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SECTION 2: MASSES
OF ATOMS
Chapter 17: Properties of Atoms
WARM-UP:
Which
of the following can change
within an element and not change
the identity of that element?
 Proton
 Neutron
 Electron
LEARNING GOALS
Compute
the atomic mass and mass
number of an atom.
Identify
the components of isotopes.
Interpret
the average atomic mass of
an element.
ATOMIC MASS
The
nucleus contains most of the
mass of the atom.
 Protons and neutrons are far more
massive than electrons
 The mass of a proton and a neutron is
almost 1 atomic mass unit (amu)
ATOMIC MASS
Different
elements are distinguished
by the number of protons that their
atoms have
 Every carbon atom has six protons
 Every oxygen atom has eight protons
Atomic
Number: the number of
protons in an atom
ATOMIC MASS
Mass
Number: the sum of the number of
protons and the number of neutrons in a
single atom
 If you know the mass number and the
atomic number, you can calculate the
number of neutrons
 # of neutrons = mass number - atomic
number
ATOMIC MASS
Example:
An atom of potassium (K) has a
mass number of 40. How many neutrons
does it have?
ISOTOPES
Not
all atoms of an element have the
same number of neutrons.
 Isotopes: atoms of the same element
that have different numbers of
neutrons
ISOTOPES
Suppose
you have a
sample of boron (B):
 Naturally occurring
atoms of boron have
mass numbers of 10
or 11. How many
neutrons are in each
isotope of boron?
ISOTOPES
Because
most elements have more
than one isotope, each element has
an average atomic mass.
 Average atomic mass: the weighted
average of mass of the mixture of its
isotopes
 The average atomic mass for each
element is listed in the periodic table!
ISOTOPES
Four
out of every five atoms of boron
are boron-11 and one out of every five
are boron-10.
11
11
11
11
10
4/5(11 amu) + 1/5(10 amu) = 10.8 amu
CHECK-IN:
What
is the mass number of an
isotope of neon (Ne) that has 11
neutrons? What is the average atomic
mass of neon?