Mass of individual atoms
Lesson 1 – introduction to project and
atomic structure
Atomic particle
Real mass (g)
Relative mass
(amu)
Proton
p+
1.673 x 10
-24
1
Neutron
n0
1.675 x 10
-24
1
Electron
e-
9.109 x 10
-28
0
 The
mass of an atom is measured relative to the
mass of a specifically chosen atom:

Carbon-12
1
atomic mass unit (amu) = 1/12th the mass of
Carbon-12
1
amu is very close to the mass of one p+ or n0
So amu is…
the unit of an atom’s mass
That’s because……
……………..it’s an AVERAGE!
Isotopes of
Chlorine
Mass (amu)
Percentage
abundance
Cl-35
35
75%
(75/100 = 0.75)
Cl-37
37
25%
(25/100 = 0.25)
 (mass
x % abundance) + (mass x % abundance)
= (35 amu x 0.75)
+ (37 amu x 0.25)
= 26.25 amu
+ 9.25 amu
= 35.5 amu
Magnesium-24 written in symbol
notation is
Mass number
24
Atomic number
12
Mg
Nuclear Changes in the atom
 Chemical
reaction: involves
electrons, not the nucleus.

Element doesn’t change.
 Nuclear
reaction: involves the
nucleus.

Element changes.
 Some
 These
substances emit particles or rays.
particles are called radiation.
 Radioactivity is the release of these particles
 Atoms
emit radiation when their nucleus is
unstable.
 Stability
is determined by ratio of neutron
to protons. Too many or too few neutrons
makes an atom unstable.
 Spontaneously
emitting radiation is
radioactive decay
Why do
radioactive atoms
change from one
element to
another?
 Alpha

radiation


β0
-1
β particles are fast moving electrons
 Gamma

2
the α-particle is the same as the He nucleus
 Beta

4 radiation
(α) or He
radiation
γ0
0
γ rays are high energy radiation
Have no mass or charge
γ rays often emitted during α or β decay
 Half-life:
time
taken for ½ the
radioactive
nuclei to decay
into their stable
products.
 During
each
half-life, the
proportion of
parent atoms
decreases by ½
 Measures
rate of radioactive decay
 Half-life: Time taken for half the radioactive nuclei
to decay into their stable products.
Mass of Kanorium-136 (g)
Decay of Kanorium-136
110
100
90
80
70
60
50
40
30
20
10
0
0
20st
40
60
80
1 half life 2nd half life
100
120
140
Time (years)
160
180
200
 If





I have 10g of strontium 90 today,
in 29 years I will have half i.e. 5g
After another 29 years, 2.50 g remains
After another 29 years, 1.25 g remains
After another 29 years, 0.625 g remains
Decay continues till almost nothing is left
 Amount
remaining = (initial amount)(1/2)n
 n = number of half-lives that have passed.
Radioactivity
is a powerful tool to
measure absolute ages of rocks, past
geologic events and
HOW?!?
 If
something has radioactive material in it.
Depending on how much has broken down, we can
figure out how old it is.

The isotopes used in radiometric dating need to be sufficiently
long-lived so the amount of parent material left is measurable
Parents
Uranium 238
Uranium 234
Thorium 232
Rubidium 87
Potassium 40
Daughters
Lead 206
Lead 207
Lead 208
Strontium 87
Argon 40
Half-Life (years)
4.5 billion
704 million
14 billion
48.8 billion
1.3 billion
Igneous
rock
TODAY
Assume:
* daughters only produced by decay of
parents (no daughters to begin with).
* original rock had 100 parents.
25 parents
75 daughters
100 parents
Orignal Rock
50 parents,50 daughters
Rock at
some stage
1 half-life
Another
Half-life
25 parents,
75 daughters
Rock Today
Rock has experienced decay for two half-lives. How old is that??
If we had been using the Potassium-40 to Argon-40 dating system,
the half-life of potassium-40 is 1.3 billion years.
In this case, the rock is 2 half-lives x 1.3 b.y./half-life = 2.6 b.y.
(14C is radioactive, 12C is stable)
14C
constantly produced in
atmosphere, producing constant
14C/12C ratio.
Plants and animals incorporate carbon
of this constant ratio
When organism dies, 14C/12C begins
to decrease due to decay of 14C.
14C
has a half life of 5730 years,
useful for dating things that are
50,000 years