# The atom and its nucleus

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The atom and its nucleus
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

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