17.1. Atomic Theory of Matter
Matter is composed of atoms, or atoms that have joined to form molecules.
Ordinary states of matter — seen in every day life (i.e., not inside neutron stars):
1. Solid – rigid, closely packed atoms or molecules
2. Liquid – fluid, less closely packed atoms / molecules
3. Gas – fluid, widely spaced atoms / molecules
4. Plasma – ionized gas (at least partly so); fluid, thermodynamically similar to gas; widely
spaced atoms
What’s an example of plasma seen in everyday experience?
Poll: Which is the most prevalent observable state of matter in the universe?
————————————————–
The masses of atoms primarily arise from protons & neutrons in nuclei. (Electron masses are smaller
by about 1/1836.)
Atomic masses are measured in atomic mass units (amu), or simply u:
1 u = 1
.
66
×
10
−
27
kg [I could also write this as 1.66e27]
1 u is very close to the mass of either the proton or the neutron.
The atomic mass of an atom
A
, expressed in u, is approximately the number of protons
Z
+ the
number of neutrons
N
n
:
A
=
Z
+
N
n
Carbon12 (
12
C) has
Z
=6 (this is what makes it carbon!)
and
N
n
=6, so
A
=12.
In fact, for
Carbon12, its A is defined to be exactly 12.
Carbon14 (
14
C) has
Z
=6 and
N
n
=8, so
A
=14.
(The atomic masses of elements listed on periodic tables are not exact integers. What are some
reasons why?)
————————————————–
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What are some other atoms or molecules with which we’re familiar, and their masses in u?
————————————————–
Einstein (1905): Atoms are about 10
−
10
m (
≡
1
˚
A= 1 angstrom) across.
Ex. 171 from the book: Cu has an atomic mass of 63 u. What is its interatomic spacing?
Answer: 2
.
3
×
10
−
10
m — this is about 2
˚
A.
————————————————–
Example:
Similar to the book’s, what’s the intermolecular spacing of water at 4
◦
C?
Strategy: We want the length between molecules. Let’s go from kg / volume to get molecules /
volume, then take the cube root to get molecules / length, then take the inverse to get length /
molecule.
At this temperature, the mass density of water is
ρ
≃
1g/cm
3
, or 10
3
kg/m
3
.
Water has a molecular weight of 18 u — 16 u from O, and two 1 u’s for H.
10
3
kg
m
3
1
u
1
.
66
×
10
−
27
kg
molecule
18u
=
3
.
3
×
10
+28
molecules m
3
Check units!
Taking the cube root gives 3.2
×
10
9
molecules / m.
Taking the inverse gives 3.1
×
10
−
10
m / molecule, or 3.1
˚
A/ molecule.
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 Spring '12
 welsch
 Atom, Atomic Mass, atoms

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