Download Representing Isotopes Radioactive Decay

The Electromagnetic Spectrum
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:
– Early discoveries of radiation relied on photographic
equipment
 Marie Curie and her husband Pierre
named the energy radioactivity
 X-rays, radiation therapy and electricity generation are
beneficial.
 High energy particles and waves can do damage to DNA
in our cells.
Isotopes

39
19
K,
40
19
K,
41
19
Radium salts, after being placed on a
photographic plate, leave behind the dark
traces of radiation.
Representing Isotopes
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

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
Atomic Mass (the decimal #’s)

Atomic mass = average of the mass numbers for
all isotopes of an element.
Radioactive Decay
 Can
19
19
19
20
21
22
19
19
19
Uranium goes through many decay steps
before it becomes stable:
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)
1
 Rutherford
identified three types of radiation
using an electric field.
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
– 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.
4
2
 or 42 He
 2 protons and 2 neutrons make a mass number of 4
 it has a charge of 2+ because of the protons
Beta Radiation:
Alpha particles are big and slow. A sheet of paper
will stop an alpha particle.

A
Example: the alpha decay of Radium - 226
Beta particle, , is a high energy electron.
– negatively charged, and weigh less than alpha
particles.
– Beta particles are represented by the symbols
0
-1
226
88

Ra 
222
86
Rn + 24 
or
226
88
Ra 
222
86
Beta decay occurs when a neutron changes into a
proton + an electron.
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
e
Gamma Radiation:

Example: The beta decay of iodine - 131
0
-1
 electrons are very tiny, so beta particles are assigned a
mass of 0.
 one electron gives a beta particle a charge of 1–.
Rn + 24 He
– The proton stays in the nucleus, and the electron is
released.
 or
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
2
Nuclear Equations:

Often, other kinds of radioactive decay will also
release gamma radiation.
238
92
U 
234
90
Th + 42 He + 2
– Uranium-238 decays into an alpha particle and also
releases gamma rays.
 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:
Summary Tables
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.
Take the Section 7.1 Quiz
3