Chapter 4.3
Distinguishing Among
Atoms
Atomic Number
number of protons in the nucleus
is what defines different atoms.
► Atomic Number—the number of protons in
the nucleus of an atom of that element.
► For atoms (which are electrically neutral),
the number of protons is equal to the
number of electrons.
► Different
Mass Number
► The
total number of protons and neutrons
in the nucleus of an atom is called the mass
number.
► The
number of neutrons in an atom can be
calculated if you know the atomic number
and the mass number.
► Number of neutrons = mass number minus
atomic number.
► Shorthand
notation uses:
► Superscript—mass number
► Subscript—atomic number
► Both to the left of the chemical symbol.
► Also may be written as the chemical name
followed by the mass number.
Example:
Gold-197
Isotopes
► Atoms
of an element with different numbers
of neutrons are known as isotopes.
Because they have different number of
neutrons, they also have different mass
numbers.
Atomic Mass
► Using
grams to indicate masses of individual
atoms results in numbers that are
inconveniently small.
► Instead, the carbon-12 isotope is used as a
reference standard.
► An atomic mass unit (amu) is defined as
one twelfth the mass of a carbon-12 atom.
► The
masses of the electrons are
insignificant when calculating atomic
mass.
► The atomic mass of an element is usually
not a whole number.
► Because of an element’s isotopes, the
element’s atomic mass is a weighted
average of the relative abundance of the
isotopes as they occur in nature.
► To
calculate atomic mass, multiply the mass
of each isotope by its natural abundance,
expressed as a decimal, and then add the
products.