1. Ch 10: Modern Atomic Theory

2. 10.1 Rutherford’s Atom
Our understanding of atoms has changed over 2000 years.
Democritus thought of atoms as small pieces of matter than cannot be subdivided.
Over 2000 years later, Dalton’s atomic theory described atoms as indivisible spheres of matter that could be combined in specific ratios to make compounds.
Thompson discovered that atoms were composed of parts when he discovered the electron and concluded there must be a positive part of the atom.
Rutherford confirmed the existence of a small, positively charged nucleus surrounded by electrons in orbits; atoms were mostly empty space.

3. Rutherford’s model of the atom is still the popular model of the atoms today, though it is very inaccurate.
However, his model did not explain how the electrons could remain in stable orbits around the nucleus. They should fall towards the protons in the nucleus atom and merge with it, thereby collapsing the atom

4. 10.2 Electromagnetic Radiation
To understand the model of the atom further requires knowledge of light and how it transmits energy.
EMR is energy transmitted from one place to another by light.
The fundamental unit of light is the photon.
There are many forms of EMR: gamma rays, x-rays, UV rays, visible light, infra-red rays, microwaves, radio waves.
Human eyes can only see a small portion of this spectrum – the visible light part.

5. EMR (light) has both particle and wave properties.

6. The wave properties of EMR include wavelength, frequency, and speed.
Wavelength is the distance between two consecutive wave peaks. (Think of water waves as analogous to EMR waves.
The frequency of the wave refers to how many wave peaks pass a given point per given time period.
The speed of a wave indicates how fast a given peak travels through its medium. For light, that speed is 186,000 miles per second (in a vacuum). The speed of light in a constant.

7. Different wavelengths of EMR (light) carry different amounts of energy (see EMR spectrum).
For example, photons that correspond to red light have less energy than photons that correspond to blue light. That’s because the wavelength of red light is longer than that of blue light.
This is why we are not easily damaged by visible light rays (ROY G BIV) but can be injured by light of shorter, and therefore more powerful, wavelengths.

8. UV light has enough energy to cause damage to our skin.
X-Rays are even stronger – they can go through our bodies. Can damage cells and cause cancer.
Gamma rays are even stronger and can seriously damage or kill organisms even with brief exposures.
But remember, even visible light, infra-red, microwaves and radio waves can cause damage if intense enough. After all, microwaves can be used for cooking foods!

9. 10.3 Emission of energy by Atoms
Different atoms give off different colored light when heated.
When atoms receive energy from a source they become “excited.”
They then release this energy by emitting light of a particular wavelength.
The energy of the light emitted is equal to the energy received.