Download 2 C Atomic Number Mass Number Atomic Mass and Isotopes

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.
Atoms have no overall electrical charge
so,
an atom must have as many
electrons
as there are
protons
in its nucleus.
The atomic number of an element also
equals
the number of electrons
in a neutral atom of that element.
Ex: Sodium
 What is the atomic number of
Sodium?
 How many protons does sodium
have?
 How many electrons does sodium
have?
Mass Number (A):
• The sum of the protons and neutrons
in the nucleus.
A=#p + #n
Notation
Other ways to write elements:
Mass Number
Mass Number
36
17Cl
Atomic Number
Cl-36
Nucleons:
protons and neutrons
Isotopes
of an element have
different mass numbers
because
they have
different numbers of neutrons,
but they have
the same atomic number.
Example:
Isotopes of Carbon and Hydrogen
Isotopes of Hydrogen
protium
H
deuterium
H
Isotopes of Carbon
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.
1 u = 1/12 the mass of a Carbon-12
atom.
Atomic Mass
is the weighted average
mass
of all the naturally occurring
isotopes
of that element.