Nuclear Physics

The atom and its nucleus

Discovery of the nuclear atom

1909-Geiger and Marsden, working under Rutherford Scattering of alpha particles shot at a thin gold foil Alpha particle 4x mass of H atom Alpha particles-emitted when unstable elements

decay Used radon – source of alpha particle

Rutherford, Geiger, and Marsden

Found that sometimes alpha particles were detected at very large scattering angles

This couldn’t be understood in terms of the prevailing model Thomson

Geiger-Marsden-Rutherford experiment

Deflection indicative of enormous force of repulsion between alpha particle and carrier of positive charge of atom

Positive charge resided on a tiny object Alpha particle could approach + charge at a

small d, and the Coulomb force of repulsion, would be enormous

Rutherford model

Massive, positively charged nucleus Electrons orbited nucleus Force keeping electrons in orbit

Electrical force between negative electron charge and positive nuclear charge

Couldn’t explain why matter is stable, i.e. why atoms exist

Bohr model

Examined hydrogen Realized electron could exist in certain specific

states of definite energy, without radiating away energy

Electron lose energy when makes transition from one state to a lower

Emitted energy is difference between states Evidence-emission and absorption spectra

Spectra

Normal conditions-lowest energy level Atoms excited higher level As soon as they do, transition back down to

lower state Energy allows wavelength of emitted light to

be calculated

Spectra

Emission spectrum set of wavelengths of light emitted by the atoms of an element

Send a light of specific wavelength through an atom Doesn’t correspond to any of the wavelengths Light transmitted without absorption

Nuclear Structure

Protons and neutrons # of protons is Z (atomic number) Protons + neutrons = mass (A) Electric charge is Z IeI # of neutrons N= A-Z Nucleon

Proton or neutron

Isotopes

Nuclei that have the same number of protons, but different number of neutrons

Same number of electrons Identical chemical properties, different

physical Mass spectrometer

Forces within the nucleus

Nucleons bound by nuclear force Attractive force Stronger than electrical force if separation

between 2 nucleons is small Larger separation-small force

Nuclear radius R = 1.2 A1/3 x 10-15m A is total number of p and n

Forces within the nucleus

Weak nuclear force Responsible for decay of a neutron into a proton Called beta decay