A short lecture on
Isotopes
by Mr. C
101
Isotopes and the Periodic Table
• An element is identified by the
number of its protons
• An atom’s mass is determined by
the number of protons + neutrons
• Therefore, two atoms of the same
element can have different masses
(ISOTOPES)
HOW?
Isotopes:
Once again:
• An element is identified by the number
of its protons
• And atom’s mass is determined by the
number of protons + neutrons
• Therefore, two atoms of the same
element can have different masses
(ISOTOPES) if they have the same
PROTONS but different
number of
numbers of NEUTRONS
Isotopes:
• So: 2
atoms of the same element can
have different masses (ISOTOPES) if
they have the same number of protons
but different numbers of neutrons.
• Example - HYDROGEN
• Hydrogen-1 and Hydrogen-3
p+
p+ n n
• Hydrogen-1 is the most common hydrogen isotope;
Hydrogen-3 (also known as tritium) is part of
“heavy water” waste from nuclear power plants
• Example - CARBON
Carbon-12
Carbon-13
Carbon-14
(6 p+ 6 n)
(6 p+ 7n)
(6 p+ 8n)
p+p+p+p+p+p+ p+p+p+p+p+p+ p+p+p+p+p+p+
nnnnnn
nnnnnnn
nnnnnnnn
C-12 occurs most often in nature,
• C-13 occurs in about 5 % of all carbon,
• C-14 is used in carbon dating.
ISOTOPES – so what?
• Because of the existence of 3
isotopes of hydrogen its atomic
mass is 1.0079 amu and not exactly
1 amu.
• This is why in the periodic table
most elements’ atomic masses are
not whole numbers (they have 2 or
more isotopes).