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Isotopes
Isotopes
• Isotopes are atoms that have the same
number of protons but different numbers of
neutrons
• Most elements in the first two rows of the
periodic table have at least 2 isotopes with
one being more common than the other
• In nature, elements are almost always found
as a mixture of isotopes
Isotopes of Hydrogen
•For example, the most common isotope of hydrogen has
no neutrons at all
•There's also a hydrogen isotope called deuterium,
with one neutron, and another, tritium, with two
neutrons.
Most common isotope
Neutron
Proton
Hydrogen
Deuterium
Tritium
More fun with Isotopes
• To identify an isotope more specifically, chemists add
a number after the elements name.
ex. Carbon-11 Carbon-12 Carbon-14
• This number is called the isotope’s mass number and
is the sum of the isotope’s number of protons and
neutrons.
• For example, an atom with 17 protons and 20
neutrons has a mass number of 37.
Q. What is the mass of the
Nitrogen isotope with 7 protons
and 12 neutrons?
A. Nitrogen-19
Examples
Atom
Protons
Electrons
Neutrons
iodine-128
53
53
75
potassium-40
19
19
21
Isotope
Symbols
Mass
number
141
Atomic
number
Ba
56
2+
Charge
How many protons, neutrons, and
electrons are present?
56
Fe
2+
26
STEP 1: The atomic number in the lower left
corner = 26 protons. The difference between the
mass number and the atomic number is 56 – 26 =
30 neutrons
STEP 2: The charge on the ion, 2+ shows us that
there are 2 more protons than electrons,
indicating that there are 24 electrons.
Isotope Symbols
Mass number
141
Atomic number
Ba
56
2+
Step 1) 141- 56=
85 neutrons
Step 2) 56 electrons -2 = 54 electrons
Charge