Subatomic Particles
Atomic Number
Number of protons in a
nucleus of an atom
 Defines the element. Never
changes … if it does, the
identity of the element
changes.
Atomic Number
Mass Number
The
sum of protons and
neutrons in an atom.
The unit of measurement is
atomic mass units or amu’s.
+
o
Mass No. = (# p) + (# n)
**Mass No. is NOT
found on the
periodic table!
NOT
mass number !!!
 How
many protons?
 How many neutrons?
 What is the mass number?
Isotopes
Atoms
of the same element
+
(same # p ), but different
number of no.
Isotope Notation
Z = mass number
A = atomic number
X = element symbol
107
47
Ag
Name:
element - Z
107
47
 Example:
14
C
6 protons
8 neutrons
6 electrons
14 mass number
Name: carbon-14
Ions
Atoms
of the same element
with different number of
electrons
Have either a pos. (lose e )
or neg. charge (gain e )
Example
+2
Ca
# p+ = 20
# e- = 18
-3
P
# p+ = 15
# e- = 18
Finding Particles
Atomic Number = # protons
Electrons:
If atom is neutral (Ca):
#protons = # electrons
If
+2
(Mg ):
atom is charged
Charge= # protons + # electrons
Mass number (amu)=
# protons + # neutrons
Atomic Weight
Usually,
one isotope is more
abundant than others.
The weighted average of
the isotopes of an element.
Example:
Chlorine has 2 isotopes:
75% …. 35Cl
25% …. 37Cl
therefore:
35 amu X 0.75 = 26.25 amu
37 amu X 0.25 = _9.25 amu
35.5 amu