing atomic theory as we know it today. However, scientists
continue to study the atom. They have found many more details about atoms. As we learn about this
building block of matter, the nuclear model of the atom will continue to change.
1. The root word “di” means “two,” so the word “diatomic” refers to _____.
a. Positively charged atom
b. Negatively charged atom
c. A molecule with 2 atoms
d. Forces within the atom
2. In paragraph 2, attraction and repulsion refer to _____.
a. Solid atoms
b. Tiny particles
c. Atomic theories
d. Electric and magnetic fields
3. How did Ernest Rutherford’s experiment relate to J.J. Thomson’s work?
a. Rutherford looked for magnetic fields while Thomson looked for electric fields.
b. Rutherford repeated Thomson’s experiment and confirmed the results.
c. Rutherford tested a conclusion based on Thomson’s results.
d. Rutherford had not known about Thomson’s experiment.
4. Which scientist discovered the neutron and gave us the current basic model of the atom?
a. James Maxwell
b. John Dalton
c. Democritus
d. James Chadwick
5. What contribution did J.J. Thomson make to the development of atomic theory?
a. Said that atoms are solid masses
b. Showed that electrons surround a dense nucleus
c. Found that atoms are made of smaller particles
d. Proposed that atoms contain electric fields