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Atomic Emission Spectra
Ck12 Science
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Printed: July 22, 2014
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Atomic Emission Spectra
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Atomic Emission Spectra
• Define ground state.
• Define excited state.
• Describe how atomic emission spectra are produced.
How much energy does it take to shoot an arrow?
Archery as a sport or a means of defense has existed for centuries. At rest, there is no tension on the bowstring and
no force on the arrow. When the string and arrow are pulled back, we now have a situation where kinetic energy
(pulling of the string) has been converted to potential energy (the tension on the string). The archer releases the
arrow and the potential energy is translated into kinetic energy as the arrow moves. It turns out that electrons behave
the same way when energy is put into the system or released from the system.
Atomic Emission Spectra
The electrons in an atom tend to be arranged in such a way that the energy of the atom is as low as possible. The
ground state of an atom is the lowest energy state of the atom. When those atoms are given energy, the electrons
absorb the energy and move to a higher energy level. These energy levels of the electrons in atoms are quantized,
meaning again that the electron must move from one energy level to another in discrete steps rather than continuously.
An excited state of an atom is a state where its potential energy is higher than the ground state. An atom in the
excited state is not stable. When it returns back to the ground state, it releases the energy that it had previously
gained in the form of electromagnetic radiation.
So how do atoms gain energy in the first place? One way is to pass an electric current through an enclosed sample
of a gas at low pressure. Since the electron energy levels are unique for each element, every gas discharge tube will
glow with a distinctive color depending on the identity of the gas (see Figure 1.1).
“Neon” signs are familiar examples of gas discharge tubes. However, only signs that glow with the red-orange color
seen in the figure are actually filled with neon. Signs of other colors contain different gases or mixtures of gases.
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FIGURE 1.1
Gas discharge tubes are enclosed glass tubes filled with a gas at low pressure through which an electric current
is passed. Electrons in the gaseous atoms first become excited, and then fall back to lower energy levels, emitting
light of a distinctive color in the process. Shown are gas discharge tubes of helium, neon, argon, krypton, and
xenon.
Scientists studied the distinctive pink color of the gas discharge created by hydrogen gas. When a narrow beam
of this light was viewed through a prism, the light was separated into four lines of very specific wavelengths (and
frequencies since λ and v are inversely related). An atomic emission spectrum is the pattern of lines formed when
light passes through a prism to separate it into the different frequencies of light it contains. The Figure 1.2 shows
the atomic emission spectrum of hydrogen.
FIGURE 1.2
When light from a hydrogen gas discharge tube is passed through a prism,
the light is split into four visible lines. Each
of these spectral lines corresponds to a
different electron transition from a higher
energy state to a lower energy state. Every element has a unique atomic emission
spectrum, as shown by the examples of
mercury (Hg) and strontium (Sr).
Classical theory was unable to explain the existence of atomic emission spectra, also known as line-emission spectra.
According to classical physics, a ground state atom would be able to absorb any amount of energy rather than only
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Chapter 1. Atomic Emission Spectra
discrete amounts. Likewise, when the atoms relaxed back to a lower energy state, any amount of energy could
be released. This would result in what is known a continuous spectrum, where all wavelengths and frequencies
are represented. White light viewed through a prism and a rainbow are examples of continuous spectra. Atomic
emission spectra were more proof of the quantized nature of light and led to a new model of the atom based on
quantum theory.
Summary
• Atomic emission spectra are produced when excited electrons return to ground state.
• The emitted light corresponds to energies of the specific electrons.
Practice
Questions
Use the link below to answer the following questions:
http://chemistry.bd.psu.edu/jircitano/periodic4.html
1. How many spectral lines are there for lithium?
2. How many spectral lines are there for beryllium?
3. Which element would you expect to have more lines: Na or Mg?
Review
Questions
1. What is the ground state of an atom?
2. What is an excited state?
3. Why do we see emission lines when electrons return to the ground state?
• atomic emission spectrum: The pattern of lines formed when light passes through a prism to separate it into
the different frequencies of light it contains.
• continuous spectrum: All wavelengths of light are present.
• excited state: A state where the potential energy of the atom is higher than the ground state.
• ground state: The lowest energy state of the atom.
References
1. Image copyright l i g h t p o e t, 2014. http://www.shutterstock.com .
2. User:Jurii/Wikimedia Commons, Heinrich Pniok (Wikimedia: Alchemist-hp). http://commons.wikimedia.org
/wiki/File:Glowing_noble_gases.jpg .
3. CK-12 Foundation - Christopher Auyeung, using emission spectra available in the public domain. H spectr
um: http://commons.wikimedia.org/wiki/File:Emission_spectrum-H.svg; visible spectrum: http://commons
.wikimedia.org/wiki/File:Linear_visible_spectrum.svg; He spectrum: http://commons.wikimedia.org/wiki/Fil
e:Helium_Emission_Spectrum.svg; Fe spectrum: http://commons.wikimedia.org/wiki/File:Emission_spectru
m-Fe.svg .
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