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Atomic Physics
Maria Luisa de Carvalho
Departamento Física
Universidade de Lisboa
Importance of Hydrogen
Atom
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Hydrogen is the simplest atom
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Nº Atómico e Nº de Massa
This enables us to understand the
periodic table
Importance of Hydrogen
Atom
„
Hydrogen is the simplest atom
Modelo do Átomo
„
Rutherford, 1911
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Modelo do Sistema
planetário
Positive charge is
concentrated in the
center of the atom,
called the nucleus
Electrons orbit the
nucleus like planets
orbit the sun
Bohr’s Assumptions, cont
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Only certain electron orbits are stable
Radiation is emitted by the atom when
the electron “jumps” from a more
energetic initial state to a lower state
Bohr’s Assumptions, final
„
Na transição do electrão:
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A energia é dada por
Ei – Ef = h ƒ
Specific Energy Levels,
cont
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The ionization energy is the energy
needed to completely remove the
electron from the atom
Atomic Transitions –
Energy Levels
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An atom may have many
possible energy levels
At ordinary temperatures,
most of the atoms in a
sample are in the ground
state
Só fotões com energia
igual à diferença de
energia entre dois
níveis podem ser
absorvidos
Energy Level Diagram
Espectros de Emissão
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A gas at low pressure has a voltage
applied to it
A gas emits light characteristic of the gas
When the emitted light is analyzed with a
spectrometer, a series of discrete bright
lines is observed
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Each line has a different wavelength and color
This series of lines is called an emission
spectrum
Examples of Emission
Spectra
Spectral Lines of Hydrogen
More Reasons the Hydrogen
Atom is so Important
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The hydrogen atom
Comportamento de metal alcalino
e de gás (halogéneo)
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1
1H;
2
1H;
3
1H
He+ and Li2+
Absorption Spectra
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An element can also absorb light at
specific wavelengths
An absorption spectrum can be
obtained by passing a continuous
radiation spectrum through a vapor of
the gas
Energy Bands in Solids
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In solids, the discrete energy
levels of isolated atoms broaden
into allowed energy bands
separated by forbidden gaps
The separation and the electron
population of the highest bands
determine whether the solid is a
conductor, an insulator, or a
semiconductor
Energy Bands, Detail
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Sodium example
Blue represents energy
bands occupied by the
sodium electrons when the
atoms are in their ground
states
Gold represents energy
bands that are empty
White represents energy
gaps
Electrons can have any
energy within the allowed
bands
Electrons cannot have
energies in the gaps
Energy Level Definitions
„
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The valence band is the highest filled
band
The conduction band is the next higher
empty band
The energy gap has an energy, Eg,
equal to the difference in energy
between the top of the valence band
and the bottom of the conduction band
Conductors
„
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When a voltage is applied
to a conductor, the
electrons accelerate and
gain energy
In quantum terms, electron
energies increase if there
are a high number of
unoccupied energy levels
for the electron to jump to
For example, it takes very
little energy for electrons to
jump from the partially
filled to one of the nearby
empty states
Insulators
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The valence band is
completely full of
electrons
A large band gap
separates the valence
and conduction bands
A large amount of
energy is needed for an
electron to be able to
jump from the valence
to the conduction band
„
The minimum required
energy is Eg
Semiconductors
„
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A semiconductor has a
small energy gap
Thermally excited
electrons have enough
energy to cross the band
gap
The resistivity of
semiconductors decreases
with increases in
temperature
The white area in the
valence band represents
holes
Movement of Charges in
Semiconductors
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An external voltage
is supplied
Electrons move
toward the positive
electrode
Holes move toward
the negative
electrode
There is a
symmetrical current
process in a
semiconductor
Interacção da radiação com a matéria
„
Absorção da Radiação
Se uma radiação de intensidade inicial I0 atravessa um material de
espessura x a radiação é absorvida e a intensidade final será I
I = Io e-µx
Io
x
I
Um dos processos principais de absorção da radiação é o efeito
Fotoeléctrico. A intensidade inicial e final da radição são dadas pela
relação 1, em que:
µ é o coeficiente linear de absorção característico de uma dada substância