### Mass of individual atoms

```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.
 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
Why do
change from one
element to
another?
 Alpha



β0
-1
β particles are fast moving electrons
 Gamma

2
the α-particle is the same as the He nucleus
 Beta

(α) or He
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
nuclei to decay
into their stable
products.
 During
each
half-life, the
proportion of
parent atoms
decreases by ½
 Measures
 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.
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
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
```