CHAPTER 2: Atoms and
Molecules
ELEMENTS IN THE HUMAN BODY:
H
O
C
N
Ca
P
K
S
Cl
Na
Mg
• Use atomic weights to calculate molecular weights
• Understand how the numbers and types of
subatomic particles determines the structure of
isotopes
• Use the mole concept to obtain the relationship
between the number of moles and the mass of an
element or compound
2.1 SYMBOLS and FORMULAS
NaH2PO4
2 atoms of
HYDROGEN
10.0
64.8
18.0
3.1
1.8
1.4
0.4
0.3
0.2
0.1
0.04
major biomolecules
major biomolecules
major biomolecules
major biomolecules
bones, electrolytes
bones, biomolecules
electrolytes
major biomolecules
electrolytes
electrolytes
electrolytes
2.2 INSIDE THE ATOM
ELECTRON CLOUD
• Each element has a unique SYMBOL (Table 2.1)
• Each chemical compound is described by a
FORMULA which tells
— The kinds of elements present
— The number of atoms of each element
1 atom of
SODIUM
Abundance,
Wgt %
Element
• Use element symbols to write chemical compound
formulas
Electrons (-)
Little mass
NUCLEUS
Protons (+) and neutrons (0)
Most of the mass
4 atoms of
OXYGEN
ATOM
ATOM
1 atom of
PHOSPHORUS
The
The smallest
smallest unit
unit of
of an
an element
element that
that
retains
retains its
its chemical
chemical properties
properties
The Nucleus
RELATIVE
RELATIVE
MASS
MASS
Subatomic particles:
The nucleus is a small part of an atom.
If
If the
the nucleus
nucleus were
were the
the size
size
of
of aa marble,
marble, the
the atom
atom would
would
fill
fill the
the Rubber
Rubber Bowl
Bowl
The
The nucleus
nucleus would
would weigh
weigh
over
over 300
300 tons
tons
Page 2-1
PARTICLE
SYMBOL
CHARGE
MASS,
g
MASS,
amu
ELECTRON
e
-1
9.11 × 10-28
0.00054
PROTON
p
+1
1.67 × 10-24
1
NEUTRON
n
0
1.67 × 10-24
1
2.5 ISOTOPES and
ATOMIC WEIGHTS
THE ATOMIC SYMBOL
A
Z
X
Q
TWO KINDS OF WATER (H2O)
• “Normal” Water
— Melts at 0°C
— Boils at 100°C
1
1H2O
• “Heavy” Water
— Melts at 4°C
— Boils at 101°C
2
1H2O
A = ATOMIC MASS/WEIGHT (protons + neutrons)
Z = ATOMIC NUMBER (protons)
Q = CHARGE (protons - electrons)
Unique
Unique for
for each
each element
element
Atoms
Atoms of
of the
the same
same element
element (Z)
(Z) that
that have
have
different
different atomic
atomic weights
weights (A)
(A) due
due to
to different
different
numbers
numbers of
of neutrons
neutrons
Hydrogen
1
1H
12
6C
Carbon
2
1H
3
1H
13
6C
14
6C
2.6 THE MOLE and
CHEMICAL FORMULAS
55.85 amu
55.85 g
55.85 lb
55.30 g
(0.015%)
ELEMENT
STABLE
ISOTOPES
ATOMIC
WEIGHTS
H
2
1, 2
C
2
12, 13
O
3
16, 17,18
Sn
10
112,
115,
117,
119,
122,
114,
116,
118,
120,
124
The Chemist’s Dozen:
Mole is a practical unit that relates a weight to a
relative number of atoms or molecules.
Samples
Samples that
that contain
contain the
the same
same relative
relative weights
weights
(in
(in terms
terms of
of atomic
atomic weights)
weights) contain
contain the
the same
same
numbers
numbers of
of atoms
atoms
1.01 amu
1.01 g
1.01 lb
1.00 g
2
1H
Atomic
Atomic weights
weights are
are averages
averages of
of the
the
weights
weights of
of each
each element’s
element’s isotopes.
isotopes.
(Chemistry
(Chemistryisisthe
thebehavior
behaviorof
ofelectrons,
electrons,not
notof
ofprotons
protonsand
and
neutrons)
neutrons)
Fe
(99.985%)
Most elements occur in nature as
mixtures of isotopes:
ALL
ALL OF
OF THE
THE ISOTOPES
ISOTOPES OF
OF AN
AN ELEMENT
ELEMENT
HAVE
HAVE THE
THE SAME
SAME CHEMICAL
CHEMICAL BEHAVIOR.
BEHAVIOR.
H
1
1H
12C)
Number
Number of
of atoms
atoms in
in 12.000
12.000 gg of
of carbon-12
carbon-12 ((12
6 C)
⇒ 1 mol = 6.022 × 1023 atoms
Avogadro’s number (N)
THE
THE FORMULA
FORMULA WEIGHT
WEIGHT OF
OF AA SUBSTANCE
SUBSTANCE
EXPRESSED
EXPRESSED IN
IN GRAMS
GRAMS
Sum of the atomic masses of all elements in
a compound based on its chemical formula
Page 2-2
A mole of (6.022 × 1023) cherries:
• 1 mol S = 6.022 × 1023 S atoms = 32.06 g S
• 1 mol H2O = 6.022 × 1023 H2O molecules
contains 2 mol H atoms = 12.04 × 1023 H atoms
contains 1 mol O atoms = 6.022 × 1023 O atoms
2 cm
• 1 mol H2O = 18.02 g H2O
contains 2.02 g H and 16.00 g O
1500
km
We’ll
We’ll be
be using
using this
this type
type of
of relationship
relationship
FORMULA
FORMULA WEIGHT
WEIGHT (FW)
(FW)
Sum
Sum of
of the
the atomic
atomic masses
masses of
of all
all elements
elements in
in
aa compound
based
on
chemical
compound based on chemical formula
formula
Formula and Atomic Weights
2H2
+
FORMULA
FORMULA WEIGHT
WEIGHT
Sum
Sum of
of atomic
atomic weights
weights in
in aa formula
formula
H H
Empirical formula: NaCl
22.99 + 35.45 = 58.44
O
H
H
O
H
1 O2
molecule
2 H2O
molecules
WEIGHTS
2 × 2.02 amu
= 4.04 amu
1 ×32.00 amu
= 32.00 amu
2 × 18.02 amu
= 36.04 amu
H H
H
H H
HHHH
H H
H H
H H
Page 2-3
O O
2 H2
molecules
HH HH
Molecular formula: H2O2
2 × 1.01 + 2 × 16.00 = 34.02
→ 2H2O
NUMBERS
H H
MOLECULAR
MOLECULAR WEIGHT
WEIGHT
Formula
Formula weight
weight if
if
chemical
formula
=
molecular
chemical formula = molecular formula
formula
O2
H
H H
H H
H H
O O
O O
O O
O OO O
O O
O
H
H
H
HO
OH H
O
H
O H HOOH HO
H
H
H
O O HH
H
H
HO
O
HH
H
H
NUMBERS
2 mol H2
1 mol O2
2 mol H2O
WEIGHTS
2 × 2.02 g
= 4.04 g
1 ×32.00 g
= 32.00 g
2 × 18.02 g
= 36.04 g