Development of the Atomic Theory Book K--Chapter 4 Section 1 Pages 82-87

The Beginning of Atomic Theory: Greek Philosopher Democritus Greek Philosopher Democritus thought that if you kept cutting something “forever” eventually you would end with something that could not be cut and called it “an atom”. Atom is Greek meaning “not able to be divided. Made of small and hard particles Made of a single material forms into different shapes & sizes.

The Beginning of Atomic Theory: Greek Philosopher Aristotle -Disagreed with Democritus -Aristotle believed you would end with something that couldn’t be cut -Because of Aristotle’s influence – people believed he was correct for a long time but was eventually proven wrong. Matter is made of small particles, which are called atoms.

The Beginning of Atomic Theory: Atom-the smallest unit of an element that maintains the properties of that element. Figure 1 shows Aluminum cans that are made solely from Aluminum atoms.

Dalton’s Atomic Theory Based on Experiments In the late 1700’s scientists learned that elements combined in proportions based on mass to create compounds. FOR EXAMPLE: Hydrogen and Oxygen always combine in the same proportion to form water (H20). John Dalton was a British chemist and schoolteacher.

Dalton’s Atomic Theory Based on Experiments The Atomic Theory of Matter states: All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances. Although Dalton’s new ideas were accepted, as time and technologies improved, Dalton’s theory was modified and made more thorough (correct).

Thomson’s Discovery of Electrons 1897 – JJ Thomson showed a mistake in Dalton’s theory. He discovered that there are particles within the atom; it would be divided into smaller particles. Thomson used a cathode-ray tube and found that positively charged particles attracted a beam that was created with a negative charge (kind of like magnets-the positive and negative sides of magnets attract one another while like charges repel).

Thomson’s Discovery of Electrons Thomson changed the picture of the atom and people called it the “plum pudding” model (now might be called chocolate chip ice cream model) because he believed that the electrons were “mixed” throughout like chocolate chips in ice cream!

Rutherford’s Atomic “Shooting Gallery” 1909 –A former student of Thomson named Ernest Rutherford tested Thomson’s theory. Rutherford created the gold foil experiment and found out that particles passed through the foil, deflected from the path & a few bounced back.

Where are the Electrons? Due to the gold foil experiment, it was decided that atoms must be “empty space”. Rutherford updated the atomic theory that there is a nucleus at the center of the atom. NUCLEUS- an atom’s central region which is made of protons and neutrons. Rutherford calculated that the diameter of an atom was 100,000 times smaller than the diameter of the gold atom.

Where are the Electrons? 1913-Niels Bohr believed that electrons moved around the nucleus (center) of the atom in specific paths. It was found that electrons do not travel in specific paths, the path cannot be predicted.

Where are the Electrons? Electrons travel in regions outside the nucleus of the atom and these regions are called electron clouds. Electron clouds- a region around the nucleus of an atom where electrons are likely to be found.