17.2 Masses of Atoms
Atomic Mass
 The nucleus contains most of the
mass of the atom bc P and N are far
more massive than E.
 P & N are about the same size.
Atomic mass
 Every P & N is about 1,836 times
greater than the mass of one E.
 That means that the E’s mass is so
small that it is considered negligible
when finding the mass of an atom.
Atomic Mass
 The unit of measurement used for atomic
particles is the atomic mass unit (amu).
 The mass of a proton or a neutron is
almost equal to 1 amu.
 The atomic mass unit is defined as onetwelfth the mass of a carbon atom
containing six protons and six neutrons.
Protons Identify the Element
 The # of P’s tell you what type of atom you
have.
 Ex: every Carbon atom has 6 protons
 Ex: every atom with 6 protons is a Carbon
atom
The # of P in an atom is equal to a number
called the atomic number.
Atomic Number
 This means that if you are given…
 The name of the element,
 The number of P in the element
 The atomic number of the element
…then, you can determine the other two.
Mass Number
 The mass number of an atom is the sum
of the number of P and the number of N
in the nucleus of an atom.
 If you know the mass number and the
atomic number of an atom, you can
calculate the number of neutrons.
 Number of neutrons = mass # - atomic #
Isotopes
 Not all the atoms of an element have
the same number of Neutrons.
 Atoms of the same element that
have different numbers of neutrons
are called isotopes.
Identifying
Isotopes
 Boron isotopes shown
 BC the # of N is different
in the isotopes, the mass
numbers are different too.
 You use the name of the
Element followed by the
mass number of the
isotope to identify each
isotope
 Boron-10
 Boron-11
Identifying Isotopes
 The average atomic mass of an element is the
weighted-average mass of the mixture of its isotopes.
 EX: 4 out of 5 atoms of boron are boron-11 and one out
of 5 is boron-10.
 4/5 (11 amu) + 1/5 (10 amu) = 10.8 amu
 If the average atomic mass is not a whole number,
that’s an indication that it has isotopes in nature.
Isotope Notation- Add to
Notes
 Isotopes can be notated in two different ways