The Atom and its Nucleus

1. Rutherford Experiment: It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge. —Ernest Rutherford

2. Rutherford Model of an atom and limitations.

• Electrons orbiting the nucleus.
• Can't work because electrons radiate electromagnetic waves and lose energy because they are accelerating.
• This would lead them to spiral into the nucleus.

3 Bohr Model of an atom and limitations, video1 2.

• Electrons can exist at certain specific states each with a well-defined energy level.
• Electron energy is discrete as opposed to continuous
• Electron can only lose energy when it makes a transition from one state to another of lower energy level.
• Supported by the existance of emission and absorption spectra
• Emission spectra is the set of wavelengths of light emitted by the atoms of an element. Each element emits specific wavelegths.

• Absorption spectra consist of the same wavelengths as the emission spectra. Yet the absence of light represents which wavelengths were aborbed ... video.
• Energy is related to light by the relationship of E=hf, where h is Plank's constant. Each particle of light is a photon ... more on photons.

4. Nucleons ... a proton or a neutron.

Nuclide – a particular type of nucleus with a certain number of protons and neutrons.

5. Z is the atomic number and it represents the number of protons in the nucleus. Also represents the electric charge in units e.

6. A is the mass number and represents the total number of nucleons.

7. N = A - Z and provides the number of neutrons

8. Isotopes. Nuclei with the same number of protons and electrons but different number of neutrons. It is the same chemical but has different physical properties.

9. Strong Nuclear Force. Attractive force that binds the nucleons together. At a short range it is much larger than the electrical force. Yet for larger separations the force is nearly negligible.

10. Nuclear radius. R = 1.2 x A1/3 x 10-15m. A is the mass number and this result tells us that the nuclear density is the same for all nuclei.

HW. PG 372: 3-9, 11.

Geiger-Marsden Experiment