Download PS 2.2

Warm-up
1/24/08
PS-2.2
Illustrate the fact that the atoms of elements
exist as stable or unstable isotopes.
The atomic number of an element is equal
to the number of protons.
The atomic number is always the same
for a given element.
The atomic number of an element can be
found on the periodic table.
Atoms of the same element may have
different numbers of neutrons.
The mass number of a particular atom is
the sum of that atom’s nucleons (protons
and neutrons).
The mass number cannot be found on
the periodic table.
The mass number is not the same as
atomic mass and cannot be found by
rounding off atomic mass.
The Atomic mass of an element as seen is
the weighted average of the masses of the
naturally occurring isotopes of an element.
The atomic mass of an element can be
found on the periodic table.
Since it is an average, it is usually not a
whole number.
Isotopes are defined as two or more atoms
of the same element having the same
number of protons but different numbers of
neutrons (and therefore different masses)
H–1
H–2
H–3
Atomic Symbol
Mass Number
Atomic Number
In order for a nucleus to be stable, there must
be enough neutrons present to block the
repulsive forces among the protons.
An unstable isotope of an element is
radioactive.
Home Work
Define Chapter 17 Vocabulary
• 15 terms on p. 530