The Atom
Atomic Number and Mass
Number
Isotopes
Average Atomic Mass
•
Democritus
•
•
•
named atom
tiny indivisable particles making all matter
Dalton – atomic theory
•
•
•
Elements “pure” – each has atoms w/ dif.
masses
Elements combine in fixed ratios
Thomson
•
•
Atom as
solid object
Discovered the electron
“plum pudding” model of atoms
•
Rutherford
• Said atom is mostly empty space
• Protons – mass of atom – in nucleus
•
Chadwick
Alpha
• neutrons
particles
• Also in nucleus (mass)
went
through
spaces.



 

Basic Structure of Atoms
e--
Proton
Positive Charge
B. Neutron
1 AMU
1 AMU
+
+
1 AMU
1 AMU
1 AMU
1 AMU
e--
No Charge
C. Electron
AMU: atomic mass unit
Negative Charge
Electrons have virtually no mass
Atomic Number
•
•
Is # of protons
# determines the element
• All atoms of an element have same #
protons
•
•
Can’t be changed by ordinary
means (physical or chemical)
Periodic table arranged by atomic #
Mass Number
•
# of protons + neutrons in
atom
Always a whole #
# of neutrons in atoms can
vary within an element
•
•
•
Carbon atoms with 6 protons and 6
neutrons are written as carbon-12 or C-12
•
The -12 is the mass number
Isotopes
“Types” of an element
+
same # of p
varying # of no
varying mass #
Isotope Names and Symbols
Chlorine Isotopes - Names
Chlorine-35
Mass Number is 35
Chlorine-37
Mass Number is 37
Symbols
35
17
Cl
Element symbol
Atomic Number
Mass Number
37
17
Cl
Using Isotope Symbols
35
17
Cl
How many
17 (atomic number)
protons?
How many
electrons?
17 (equal to protons)
How many
neutrons?
18 (35 AMUs, 17 of them
protons, so 35-17 = 18)
37
17
Cl
17 (atomic number)
17 (equal to protons)
20 (37 - 17 = 20)
Mass Number
Mass # is actual # of protons &
neutrons in an isotope:
Mass # =
n0
+
p
e-
+
0
n
Isotope
p+
Oxygen - 18
Arsenic - 75
8 10 8 18
33 42 33 75
Phosphorus - 31
15 16 15
Mass #
31
Atomic Mass
atomic mass units (amu)
average of each element’s isotopes
Some isotopes more common than
others
Relative abundance %
Always has decimal places
Na
22.99
Calculating Atomic Mass
Set up the problem
•
6
X has amu of 6 and relative
abundance of 7.5%
7
• X has amu of 7 and relative
abundance of 92.5%
rel. abd  
rel. abd 

 amu 
   amu 

100  
100 

Calculating Atomic Mass
1. Solve the problem
• Calculate weighted average
7
amu of X
6
amu of X
6  0.075  7  0.925
6
rel abd of X
7
rel abd of X
= 6.93 amu
Try this…
Gallium is a metallic element found
in small lasers used in compact
disc players. In a sample of
gallium, there is 60.2% of gallium69 (68.9 amu) atoms and 39.8% of
gallium-71 (70.9 amu) atoms. What
is the atomic mass of gallium?
Solution
Ga-69
68.9 amu x
60.2
=
41.5 amu for
69Ga
=
28.2 amu for
71Ga
100
Ga-71
70.9 amu x 39.8
100
Atomic mass Ga = 69.7 amu
Atomic Mass of Magnesium
Isotopes
Mass of Isotope
Abundance
24Mg
= 24.0 amu
78.70%
25Mg
= 25.0 amu
10.13%
26Mg
= 26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
Homework Assgt 14
• #66: Chlorine, which has an atomic
mass of 35.453 amu, has two
naturally occurring isotopes, Cl-35 and
Cl-37. Which isotope occurs in
greater abundance? Explain
• #78: Boron-10 and Boron-11 are the
naturally occurring isotopes of
elemental boron. If boron has an
atomic mass of 10.81 amu, which
isotope occurs in greater abundance?
Ions
• Ions are atoms with the same number of
protons, but different numbers of electrons
• Atoms of the same atomic number, but
different charges (since p+ ≠ e-)
– Na+ has one more proton than electron
– Cl- has one less proton than electron