http://www.ck12.org Chapter 5. Electrons in Atoms
Practice Problem- A certain photon of radiation has an energy of 8.72× 10 −^21 J. Calculate the frequency and wavelength (in m)
of this radiation.
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 stateof an atom is the lowest energy state of the atom.When an atom is supplied with a sufficient amount
of energy, the electrons can absorb that energy and move to a higher energy level. The energy levels of the electrons
in atoms are quantized. An atom cannot absorb an arbitrary amount of energy, it can only do so in amounts that are
equal to the difference between two allowed energy levels.An atom is in anexcited statewhen its potential energy
is higher than that of the ground state.An atom in an 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.
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 (Figure5.7).
FIGURE 5.7
A gas discharge tube is produced when an electric current is passed through an enclosed glass tube that is filled
with a gas at low pressure. Electrons in the gaseous atoms first become excited by the current and then fall back
to lower energy levels, emitting light of a distinctive color in the process. Gas discharge tubes filled with helium,
neon, argon, krypton, and xenon are shown.“Neon” signs are familiar examples of gas discharge tubes. However, only signs that glow with the red-orange color
(Figure5.7) are actually filled with neon. Signs of other colors contain different gases or mixtures of gases.
Scientists have exhaustively studied the distinctive pink color of the gas discharge created by hydrogen gas. When
a narrow beam of this light is viewed through a prism, the light can be separated into four lines of very specific
wavelengths (and frequencies, sinceλandνare inversely related). Anatomic emission spectrumis the pattern of
lines formed when the light emitted from an atom passes through a prism to separate it into the different frequencies
of light it contains.Pictured below is the atomic emission spectrum of hydrogen (Figure5.8).
The four visible lines of hydrogen’s atomic emission spectrum are violet (410 nm), blue (434 nm), blue-green (486
nm), and red (656 nm). The pinkish color that our eyes see is a combination of these four colors. Every element