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Atomic Theories Democritus (300 B. C.)

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1 Atomic Theories Democritus (300 B. C.)
Matter can be cut into smaller and smaller pieces, but eventually you will end up with tiny particles (called atoms) that are indivisible.

2 1. All matter is made up tiny particles called atoms.
Dalton’s Atomic Model (1808) 1. All matter is made up tiny particles called atoms. 2. Atoms cannot be created, destroyed, or subdivided in chemical changes. (Law of Conservation of Mass) 3. All the atoms of one element have the same properties, such as mass and size. These properties are different from the properties of the atoms of any other element. 4. Atoms of different elements combine in specific proportions to form compounds. (Law of Definite Proportions)

3 Electrons cathode ray tube J. J. Thomson paddlewheel + -

4 Protons Eugen Goldstein

5 J. J. Thomson (1904)
“pudding” (electrons)

6 Rutherford’s Gold Foil Experiment
tiny, dense positive core (called the nucleus) alpha particles

7 Rutherford’s Nuclear Model (1911)
nucleus (contains protons) electron empty space

8 Problems With Rutherford’s Model
1. Why doesn’t the electron spiral into the nucleus?

9 2. How do you explain line spectra?

10 Quantum Theory of Light
Light consists of a stream of energy packets (or quanta) called photons. Fire photon torpedo, Mr. Sulu!

11 Evidence for the Quantum Theory
Photoelectric Effect : the release of electrons from a substance due to light striking the surface of a metal According to the classical theory of light, the brightness (intensity) of the light shone on the metal would determine the kinetic energy of the liberated electrons. This prediction was shown to be false. The frequency (colour/energy ) of the light was the most important characteristic of the light producing the effect.

12 Einstein’s Explanation for the Photoelectric Effect (1905)
Light is transmitted in the form of energy packets called photons. 2. The energy of a photon depends on its frequency.

13 Neils Bohr An electron is so small that it DOESN’T obey the classical
(Newtonian) laws of motion. Therefore, an electron does NOT necessarily lose (radiate) energy as it orbits the nucleus. 2. An electron can only have certain energies, just as the gearbox in a car can only have certain gears. (Its energy is quantized.)

14 Bohr’s Model of the Hydrogen Atom (1914)
electron dropping to a lower orbit orbits electron in excited state photon of light emitted when electron drops electron in ground state

15 Line Spectrum of Hydrogen

16 The line spectrum of each element is unique, like a fingerprint.

17 Helium was first discovered when scientists looked at the
pattern of spectral lines during a solar eclipse.

18 The colours in fireworks are a result of electrons becoming
“excited” and then dropping back down to orbits closer to the nucleus.

19 Problems with Bohr’s Model
Although Bohr’s model successfully predicted the line spectrum for hydrogen, the extension of his model to atoms with two (or more) electrons did not agree with experimental evidence. Gimme a break!!!

20 The Wave Mechanics Model of the Atom (Schrodinger)
A model of the atom based on the fact that the electron exhibits properties of a particle AND a wave (but never both simultaneously).

21 The idea that an electron had a dual nature was first proposed
by Louis de Broglie in 1923.

22 Particle-Like Properties of Electrons
+ Electrons are deflected by magnetic or electric fields. - 2. Electrons can make a paddlewheel turn.

23 Wave-Like Properties of Electrons
Diffraction of electrons through a crystal. diffraction : the spreading out of a wave when passing through a small opening

24 Moving Waves

25 Standing Waves

26 Since electrons are “tied down” by the nucleus, they create
standing wave patterns like a guitar string called orbitals. Unlike the standing wave on a guitar string, these standing electron waves/orbitals occur in 3-D.

27 Heisenberg Uncertainty Principle
If the velocity of an electron is known, its position cannot be determined exactly. (i.e.- While measuring the velocity of tiny particles like electrons, you will affect their position.)

28 Electron Cloud: 3-D representation of the probability of finding an
electron in an orbital. Where there are large #s of dots, the probability of finding the electron is high.

29 Neutrons James Chadwick

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