大学物理西尔斯

1. Atomic Spectra of Hydrogen
1) Atomic Spectra ： emission spectrum：the spectrum formed during the process of radiation of electromagnetic waves.
absorption spectrum： the spectrum formed during the process of absorption of electromagnetic waves with certain wavelengths.
1) Ground-state level : Every atom has a lowest energy level that includes the minimum internal energy state that the atom can have.
2) Excited level :
Limit of wavelength of Balmer series
n , B 364.6nm
1890:
1
R(
1
2
2
1
2
)
n
n 3, 4,5,
R＝ 4
1.097
373
7 1
10 m
B
➢ Atomic Spectra of Hydrogen
Lyman series:
UV
~ R( 1 1 )
Q1：There must be something with positive charge in the atom. How does the mass of the atom distribute? Q2：How does the charges of the atom distribute?
3) Lifetime:
The average time spent in an excited level is called the lifetime of the level.
4) absorption spectrum :
In general, a photon emitted when an atom makes a transition from an excited level to a lower level can also be absorbed by a similar atom that is initially in a lower level.
玻尔（Niels henrik David Bohr，1885-1962）
丹麦理论物理学家，现代物理学的创始人之一。 在1913年发表了《论原子结构与分子结构》等三 篇论文，提出了在卢瑟福原子有核模型基础上的 关于原子稳定性和量子跃迁的三条假设，从而圆 满地解释了氢原子的光谱规律。
玻尔的成功，使量子理论取得重大发展，推动了 量子物理的形成，具有划时代的意义。
Limitation :
• It cannot explain why the positive and negtive charges do not neutralize;
• It cannot explain the presence of series of the hydrogen spectrum；
1903: J.J. Thomson
The positive charges and the mass of the atom distribute uniformly in a sphere with the radius 10-10 m, and the electrons are immersed in the sphere.
~ R( 1 1 ) n 6,7,8,
52 n2
Similar regularity has been found in the spectra of atoms of other elements.
2. Rules of Atomic Spectra
1
v
R
1
2
nf
1
2
ni
,
ni n f
expressed by the difference of two spectral terms.
Ritz combination principle：
The wave number of any line spectrum can be expressed by the difference of two spectral terms which comes from the many inherent spectral terms of the element.
Experiments show that the emission spectrum is corresponding to absorption spectrum： Different atoms have different spectra which are the characteristic to identify the atoms.
玻尔于1922年12月10日诺贝尔诞生100周年之际， 在瑞典首都接受了当年的诺贝尔物理学奖金。 1937年，他来中国作学术访问，表达了对中国人 民的友好情谊。§2Leabharlann .4 the nuclear atom
1 . Two important models for atoms:
1) Thomson model：raisins-cake model
nf 1, 2, ; ni nf 1，nf 2,
unified formula
Experiments show that:
• Atoms have line spectrum； • There is some relationship between every line; • The wave number of each line can be
All higher levels are called excited level.
A photon corresponding to a particular Spectrum line is emitted when an atom makes a transition from a state in an excited level to a state in a lower excited level or the ground level.
Feature of the atom spectra: line spectra
➢ Balmer series We can get four lines
H H H H
within the visible light in
an electric discharge tube. 条
H ： 656.210nm H ： 486.074nm H ： 434.010nm H ： 410.120nm
electron
Quantum physics
Newtonian mechanics for Microworld
electric neutrality
atom electron
Electron is negatively charged. Its mass is about one two-thousandth of that of Hydrogen atom.
during the emission of a photon, the internal energy of an atom changes by an amount equal to the energy of the photon.
Each atom must be able to exist only with certain specific values of internal energy---energy level.
In 1885 the Swiss teacher Johann Balmer found formula that gives the wavelength of three lines, which are now called the Balmer series.
＝B
n2 n2
22
n 3,4,5,
(This was discovered by Ritz in 1908.)
~ T (nf ) T (ni )
T (nf )
R
2
,
nf
T (ni )
R
2
ni
3. Energy levels :
1913, Bohr
The line spectrum of an element results from the emission of photons with specific energies from the atoms of the element.
2) the nuclear atom
Planet model of atom
1911: Rutherford
The positive charges all concentrated in a small space-----nucleus. It occupies only about 10-12 of the total volume of the atom or less. It contains more than 99% of the total mass of the atom. Electrons revolve around the nucleus. This mode can explain the largeangle scattering of α-particles.
• It cannot explain the large-angle scattering of α-particles.
➢ Scattering of α-particle:
FP T
S
θ
R
O
The experiment of large-angle scattering of α-
particles proved the Thomson’s model incorrect.
§28-3 Hydrogen Atomic line Spectra and energy levels
Introduction：
1) the quantum theory In 1900，Plank put forward the concept of quantum , explaining the black-body radiation, which laid foundation to the quantum theory. In 1905，Einstein explained the photoelectric effect by the concept of photon, which led the the quantum theory to a new stage; In 1920~1926，the discovery and analysis of Compton effect proved the theory of photon to be correct. The theoretical analysis coincided experiments perfectly.
Most α-particles are scattered through only a small angle when passing through the gold foil. A few α-particles are scattered through an angle greater than 900，even nearly 1800 .
12 n2
Paschen series: visible light
~ R( 1 1 )
32 n2
Rydberg constant
n 2,3,4, n 4,5,6,
Brackett series : near-infrared
~ R( 1 1 ) n 5,6,7,
42 n2
Pfund series:
2) Spectrum With the development of spectroscopy
people realized that spectra actually discover the interior structure of atoms. 3) electron • In 1897，J.J. Thomson discovered electron, leading people to the microworld. What