Download 1030133Notes 4.3

Section 4.3
How atoms differ
Atomic Number
Represents three things in a
neutral atom:
1. What element it is
2. The number of protons in
each atom
3. The number of electrons in
each atom
Atoms
 Must have neutral charge so
positive charge = negative
charge
 This means:
#
+
p
=#
e
Finding Mass Number
+
0
Mass p = mass n
A proton or neutron
has a mass of 1 amu
Mass e ~ 0
Mass number
 Will be equal to the number of
protons + number of neutrons
 If not given to you, round the
atomic mass # off to a whole #
 Atoms of the same element can
have different mass #s
Neutrons
Since mass number =
+
o
# of p + # of n
o
Then # of n =
+
mass number – # of p
Gold
Atomic
number
79
Element
symbol
Au
196.967
Atomic Mass
How many
+
0
p ? e? n ?
Isotopes
 Atoms of the same element
with a different number of
neutrons (different mass #)
 Still have same # of protons
(that determines the element)
 Same atomic number
Neon (atomic # = 10)
3 isotopes
+
0
Ne-20: 10p & 10n
+
0
Ne-21: 10p & 11n
+
0
Ne-22: 10p & 12n
Isotopes of H
H-1 hydrogen
H-2 deuterium
H-3 tritium
Atomic Mass
Different from mass #
A weighted average of all
isotopes of that element
The mass given on the
periodic table
Atomic Mass
Isotopes occur in different
amounts or % - not in
equal amounts
So atomic mass has to be
a weighted average
Finding a weighted
average atomic mass
(% abundance isotope 1 x mass isotope 1 )
+ (% abundance isotope 2 x mass isotope 2)
+ (% abundance isotope 3 x mass isotope 3)
+ ……..Continue for all the isotopes you have
Finding a weighted
average atomic mass
Element X has 2 isotopes:
X-6 has a mass of 6.015 amu
and is 7.5% abundant
X-7 has a mass of 7.016 amu
and is 92.5% abundant
Find the average atomic
mass for element X
Abundance
Which isotope must
be more abundant
Cu-63 or Cu-65?
How did you decide?
The Standard
C-12 is the standard
that atomic mass is
based on
C = 12 amu (atomic
mass units)