By convention there is color,
By convention sweetness,
By convention bitterness,
But in reality there are atoms and space.
-Democritus (c. 400 BCE)
AT O M
ATOM
NUCLEUS
NUCLEUS
ELECT RONS
ELECTRONS
PROT ONS
PROTONS
NEUT RONS
NEUTRONS
P
O S IT IV E
POSITIVE
N
E U T R AL
NEUTRAL
C
H AR G E
CHARGE
C
H AR G E
CHARGE
Mass Number
N
E G AT IV E CHARGE
C H AR G E
NEGATIVE
Atomic Number
equals the # of...
Scale of
the atom
atomic mass unit (amu) =
the mass of one proton
1.66 
-27
10 g
Sum of the protons and neutrons in the
nucleus of an atom
Always a whole number
# of neutrons = mass # - atomic #
The number of protons within the nucleus
Atoms of the same element with
different numbers of neutrons
Isotope symbol:
Mass #
Atomic#
6
C
“Carbon-12”
How to calculate the average atomic mass of an element:
List all isotopes, mass numbers, and percent relative
abundance of an element
Multiply the mass number of each isotope by its
relative abundance
Add all the products together = atomic mass
An example:
Chlorine-35 (35Cl) has 17 protons and 18
neutrons and has a relative abundance of 75.8%
Chlorine-37 (37Cl) has 17 protons and 20
neutrons and appears 24.2% of the time.
Multiply the mass number of each isotope
by its relative abundance
(Chlorine-35) 35 amu × 0.758 = 26.53 amu
(Chlorine-37) 37 amu × 0.242 = 8.95 amu
Add all the products together = atomic mass
26.53 amu + 8.95 amu = 35.48 amu