Development of Atomic Models

1. Development of Atomic Models

2. Greek Model • Democritus • 2400 years ago • Believed that there was a smallest piece of matter that couldn’t be broken down any further • Name came from the Greek word “atomos” which meant “not to be cut” or “indivisible” • Group that shared his belief was group called the atomists • They believed that atoms were small hard particles • Atoms were made of same material • Believed atoms were different sizes and shapes • Believed atoms were infinite in number • Atoms were always moving • Capable of joining together

3. John Dalton • Early 1800’s • John Dalton was an English chemist who was interested in meteorology(study of weather) • He was investigating the properties of gases • Discover that gases combined as if they were made of individual particles • THESE PARTICLES WERE THE ATOMS OF DEMOCRITUS

4. Dalton’s Atomic Model 1803 • All elements are composed of atoms. Atoms are indivisible and indestructible. • Atoms of the same element are exactly the same. • Atoms of different elements are different. • Compounds are formed by the joining of atoms of 2 or more elements • Answer questions at bottom of page 92 1-4

5. Thomson’s Model • 1897 • English scientist J.J. Thomson • Atom WAS divisible and made up of smaller particles • Was doing experiment passing an electric current through a gas. Gas gave off rays made of negatively charged particles • He called the negatively charged particles “corpuscles”(today we call them electrons)

6. Rutherford’s Model 1908 • English • Devised(came up with) an experiment to test Thomson’s model • Fired tiny positively charged particles at a very thin sheet of gold. Surrounding the gold was a screen coated with material that glowed whenever a positively charged particle hit the screen