1. Chapter 2- Introduction to Atoms
© Fall 2005, Pflugerville ISD, 8th Grade

2. Development of the Atomic Theory
Democritus was a Greek philosopher who theorized that all matter was made of invisible particles called atoms.
Democritus of Abdera,
about BCE
Unit A : Chapter 2

3. Development of the Atomic Theory
Dalton, a British chemist and teacher. Noticed that elements combine in specific proportions to form compounds, and theorized that their atoms combine at the same proportions
John Dalton
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4. Development of the Atomic Theory
Rutherford, a former student of Thomson’s from New Zealand, tested his teacher’s theories in his Gold Foil Experiment.
Ernest Rutherford
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5. Development of the Atomic Theory
But some of the particles were deflected or bounced straight back!
This showed that a nucleus with a positive charge makes up the center of an atom.
Rutherford’s Gold Foil Experiment
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6. Development of the Atomic Theory
Bohr, a Danish scientist described the motion of electrons around the nucleus.
Niels Bohr
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7. Development of the Atomic Theory
Bohr said that electrons orbit the nucleus at specific energy levels, and can move from one level to another.
Bohr’s Atomic Model
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8. Development of the Atomic Theory
The current atomic theory states that there are regions inside an atom where electrons are likely to be found.
These regions are called electron clouds.
Electron Cloud Model
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9. Parts of the Atom
The nucleus is the small, dense, positively charged center of the atom.
Protons are positively charged particles in the nucleus
Neutrons are particles in the nucleus that have no charge
Nucleus
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