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Notes on Isotopes
• Remember Protons are (+) and
Electrons are (-).
• Neutrons were the last subatomic particles to be discovered
because they have no electrical
charge.
• All atoms of an element have the
same number of protons. (Atomic #)
• The number of neutrons can vary
from one atom of an element to
another.
–These variations are called
ISOTOPES.
• Isotopes are atoms of the same
element that have different masses
because they have different numbers
of neutrons.
–Protons and Neutrons have about
the same mass.
–Electrons have so little mass they
don’t affect the atom’s mass.
• Mass number tells us how
many protons and neutrons
are in the nucleus of an atom.
–It is always a whole number.
Isotope Notation
• 2 Forms:
Carbon-12 (12 is the mass number)
• Carbon will always have 6 protons
How many neutrons does Carbon-14 have?
there are 14 total protons and neutrons.
6 of them are protons.
So 14 (total) – 6 (protons) = 8 Neutrons
How many neutrons does Carbon-12 have?
• Atomic Mass (atomic weight) is the
average mass of all of the known stable
isotopes of an element.
– Most have decimals because they are
averages.
– This is the # on the periodic table
• Mass Number is specific to an isotope.
– Not written on periodic table
– Rounding the atomic mass usually gives you
the most common mass number.