Light and Atoms Physics 113 Goderya Chapter(s): 7 Learning Outcomes:

The Atom Smallest part of matter Nucleus, Protons, Neutrons and Electrons The elementary particle Quark Symbol - Z X A Z = Atomic Number Number of protons A = Mass Number Electron (-) Nucleus Protons (+)NeutronsQuarks

Hydrogen Atom One proton in the nucleus one electron orbiting the nucleus Symbol - 1 H 1 Electron (-) Nucleus Proton (+) 1H11H1

Helium Atom Two proton and two neutrons in the nucleus Two electrons orbiting the nucleus Symbol - 2 He 4 2 He 4

The Chemical Elements Each chemical element has a specific atomic configuration. The periodic table will show this for any known chemical element.

The Bohr Atom Model The Energy of the electron orbits are quantized That is each orbit carries a discrete energy value The closer the orbit to the nucleus, the more negative is its energy value -E 1 -E 2 -E 3 -E 4 n1n1 n4n4 n3n3 n2n2 -E 1 < -E 2 < -E 3 < -E 4

The Bohr Atom Model Electrons can jump from one energy orbit to the other and absorb or emit energy The energy of each orbit is calculated by E n = / n 2 (Z 2 ) eV n is orbit number and Z is the atomic number eV = electron Volts 1 eV = x J -E l -E h n l n h

The Bohr Atom Model When an electron jumps from a higher orbit to a lower orbit the change in energy is ∆E = E h - E l The wavelength of emitted light is 1/  = Z 2 R (1/n l 2 - 1/n h 2 ) R is Rydberg constant = x nm -1 -E l -E h n l n h Spectral line

The Bohr Atom Model Electrons can only jump to certain defined levels when it gains or loses energy. Because n h and n l are integers -E l -E h n l n h ∆E = E h - E l

The Bohr Atom Model Emission: (colored spectral lines) Transition from n h to n l level. ∆E = E h - E l Absorption: (dark spectral line) Transition from n h to n l level. -∆E = E h - E l -E l -E h n l n h Spectral line

F4-8 The Atomic Spectrum The emission of light from an atom can be mapped in a diagram called the spectrum 300nm 700nm500nm Orbit 6->2Orbit 5->2 Orbit 4->2Orbit 3->2 Hydrogen Helium

Lines of Hydrogen Most prominent lines in many astronomical objects: Balmer lines of hydrogen

The Balmer Lines n = 1 n = 2 n = 4 n = 5 n = 3 HH HH HH The only hydrogen lines in the visible wavelength range. Transitions from 2 nd to higher levels of hydrogen 2 nd to 3 rd level = H  (Balmer alpha line) 2 nd to 4 th level = H  (Balmer beta line) …

F4-9 Finger print of an Atom Every atom and molecule has its own distinct spectrum Spectra differ in number of lines and the spacing between the lines