Atomic Number, Mass Number,
Atomic Mass and Isotopes
Atomic Number (Z):
• is the number of protons in
the nucleus of the atom.
Z=#p+
• The number of protons
(atomic number) determine
the identity of an element.
What about the electrons? How many?
•
Atoms have no overall electrical charge
(neutral) so, an atom must have as many
electrons as there are protons in its nucleus.
• # p+ =# e• The atomic number of an element also
equals the number of electrons in a neutral
atom of that element
• Z =#e-
Ex: Sodium
 What is the atomic number of Sodium?
11
 How many protons does sodium have?
11
 How many electrons does sodium have?
11
Mass Number (A):
• The sum of the protons and neutrons in the
nucleus.
A=#p+ + #n0
Standard atomic Notation
Other ways to write elements:
Mass Number
Mass Number
36
17Cl
Atomic Number
Cl-36
Nucleons:
protons and neutrons
Isotopes
Atoms of the same element with
• Same number of protons
• Same number of electrons
• Different number of neutrons
They have different mass numbers
because they have
different numbers of neutrons,
but they have
the same atomic number because they have the same
number of protons.
Isotopes
 Do isotopes have similar chemical properties?
Why?
 Yes – electrons are the same
 Do they have similar physical properties? Why?
 NO – mass is different.
Remember:
Isotopes have:
same # of protons
different # of
neutrons
Example:
Isotopes of Carbon and Hydrogen
Isotopes of Hydrogen
protium
H
Isotopes of Carbon
deuterium
H
tritium
H
Atomic Mass Unit
is a unit used to compare
the masses of atoms
and has the symbol
u
or
amu.
1 amu or u
is approximately equal to
the mass of
a single
proton or neutron.
Carbon-12
Chemists have defined
the carbon-12 atom
as having a mass of
12 atomic mass units.
For carbon: Mass = 12.01
99% 12C AND 1% 13C found in nature
12.01 = average mass based on abundance NOT just
12
Atomic Mass
is the weighted average
mass
of all the naturally occurring
isotopes
of that element.
How do we deal with the fact that
some atoms have several isotopes?
 The solution is to measure the mass of
each individual isotope in the element
and then calculate the weighted
average.
 The weighted average is known as the
atomic mass of the element
MARBLE
ACTIVITY
Types of calculations
1.
Finding atomic mass, given mass and abundance of
each
2.
Finding the mass of one of the isotope given atomic
mass and abundance.
3.
Finding abundance given atomic mass and mass of
each
 NOTE – if atomic mass is not given for 2 and 3
above use the mass in the periodic table.
Problem 1- finding atomic mass
An element consist of two isotopes. Isotope A
has an abundance of 75.00 percent, and its
mass is 14.000 atomic mass units. Isotope B
has an abundance of 25.00 percent, and its
mass is 15.000 atomic mass units. What is the
atomic mass of the element?
Atomic mass =
(14.00u x 0.7500) +
= 14.25
(15.000 u x 0.2500)
Problem 2 – finding mass of one
isotope
 Bromine has two naturally occuring isotopes. Bromine-79 has a
mass of 78.918amu and is 50.69% abundant. Using the atomic
mass reported in the periodic table, determine the mass of
bromine-81, the other isotope of bromine.
 Atomic mass = (MBr79 x Ra) + (MBr81x Ra)
 Ra for Br81 = 100 – 50.69 = 49.31
 79.90(mass from periodic table) = (78.917 x 0.5069) + (MBr81 x 0.4931)
 79.90 = 40.003 + 0.4931Br
 79.90 – 40.003 = 0.4931Br
 39.897 = 0.4931 Br
 80.91 amu = Br
Problem –
3. finding relative abundance
 Gallium consists of two naturally occuring isotopes
with masses of 68.926 and 70.925 amu. The average
atomic mass of Ga is 69.72 amu. Calculate the
abundance of each isotope.
 Note – if atomic mass is not given use the mass in
the periodic table.
Answer should be:69Ga: 60.3%,
Average atomic mass = (mass
69Ga)(x)
+ (mass
71Ga:
39.7%
71Ga)(y)
Where x and y are the relative abundances of 69Ga and
respectively. So, 69.72 = 68.926(x) + 70.925(y)
Cant have 2 unknowns?????
71Ga
Let the abundance of 1 isotope = x, then the abundance of the other
will be 1-x . In this way the problem will have 1 unknown.
(Substitute y for 1-x)
So
69.72 = (68.926) (x) + (70.925)(1-x)
69.72 = 68.926x + 70.925 - 70.925x
- 1.205 = -1.999x
x = 0.60280
= 0.603 x 100 = 60.3%