Particle and Nuclear Physics. Outline. Structure of the Atom. History of Atomic Structure. 1 Structure of the Atom

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1 Outline of 1 of

2 Atomic Spectra of Helium

3 Classical Atom of Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed. By about 1890, most physicists and chemists believed matter was composed of atoms consisting of electrons and protons. The behavior could not be explained by Newton s mechanics. Plum Pudding Model

4 Scattering Experiments of Ernest Rutherford (30 August October 1937) Rutherford expected the relatively massive α particles would pass freely through the plum-pudding atom. A small number of alpha particles were actually deflected through very large angles.

5 Review Question 1 of According to the Rutherford experiment (and later model) of the atom, most of the volume of an atom: A is empty space. B was occupied by the nucleus. C contained positive charges. D excluded electrons.

6 Review Question 1 of According to the Rutherford experiment (and later model) of the atom, most of the volume of an atom: A is empty space. B was occupied by the nucleus. C contained positive charges. D excluded electrons.

7 Planetary Model of Atom of In Rutherford s Planetary Model, the electric force between the proton and the electron supplies a centripetal force: v = ke 2 rm KE = 1 2 mv 2 = 7.5 ev 13.6 ev Value is very close to the measured ionization energy of the hydrogen atom of 13.6 ev, but:

8 Planetary Model of Atom of In Rutherford s Planetary Model, the electric force between the proton and the electron supplies a centripetal force: v = ke 2 rm KE = 1 2 mv 2 = 7.5 ev 13.6 ev Value is very close to the measured ionization energy of the hydrogen atom of 13.6 ev, but:

9 Bohr Model of Atom of Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed. Bohr Model had certain allowed orbits for electron: (works nicely for atoms with one electron, e.g. H, He +, etc.)

10 of Modern Atom Emission energies correspond to energy differences between allowed levels. Modern Model has electron cloud rather than orbit:

11 of 1 In an atom, Atomic the negative electrons orbit the positive nucleus and form standing waves known as orbitals. Each orbital can have at most two electrons in it. (Pauli Exclusion Principle)

12 of 1 In an atom, Atomic the negative electrons orbit the positive nucleus and form standing waves known as orbitals. Each orbital can have at most two electrons in it. (Pauli Exclusion Principle) 3 D Standing Waves resemble the 2 D standing waves of the drumhead

13 of 1 In an atom, Atomic the negative electrons orbit the positive nucleus and form standing waves known as orbitals. Each orbital can have at most two electrons in it. 2 An orbital s energy = kinetic + potential The energy of a photon is: E photon = h f Since energy is conserved, the energy of the photon is the difference in energy of the atom before and after emission or absorption. 3 Electrons normally reside in the lowest energy orbitals Ground state 4 Electrons can be excited to higher energy orbitals.

14 Atomic of Electrons reside in standing waves: An electron in an orbital does not emit light. An electron emits light when it changes orbitals.

15 Atomic of Electrons reside in standing waves: An electron in an orbital does not emit light. An electron emits light when it changes orbitals. Light travels as a wave (a diffuse pattern) is emitted or absorbed as a particle (a photon): Photon energy = Planck constant frequency E = h f An atom s orbitals differ by specific energies: Energy differences establish photon energies. Atom emits a specific spectrum of photons.

16 Question 2 of If a hydrogen atom, originally in its ground state of energy 13.6 ev, absorbs a photon of energy 16.0 ev, then what is the resulting kinetic energy of the electron if the proton has negligible kinetic energy? A Such a photon cannot be absorbed in this case. B 2.4 ev C 2.4 ev D 16.0 ev

17 Question 2 of If a hydrogen atom, originally in its ground state of energy 13.6 ev, absorbs a photon of energy 16.0 ev, then what is the resulting kinetic energy of the electron if the proton has negligible kinetic energy? A Such a photon cannot be absorbed in this case. B 2.4 ev C 2.4 ev D 16.0 ev