1.3 Atomic Theory

1. 1.3 Atomic Theory 1-3A Behaviour of Gases Demo

2. Early Greek Philosophers (5th century BC) • Debated what the universe was made up of • Democritus theorizes that everything made of tiny particles he calls atomos

3. Aristotle (4th century BC) • Lived 2500 years ago • Most respected Greek philosopher • Creator of the “4 elements” theory • Believed that everything in the world was made up of earth, air, fire & water

4. The Alchemists • Aristotles theory persisted for over 2000 years! • During the middle ages, everyone was attempting alchemy. • These attempts were all unsuccesful, but resulted in an enormous amount of accidental scientific discoveries! • These discoveries had many questioning Aristotles 4 elements theory

5. John Dalton (1766 – 1844) • Credited with disproving Aristotle and developing a new way of explaining matter • Video of Dalton’s candle/water/jar experiment • Since the flame ‘used up’ something in the air, obviously air could be broken down further, so was not a building block of matter!

6. Dalton’s Atomic Theory • All matter is made of small particles called atoms. • Atoms cannot be created, destroyed, or divided into smaller particles. • All atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements. • Compounds are created when atoms of different elements link together in definite proportions.

7. Dalton’s theory shows why the alchemists were never successful. • Metals are made of specific atoms, and atoms cannot be turned into other atoms!!

8. J.J. Thomson (1856-1940) • Studied electric currents in gas discharge tubes • Discovered (in 1897) that currents are streams of negatively charged particles he called electrons. • He thought that atoms must be composed of smaller particles – a mind boggling idea at the time! • British physicist

9. Thomson’s “Raisin Bun” Model of the Atom • Atom is a positively charged ball studded with negatively charged electrons

10. Thomson’s raisin bun model didn’t last long, as it was quickly followed by a new model proposed by a scientist from New Zealand… Ernest Rutherford

11. Ernest Rutherford (1871 – 1937) • Expected alpha particles to pass straight through • But every once in a while, a particle was reflected back!

12. Rutherford had discovered the nucleus! • The tiny, dense, positively charged centre of the atom. • 10 years later he determined that the nucleus was actually made up of 2 types of particles: • A proton – positively charged particle • A neutron – particle with no charge

13. Niels Bohr (1885 – 1962) • A Danish physicist who worked for Rutherford • He studied the parts of the atom surrounding the nucleus • Bohr was particularly interested in the electrons that existed in this space

14. He studied light given off by gases that glowed when an electric current was passed through them • He discovered that electron surround the nucleus in specific energy ‘levels’ or ‘shells’

15. Bohr’s Model: Electrons have different amounts of energy • Basically, electrons orbit the nucleus on specific paths • If energy is supplied, they can jump to higher energy level paths • When the energy supply stops, the electrons ‘fall’ back to their original path

16. Inside the Atom… • An atom is the smallest particle of an element that retains the properties of the element • Atoms are made up of 3 types of subatomic particles: • Proton (positively charged, found in the nucleus) • Neutron (no charge, found in the nucleus) • Electron (negatively charged, found outside of the nucleus)

17. The Subatomic Particles • The nucleus contains protons and neutrons • It is extremely tiny but incredibly dense • Takes up 0.01% of the space in an atom but accounts for 99% of the mass! • Electrons have almost no mass but take up a huge amount of space (99.99% of the atom)