Early Atomic Models

1. Democritus World is made of two things: empty space and indivisible particles, called atoms There are different atoms for every material in the world Aristotle Proposed that matter was continuous and not made of smaller particles of matter He called this continuous flow of matter hyle This was accepted until 17th century Early Atomic Models

2. John Dalton (1800s) • Elements consisted of very tiny particles, called atoms, which cannot be broken into smaller particles by chemical processes • All atoms of an element were identical and they had the same mass • Compounds consisted of atoms of different elements combined together

3. John Dalton’s Model • Atoms were tiny, indivisible, indestructible particles • Each one had a certain mass, size and chemical behavior

4. Dalton used these to prove: • Law of Definite Proportions- specific substances always contain elements in the same ratio by mass • Example: NO, NO2, N2O • Law of Conservation of Matter- matter cannot be created or destroyed. A chemical reaction simply changes the way atoms are grouped together.

5. J.J. Thompson (1890s) • showed that atoms of any element have negative particles with a mass. • Theorized that positive and negative charges were distributed through the atom, called the plum pudding model.

6. Cathode Ray Tube Parts Tube- sealed glass tube from which most of the air is removed. It contains an a metal screen with material that emits light when struck by electrons Cathode- negative electrode where electrons originate Anode- positive electrode which electrons travel towards For a cathode ray tube to work you must have power source • Cathode Ray Tube

7. Experiment • Scientists found that the beam would bend when it was in the presence of a magnet. Therefore, he concluded that all atoms have particles with a mass. • The beam would bend toward a positively charged plate. Therefore he concluded that atoms have particles with a charge.

8. Experiment continued.. • Finally, Thomson measured the charge to mass ratio and found that it was very large. This meant the particles were a thousand times smaller than an atom. • He found that charge to mass ratio was the same for every element he tried and concluded that all elements have this negatively charged particle that has mass. • WE CALL THE PARTICLE THOMSON DISCOVERED THE ELECTRON. THIS WORK WON JJ THOMSON THE PHYSICS NOBEL PRIZE IN 1906.

9. The Plum Pudding Model • JJ Thomson- Protons were found later in a modified cathode ray tube • From this Thomson came up with the plum pudding model. This showed that atoms are composed of negative charges spread in a positve “pudding” .

10. Other Experiments • Millikan- Obtained first accurate measurement of an electrons charge using the oil drop experiment • Millikan Experiment • Roentgen discovered X-rays • Bequerel discovered radioactivity • Chadwick- discovered neutrons (1932)

11. Lord Rutherford and the Gold Foil Experiment (1909) • Used gold foil experiment to find positively charged “core” of the atom which he called the nucleus • Found that the rest of the atom is composed of empty space Gold Foil Experiment

12. Rutherford-Bohr Atom- “Planetary Model” • Suggested electrons orbit nucleus the way that the planets orbit the sun. • The more energy an electron has, the farther it will exist from the nucleus. We use a lot of the same principles in our modern day atomic theory, but not that electrons orbit the nucleus!