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The Development of a New Atomic Model
In the early twentieth century, a new atomic model evolved as
a result of investigations into the absorption and emission of
light by matter
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This led to the
discovery of the nature of energy, matter, and atomic
structure
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The Wave Description of Light
Visible light is a kind of electromagnetic radiation, which is a
form of energy that exhibits wavelike behavior as it travels through
space
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All of
these forms of electromagnetic radiation form the electromagnetic
radiation
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Electromagnetic radiation travels in the
form of waves covering a wide range of
wave lengths and frequencies
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Only a small portion of the spectrum, from
400 nm to 700 nm, is visible to the human
eye

The significant feature for wave motion is its repetitive nature, which can
be characterized by the measurable properties of wavelength and
frequency
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The unit for wavelength is a distance unit
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Frequency is expressed in waves/second
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The distance between any two corresponding
points on one of these water waves, such as
from crest to crest, is the wave’s wavelength
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Frequency and wavelength are mathematically related to each other
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c= λν

In the equation, c is the speed of light (in m/s)
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Because c is the same for all
electromagnetic radiation, the product is constant
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In other words, as the
wavelength of light decreases, its frequency increases, and vice versa
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The Photoelectric effect refers to the emission of electrons from a metal
when light shines on the metal
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For a given metal,
no electrons were emitted if the light’s frequency was below a certain
minimum—regardless of the light’s intensity
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But
the wave theory of light predicted that light of any frequency could supply
enough energy to eject an electron
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The Particle Description of Light

Max Planck proposed that a hot object does not emit electromagnetic energy continuously
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A quantum of energy is the minimum quantity of energy
that can be lost or gained by an atom
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E = hv
In this equation, E is the energy, in joules, of a quantum of radiation, v is the frequency, in s , of the radiation emitted,
and h is a fundamental physical constant now known as Planck’s constant; h= 6
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Albert Einstein expanded on Planck’s theory by introducing the idea that elcromagnetic radiation has a dual wave-particle
nature
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Each particle of light carries a quantum of energy called photons
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The energy of a
particular photon depends on the frequency of the radiation
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In order for an
electron to be ejected from a metal surface, the electron must be struck by a single photon possessing at least the
minimum energy required to knock the electron loose
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Electrons in different metals are bound more or less tightly, so needing
different minimum frequencies to exhibit the
photoelectric effect
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The lowest energy state of an atom
is its ground state
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When an excited atom
A series of specific wavelengths of emitted light makes up hydrogen’s
returns to its ground state, it gives off the energy
it gained in the form of electromagnetic radiation
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The letters below the lines label hydrogen’s
various energy-level transitions
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The production of colored light in neon signs is an example of this process
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When a narrow beam of the emitted light was shined through a prism, it was separated
into four specific colors of the visible spectrum
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Additional series of lines were discovered in the ultraviolet and infrared regions of hydrogen’s lineemission spectrum
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Classical theory predicted that hydrogen atoms would be excited
by whatever amount of energy was added to them
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Whenever an excited hydrogen atom falls to its ground state or to a
lower-energy excited state, it emits a photon of radiation
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The fact
that hydrogen atoms emit only specific frequencies of light
indicated that the energy differences between the atom’s energy
states were fixed, which suggested that the electron of a hydrogen
atom exists only in very specific energy states
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Bohr Model of the Hydrogen Atom

The hydrogen-atom spectrum was solved in 1013 by Niels Bohr
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When the electron
is in one of these orbits, the atom has a definite, fixed energy
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This orbit is separated from the nucleus by a large empty
space where the electron cannot exist
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While in a given orbit, the electron is neither gaining nor losing energy
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When a hydrogen atom is in an excited state, its electron is in one of the higher-energy orbits
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The photon’s energy is equal
to the energy difference between the initial higher energy level and the final lower energy level
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The energy of
each absorbed or emitted photon corresponds to a particular frequency of emitted radiation, Ephoton =hv
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(a) Absorption and (b) emission of a
photon by a hydrogen atom according to
Bohr’s model
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Based on the different wavelengths of the hydrogen line-emission spectrum, Bohr calculated the allowed energy levels for
the hydrogen atom
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The five lines of the Lyman series for example, were shown to be the result of electrons dropping from energy levels E2,
E3, E4, and E6 to the ground-state energy level E1
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This energy-state diagram for a hydrogen
atom shows some of the energy transitions
for the Lyman, Balmer, and Paschen
spectral series
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