University:
California State University, Northridge
Course:
PHYS 100B | General Physics II
Academic year:

2008
Views:

227

Pages:

15
Author:

Layla Bonilla

31 Atomic Physics Outline 31-1 Early Models of the Atom 31 2 31-2 The Spectrum of Atomic Hydrogen 31-3 Bohr’s Model of the Hydrogen Atom 31-1 Early Models of the Atom The “solar system” model has been applied to the atom : electrons orbit a small, positively charged nucleus. The electrons move in centripetal motion, and the force is p provide by y the Coulomb force. Electron Energies include: • Electric Potential energy energy. • Kinetic energy. • Spectrum (photon) can be created when electron jumps from one orbit to other orbit. • The photon wavelength is determined by hf. 31-2 The Spectrum of Atomic Hydrogen The creation of spectral lines: Emission spectrum. ((hydrogen y g g gas)) Figure 31-3 The Line Spectrum of an Atom There are two types of spectral lines: Emission spectrum: bright lines. Direct radiation from hot gas. Absorption Spectrum: dark lines. Absorption of gas atom when white light goes through the gas. Why spectrum is the fingerprint of an atom and why it is discontinuous? 1) Since the electron of each atom has its specific energy level in its orbits that is discontinuous; 2) When the electron jump from one energy level to another energy level, it gives off or absorbs photons: ΔE = hf . Figure 31-4 The Line Spectrum of Hydrogen Emission spectrum lines (a); absorption spectrum lines (b). Spectrum of the Sun (In 1885) Swiss school teacher, Balmer developed an empirical formula o u a for o tthe e ca calculation cu at o Hydrogen yd oge spect spectrum u lines. es This s formula o u a is s expressed as: 1 λ = R( 1 1 − 2 ), 2 2 n n = 3,4,5,⋅ ⋅ ⋅ ⋅ ( Balmer series) 31 − 1 Unit: meter. Where R is Rydberg constant: R = 1.097 x 107 m-1 The collection of lines by the above formula is called Balmer series. For example, n=5: 1 1 1 = (1.097 × 10 m )( 2 − 2 ) λ 2 5 so, λ = 4.341× 10 −7 m 7 −1 Example 31-1 The Balmer Series Find the longest and shortest wavelengths in the Balmer series of the spectral lines. Solution: 1). The longest wavelength in Balmer series is n = 3, 1 1 1 1 1 −1 7 = R( 2 − 2 ) = (1.097 ×10 m )( 2 − 2 ), λ 2 n 2 3 λ = 653.3 ×10 −9 m = 653.3 nm 2). The shortest wavelength in Balmer series is n → ∞, 1 1 1 = (1.097 ×10 m )( 2 − 2 ), λ 2 0 λ = 364.6 ×10 −9 m = 364.6 nm 7 −1 Equation 31-1 only calculate the Balmer Series spectral lines. There are other spectra: Figure 31-5 The Lyman, Balmer, and Paschen Series of Spectral Lines The formula that that gives the spectra in all wavelength series of h d hydrogen iis 1 1 1 = R( 2 − 2 ), λ n' n n ' = 1, 2,3, ⋅⋅⋅⋅ n = n '+ 1, n = n '+ 2, Table 31-1 Common Spectral Series of Hydrogen n’’ S i name Series 1 Lyman 2 Balmer 3 Paschen 4 Brackett 5 Pfund 31 − 2 Exercise 31-1 Find (a) the shortest wavelengths in the Lyman series and (b) the longest wavelength in the Paschen series. Exercise 31-1 Find (a) the shortest wavelengths in the Lyman series and (b) the longest wavelength in the Paschen series. Solution: 1). Substitute n’ =1 (Lyman) and n → ∞ in equation 31-2: 1 1 1 1 1 7 −1 = R( 2 − 2 ) = (1.097 ×10 m )( 2 − 2 ), ) λ 1 n 1 ∞ λ = 91.16 ×10 −9 m = 91.16 nm 2). Substitute n’ =3 (Paschen) and n=4 in equation 31-2: 1 1 1 1 1 −1 7 = R( 2 − 2 ) = (1.097 ×10 m )( 2 − 2 ), λ 3 n 3 4 λ = 1875 ×10 −9 m = 1875 nm Summary The formula that that gives the spectra in all wavelength series of hydrogen is 1 1 1 = R( 2 − 2 ), λ n' n n ' = 1, 2,3, ⋅⋅⋅⋅ n = n '+ 1, n = n '+ 2, Table 31-1 C Common S Spectral t l Series S i off Hydrogen H d n’ Series name 1 Lyman 2 Balmer 3 Paschen 31 − 2