Download Electromagnetic Radiation and Atomic Physics

Electromagnetic Radiation
and Atomic Physics
Properties of Electrons, Protons, and Neutrons
(The Main Constituents of Ordinary Matter)
• Mass
Electrons have a mass of 9.11×10-31kg.
The mass of a proton is 1836 times the mass of an electron.
The mass of the neutron is 1839 times the mass of the electron.
• Charge
Electrons are negatively charged.
charged
Protons are positively charged.
Neutrons are uncharged.
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How do electrically-charged particles influence each
other?
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There are two kinds of electrical charge – positive and negative.
Charges with the same sign repel each other.
Charges with opposite signs attract each other.
Uncharged particles exert no electrical force on each other or on charged
particles.
+
+
+
kq q
F = 12 2
r
( Coulomb's Law )
An electric field is the property of space by means of which one electrically charged particle exerts a force on another electrically-charged particle.
The charge of particle 1 changes the space around it, giving it the property we call
the electric field. The electric field of particle 1 exerts a force on particle 2.
The electric force on a positively charged particle is in the direction of the
electric field.
A magnetic field is the property of space by means of which one moving charged
particle exerts a magnetic force on another moving charged particle.
The motion of particle 1 produces a magnetic field in space. That magnetic field
exerts a magnetic force on moving particle 2.
The magnetic force on a moving charged particle is in the direction
perpendicular to the magnetic field and to the particle’s velocity. Because of
this, charged particles tend to spiral around magnetic field lines.
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A wave is something that transfers energy and momentum from one point to another
without the motion of mass between the two points. A wave transfers a “disturbance” rather
than matter.
•
•
W t waves
Water
A pebble dropped into some water disturbs the water, causing ripples to move
away from the point of impact. The disturbance is an up and down motion of
the water.
Sound Waves
The vibration of the head of a drum alternately compresses and rarifies the air
in contact with it. The series of compressions and rarefactions moves away
from the drumhead. When these compressions and rarefactions reach an
eardrum the eardrum moves back and forth with the same frequency as the
eardrum,
drumhead.
Electromagnetic Waves and Photons
An electromagnetic wave consists of oscillating electric and magnetic fields.
f=
E Field
B Field
c
λ
c = speed of light in vacuum
λ = wavelength
n = index of refraction
λ
Direction
of Wave
Motion
v = speed of light in matter
v=
c
n
An electromagnetic wave consists of a stream of photons.
E = energy of a photon
E = hf =
hc
λ
h = 6.626 × 10−34 J ⋅ s
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The Electromagnetic Spectrum
Electromagnetic radiation with wavelengths between 400 nm and 700 nm is visible. In
order of decreasing wavelength (increasing frequency), the colors are red (700 nm),
orange, yellow, green, blue, violet (400 nm).
← γ − ray
X-ray
UV
Infrared
μ wave
→ radio
Visible
o
o
4000A
7000 A
Types of Electromagnetic Radiation
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Visible (400 nm – 700 nm)
Near infrared (700 nm – 40000 nm)
Far infrared (more than 40000 nm)
Near Ultraviolet (400 nm – 290 nm)
Far ultraviolet (290 nm – 10 nm)
y ( 10 nm– 0.01 nm))
X rays
Gamma rays (less than 0.01 nm)
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Atoms
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A neutral atom consists of a nucleus, made of protons and neutrons, and
enough electrons to neutralize the positive charge of the protons.
The diameter of the nucleus is about 1 femtometer (1 fm = 10-15m).
m)
The “cloud” of electrons, on the other hand, occupies a volume with a
diameter of about 0.1 nm (1 nm = 10-9 m).
Since mp = 1836 me and mn = 1839 me, most of the mass of an atom is
concentrated in its nucleus.
An ion is an atom that has either a deficit or a surplus of electrons.
The process of removing an electron from an atom is called “ionization”.
The number of protons in the nucleus determines the chemical identity of the
atom.
The Bohr Model of the Hydrogen Atom
n=3
n=2
The electron in a hydrogen atom moves in a
circular orbit. The radius can only have one of
a set of discrete values, rn (n = 1, 2, 3, …).
n = 1,
1 the
th lowest
l
t possible
ibl orbit,
bit is
i called
ll d the
th
ground state of the atom. The states above the
ground state are called excited states.
n=1
The electron can transfer to a higher orbit if it
absorbs a photon of energy equal to the energy
difference between the two orbits.
En = −
13.6eV
n2
( n = 1, 2,...)
1 eV = 1.602 × 10−19 J
When an electron transfers to a lower orbit, it
emits a photon with energy equal to the energy
difference between the two orbits.
A transfer from one orbit to another is called a
transition.
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An energy level diagram is a graphical representation of the energies that are
available to the electron in the atom. The following is an energy level diagram for the
hydrogen atom.
n=5
n=4
n=3
Paschen Series - infrared
Balmer Series - visible
n=2
α β γ
Lyman Series - ultraviolet
n=1
Any atom can be described in terms of its
energy levels. He, for example, has twice as
many protons as H. Its energy levels are closer
together, and the ionization potential for
removing one electron from a neutral He atom
in its ground state is 24.5 eV.
The higher energy levels are
increasingly close together and
blend into the continuum at an
energy of 13.6 eV above the
ground state. When a hydrogen
atom absorbs a photon with
energy greater than or equal to
13.6 eV, the electron is removed
from the atom. 13.6 eV is the
ionization potential of the
hydrogen atom.
λ Hα = 656.3nm
λ Hβ = 486.1nm
λ Hγ = 434.0nm
Kirchoff’s Rules
I. A solid, liquid, or dense gas has a continuous spectrum.
II. A hot, low-density gas has an emission (bright line) spectrum.
III. When light with a continuous spectrum passes through a cool gas, the result is an
absorption spectrum.
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The dominant component of a star’s spectrum is a set of absorption lines. The visible lines
in the absorption spectrum of the Sun are shown below.
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