CHAPTER 4 Electron Configurations (current model of the atom) deBroglie, Davisson, Germer (wave nature of the electron) Heisenberg’s Uncertainty Principle Bohr in 1922 Bohr… observation of discrete emission spectra… quantized energy levels for electrons “planetary” model for the atom (used classical mechanics and electrostatics) model worked for H atoms … only… 1e-, 1p amount of radiant energy released/absorbed as electrons move between orbits matched Bohr’s predictions BUT … did not correctly predict the spectrum of any other substance so…. model must be incomplete
Bohr’s work was doomed… used classical mechanics (physics) Classical mechanics is the study of the motion of bodies (including the special case in which bodies remain at rest) in accordance with the general principles first enunciated by Sir Isaac Newton in his Philosophiae Naturalis Principia Mathematica (1687), commonly known as the Principia.
Bohr’s work was doomed… used classical mechanics (physics) … to describe a body (the electron) that does NOT OBEY Newton’s Laws… it obeys the laws of quantum mechanics why? you ask… well, the electron is not a particle … huh? what is it??!!??
how can something be both!? LEAP! The next step towards the structure of the atom (where the electrons can be) was to realize that the electron (which up to this point we have described as a particle) can behave like a wave. remember…. wave – continuous particle – discrete how can something be both!? we have looked at the wave and particle nature of light (wave-particle duality) wave properties particle properties diffraction interference reflection photoelectric effect discrete emission spectra (absorption/emission of single photons as electrons move between orbits)
tells me whether light is a wave or a particle I’ll write my check.” “Once and for all I want to know what I’m paying for. When the electric company tells me whether light is a wave or a particle I’ll write my check.”
Wave-particle Duality of Matter deBrogle 1925 hypothesis : matter has both wave and particle properties (wave-particle duality) predicted that a particle of mass m and velocity v should have a wavelength associated with it has properties of waves (continuous) has properties of particles (discrete) Planck’s constant (again!) Davisson and Germer - 1927 observed diffraction and constructive interference (wave properties) of electrons by a crystal of nickel
Diffraction and Interference of Light paths of constructive detector dark the slits have to be small AND the separation between them (d) has to be~ ~ 100’s nm (0.0000001 m) beam of light (beam of photons) diffraction grating
e- e- Diffraction and Interference of Electrons detector “dark” crystal of metal (ex: Ni / Al) each “dot” represents an atom have to have a beam of e- to observe the diffraction/ interference pattern beam of electrons e- the separation between atoms in the crystal must be ~ e- - separation b/w atoms ~10-10 m - e- ~ 10-10 m
The diffraction pattern on the left was made by a beam of x-rays passing through thin aluminum foil. The diffraction pattern on the right was made by a beam of electrons passing though the same foil.
size of an electron – re- = 2.82 10-15 m Wave Nature of Matter Louis de Broglie for an electron e- = = speed of the e- in the n=1 orbit of H atom 6.626 10-34 J s ( 9.11 10-31 kg ) ( 2.19 106 m/s ) 3.32 10-10 m (or 3.32 ) size of an electron – re- = 2.82 10-15 m e- > re- so…. Slide 11 … there’s plenty of room for the electrons to get through the “grid”
Wave Nature of Matter Louis de Broglie for a body of mass 50 kg walking - “me” me = = 6.626 10-34 J s ( 50 kg ) ( 1 m/s ) 1 10-35 m size of “me” – rme = 0.5 m me < rme to observe the diffraction (wave property) of “me” - there has to be a beam of “me’s” - and, the “me’s” have to go through slits separated by d ~ me …… IMPOSSIBLE
Wave Nature of Matter Louis de Broglie for a body of mass 50 kg walking – “me” me = 1 10-35 m to observe the diffraction (wave property) of “me” - there has to be a beam of “me’s” - and, the “me’s” have to go through slits separated by d ~ me …… IMPOSSIBLE how fast (or slow) would “me” have to be moving for me = 1 m ? 6.626 10-34 J s me (1 m) = v = ( 50 kg ) ( v ) can’t experience wave properties because need to be moving to do so 1 10-35 m/s imperceptibly slow
Is the electron a wave? or is it a particle?
e- e- go back Diffraction and Interference of Electrons detector “dark” crystal of metal (ex: Ni / Al) each “dot” represents an atom have to have a beam of e- to observe the diffraction/ interference pattern beam of electrons e- the separation between atoms in the crystal must be ~ e- - separation b/w atoms ~10-10 m - e- ~ 10-10 m go back
Heisenberg Uncertainty Principle Werner Heisenberg 1933 Heisenberg Uncertainty Principle terrifically! complex … for us, the key idea is you cannot ______________ know _________ the __________ and ___________ of a particle quantitatively… the uncertainty of position (x) ________ the uncertainty of momentum (mv) __________ simultaneously exactly position momentum (x) (mv) (x) (mv) insignificant for BIG objects (like us) for tiny objects, like the electron (me 10-25 kg, re 10-15 m) becomes significant
by the numbers….
The Uncertainty Principle qualitatively… for an electron how do you determine the position of a particle (________ the electron) and how fast it is going (two points in time) when… you can’t see the particle!
ouch... You ____ the electron untethered can’t move …. free! to find the electron precisely need to “poke” around it with something smaller than ____ the electrons say, perhaps, a photon
You ____ the electron untethered …. free! can’t move to find the electron precisely to find the edges, need to throw a “lot” of photons… fast, and all at once ____ the electron gets pushed… no longer where it was… so, in the act of finding ___ the electron…. it is lost
can’t ever know precisely where an electron is You ____ the electron untethered …. free! can’t move to find the electron precisely can’t ever know precisely where an electron is (or in fact, where anything is…)