7.1 Atomic Theory and
Radioactive Decay
Natural background radiation exists all around
us.
 Radioactivity is the release of high energy
particles or waves that we call radiation

– When atoms lose high energy particles and waves,
new atoms can be formed.
– Radiation can be good or bad:
 X-rays, radiation therapy and electricity generation are
beneficial.
 High energy particles and waves can do damage to DNA
in our cells.
See pages 286 - 287
The Electromagnetic Spectrum
Early Discoveries of Radiation
– Roentgen named X rays with an “X” 100 years ago
because they were previously unknown.
– Becquerel realized uranium emitted seemingly
invisible energy as well.
– Marie Curie and her husband Pierre named this
energy radioactivity.
 Early discoveries of radiation relied
on photographic equipment.
– Later, more sophisticated devices
such as the Geiger-Müller counter Radium salts, after being
placed on a photographic
plate, leave behind the
dark traces of radiation.
Isotopes

39
19
K,
40
19
K,
41
19
K
are atoms of the same element, with a different
number of neutrons.
– changing the # of neutrons changes the mass number
 Remember: mass # = # protons + # neutrons
– isotopes still have the same number of protons and the
same element symbol
Atomic Mass (the decimal #’s)

Atomic mass = average of the mass numbers for
all isotopes of an element.
Representing Isotopes

Isotopes are written two ways
– with the mass number at the end
– With its chemical symbol
Mass
number
Atomic
number
39
19
K,
Ex. Potassium – 40
Ex. 40 K
19
40
19
K,
41
19
K
19
19
19
20
21
22
19
19
19
Radioactive Decay

Can result in new atoms forming.
– Radioactivity results from having an unstable
nucleus.
– Radioactive decay = when nuclei break apart +
release energy from the nucleus.
 Radioactive decay continues until a stable element
forms.
 An element may have isotopes that are radioactive
called radioisotopes
– Ex. carbon-12, carbon-13 and carbon-14
(only C-14 is radioactive)
Uranium goes through many decay steps
before it becomes stable:

Rutherford identified three types of radiation
using an electric field.
– Positive alpha particles were attracted to the negative plate.
– Negative beta particles were attracted to the positive plate.
– Neutral gamma particles did not move towards any plate.
Alpha Radiation:

is a stream of alpha particles, (shown as )
– positively charged
– weighs the most
– are the same as a helium nucleus
– Alpha particles are represented by the symbols
4
2
 or He
4
2
 2 protons and 2 neutrons make a mass number of 4
 it has a charge of 2+ because of the protons
Alpha particles are big and slow. A sheet of paper
will stop an alpha particle.

Example: the alpha decay of Radium - 226
226
88
Ra 
222
86
Rn + 
4
2
or
226
88
Ra 
222
86
4
2
Rn + He
Beta Radiation:

A Beta particle, , is a high energy electron.
– negatively charged, and weigh less than alpha
particles.
– Beta particles are represented by the symbols
0
-1
 or
0
-1
e
 electrons are very tiny, so beta particles are assigned a
mass of 0.
 one electron gives a beta particle has a charge of 1–.

Beta decay occurs when a neutron changes into a
proton + an electron.
– The proton stays in the nucleus, and the electron is
released.
Example: The beta decay of iodine - 131
131
53
I 
131
54
Xe +
0
–1

or
131
53

I 
131
54
Xe +
0
–1
It takes a thin sheet of aluminum foil to stop a
beta particle.
e
Gamma Radiation:

Gamma radiation, , is a ray of high energy,
short-wavelength radiation.
– has no charge and no mass.
– is the highest energy form of electromagnetic radiation.
– It takes thick blocks of lead or concrete to stop gamma
rays.
– Gamma decay results from energy being released from
a high-energy nucleus.
60
28
Ni* 
60
28
Ni + 00
Shows unstable nucleus for
gamma decay

Often, other kinds of radioactive decay will also
release gamma radiation.
238
92
U 
Th + He + 2
234
90
4
2
– Uranium-238 decays into an alpha particle and also
releases gamma rays.
Nuclear Equations:

are written like chemical equations, but
represent changes in the nucleus of atoms.
– Chemical equations represent changes in the
position of atoms, not changes to the atoms
themselves.

Remember:
1.The sum of the mass numbers on each side of
the equation should equal.
2.The sum of the charges on each side of the
equation should equal.
Summary Tables
Take the Section 7.1 Quiz