Atom and Electromagnetic Radiation
The number of protons, neutrons, and
electrons in an atom can be determined from a set of simple
- The number of protons in the nucleus of the atom is
equal to the atomic number (Z).
- The number of electrons in a neutral atom is
equal to the number of protons.
- The mass number of the atom (M) is equal to
the sum of the number of protons and neutrons in the
- The number of neutrons is equal to the difference
between the mass number of the atom (M) and
the atomic number (Z).
Examples: Let's determine the number of protons, neutrons,
and electrons in the following isotopes.
The different isotopes of an element are
identified by writing the mass number of the atom in the
upper left corner of the symbol for the element. 12C,
13C, and 14C are isotopes of carbon (Z
= 6) and therefore contain six protons. If the atoms are
neutral, they also must contain six electrons. The only
difference between these isotopes is the number of neutrons
in the nucleus.
12C: 6 electrons, 6 protons, and
13C: 6 electrons, 6 protons, and
6 electrons, 6 protons, and 8 neutrons
Much of what is known about the structure of the electrons
in an atom has been obtained by studying the interaction
between matter and different forms of electromagnetic
radiation. Electromagnetic radiation has some of the
properties of both a particle and a wave.
Particles have a definite mass and they occupy
space. Waves have no mass and yet they carry energy as
they travel through space. In addition to their ability to
carry energy, waves have four other characteristic
properties: speed, frequency, wavelength, and amplitude. The frequency
(v) is the number of waves (or cycles) per unit of
time. The frequency of a wave is reported in units of cycles
per second (s-1) or hertz (Hz).
The idealized drawing of a wave in the figure below
illustrates the definitions of amplitude and wavelength. The wavelength
(l) is the smallest distance between repeating points
on the wave. The amplitude of the wave is the distance
between the highest (or lowest) point on the wave and the
center of gravity of the wave.
If we measure the frequency (v) of a wave in cycles
per second and the wavelength (l) in meters, the
product of these two numbers has the units of meters per
second. The product of the frequency (v) times the
wavelength (l) of a wave is therefore the speed (s)
at which the wave travels through space.
vl = s
Light and Other
Forms of Electromagnetic Radiation
Light is a wave with both electric and magnetic
components. It is therefore a form of electromagnetic
Visible light contains the narrow band of frequencies and
wavelengths in the portion of the electro-magnetic spectrum
that our eyes can detect. It includes radiation with
wavelengths between about 400 nm (violet) and 700 nm (red).
Because it is a wave, light is bent when it enters a glass
prism. When white light is focused on a prism, the light rays
of different wavelengths are bent by differing amounts and
the light is transformed into a spectrum of colors. Starting
from the side of the spectrum where the light is bent by the
smallest angle, the colors are red, orange, yellow, green,
blue, and violet.
As we can see from the following diagram, the energy
carried by light increases as we go from red to blue across
the visible spectrum.
Because the wavelength of electromagnetic radiation can be
as long as 40 m or as short as 10-5 nm, the
visible spectrum is only a small portion of the total range
of electromagnetic radiation.
The electromagnetic spectrum includes radio and TV waves,
microwaves, infrared, visible light, ultraviolet, x-rays,
g-rays, and cosmic rays, as shown in the figure above. These
different forms of radiation all travel at the speed of light
(c). They differ, however, in their frequencies and
wavelengths. The product of the frequency times the
wavelength of electromagnetic radiation is always equal to
the speed of light.
vl = c
As a result, electromagnetic radiation
that has a long wavelength has a low frequency, and radiation
with a high frequency has a short wavelength.