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Quantum Theory of Atoms

Quantum Theory of Atoms. The Bohr theory of Hydrogen(1913) cannot be extended to other atoms with more than one electron we have to solve the Schrödinger equation(1925)

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Quantum Theory of Atoms

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  1. Quantum Theory of Atoms • The Bohr theory of Hydrogen(1913) cannot be extended to other atoms with more than one electron • we have to solve the Schrödinger equation(1925) • since the Coulomb force only depends on r, we should use spherical coordinates (r,,) instead of Cartesian coordinates (x,y,z)

  2. Quantum Theory of Atoms • For a particle in a cubic or rectangular box, Cartesian coordinates are more appropriate and there are three quantum numbers (n1,n2,n3) needed to label a quantum state • similarly in spherical coordinates, there are also three quantum numbers needed • n = 1, 2, 3, ….. • l= 0, 1, 2, …,n-1 => n values of l for a given value of n • m= -l,(-l+1),…0,1,2,…,l => 2l+1 values of m for a given l • n is the principal quantum number and is associated with the distance r of an electron from the nucleus • l is the orbital quantum number and the angular momentum of the electron is given by L=[l(l+1)]1/2 ħ • m is the magnetic quantum number and the component of the angular momentum along the z-axis is Lz = m ħ

  3. Quantum Theory of Atoms • The fact that both l and m are restricted to certain values is due to boundary conditions • in the figure, l=2 is shown • hence L=(2(2+1))1/2 ħ = ħ(6)1/2 • and m= -2, -1, 0, 1, 2 • the Schrödinger equation can be solved exactly for hydrogen • the energies are the same as in the Bohr theory En = -Z2 (13.6 eV)/n2 • they do not depend on the value of l and m • this is a special property of an inverse-square law force

  4. The lowest energy has n=1 => l=0 and m=0 • the second lowest energy has n=2 => l=0, m=0l=1, m=-1,0,1 • hence 4 states! • Notation: l=0 S-state • l=1 P-state • l=2 D-state • l=3 F-state • l=4 G-state

  5. When a photon is emitted or absorbed we must have m= 0 or ±1 l = ±1 • conservation of angular momentum and conservation of energy

  6. Wave Functions • We denote the solutions of the SE as nlm (r,,) • the probability of finding the electron at some position (r,,) is P (r,,)= (nlm)2 dV where dV is the volume element • in spherical coordinates dV=r2drsindd

  7. Wave Functions • the ground state wave function is Cexp(-Zr/a0) where C is determined from normalization • Hence for the ground state, the probability density 1002 is independent of  and  and is maximum at the origin

  8. Wave Functions • What is the probability of finding the electron between r and r+dr? • in other words, in a spherical shell of thickness dr • volume of shell is 4r2 dr (area x thickness) • P(r)dr = (4r2 2)dr P(r) is the radial probability density

  9. Wave Functions • Maximum is at r = a0/Z = a0 for Hydrogen (first Bohr orbit) • not a well defined orbit but rather a cloud

  10. Probability density

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