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CONCEPTS Applied with matter is an “all-or-none” affair; that is, matter absorbs an
entire photon or none of it. The relationship between frequency
Atomic Parts (f ) and energy (E) is
Identify the number of protons, neutrons, and electrons E = hf
16
in an atom of 8 O. Write your answer before you read the
equation 8.1
solution in the next paragraph.
The subscript to the bottom left is the atomic number. where h is the proportionality constant known as Planck’s
Atomic number is defined as the number of protons in the –34
constant (6.63 × 10 J⋅s). This relationship means that higher-
nucleus, so this number identifies the number of protons
frequency light, such as ultraviolet, has more energy than lower-
as 8. Any atom with 8 protons is an atom of oxygen,
frequency light, such as red light.
which is identified with the symbol O. The superscript
to the top left identifies the mass number of this isotope
of oxygen, which is 16. The mass number is defined as
the sum of the number of protons and the number of EXAMPLE 8.1
neutrons in the nucleus. Since you already know the
number of protons is 8 (from the atomic number), then What is the energy of a photon of red light with a frequency of 4.60 ×
14
the number of neutrons is 16 minus 8, or 8 neutrons. 10 Hz?
Since a neutral atom has the same number of electrons
as protons, an atom of this oxygen isotope has 8 protons, SOLUTION
8 neutrons, and 8 electrons.
Now, can you describe how many protons, neutrons, f = 4.60 × 10 Hz
14
17
and electrons are found in an atom of 8 0? Compare your –34
h = 6.63 × 10 J⋅s
answer with a classmate’s to check.
E = ?
E = hf
14 1 _
–34
= (6.63 × 10 J⋅s)(4.60 × 10 )
s
–34 14 1 _
= (6.63 × 10 ) (4.60 × 10 ) J⋅s ×

s
8.2 THE BOHR MODEL –19
= 3.05 × 10 J
Niels Bohr was a young Danish physicist who visited
Rutherford’s laboratory in 1912 and became very interested
in questions about the solar system model of the atom. He EXAMPLE 8.2
wondered what determined the size of the electron orbits What is the energy of a photon of violet light with a frequency of
and the energies of the electrons. He wanted to know why 7.30 × 10 Hz? (Answer: 4.84 × 10 J)
–19
14
orbiting electrons did not give off electromagnetic radia-
tion. Seeking answers to questions such as these led Bohr
to incorporate the quantum concept of Planck and Einstein
with Rutherford’s model to describe the electrons in the outer ATOMIC SPECTRA
part of the atom. We will briefly review this quantum concept
before proceeding with the development of Bohr’s model of Planck was concerned with hot solids that emit electromag-
the hydrogen atom. netic radiation. The nature of this radiation, called blackbody
radiation, depends on the temperature of the source. When
this light is passed through a prism, it is dispersed into a con-
tinuous spectrum, with one color gradually blending into the
THE QUANTUM CONCEPT next as in a rainbow. Today, it is understood that a continuous
In the year 1900, Max Planck introduced the idea that mat- spectrum comes from solids, liquids, and dense gases because
ter emits and absorbs energy in discrete units that he called the atoms interact, and all frequencies within a temperature-
quanta. Planck had been trying to match data from spectros- determined range are emitted. Light from an incandescent gas,
copy experiments with data that could be predicted from the on the other hand, is dispersed into a line spectrum, narrow
theory of electromagnetic radiation. In order to match the lines of colors with no light between the lines (Figure 8.8). The
experimental findings with the theory, he had to assume that atoms in the incandescent gas are able to emit certain char-
specific, discrete amounts of energy were associated with differ- acteristic frequencies, and each frequency is a line of color
ent frequencies of radiation. In 1905, Albert Einstein extended that represents a definite value of energy. The line spectra are
the quantum concept to light, stating that light consists of dis- specific for a substance, and increased or decreased temper-
crete units of energy that are now called photons. The energy of ature changes only the intensity of the lines of colors. Thus,
a photon is directly proportional to the frequency of vibration, hydrogen always produces the same colors of lines in the same
and the higher the frequency of light, the greater the energy of position. Helium has its own specific set of lines, as do other
the individual photons. In addition, the interaction of a photon substances. Line spectra are a kind of finger print that can

208 CHAPTER 8 Atoms and Periodic Properties 8-6