Essential Question: How do
atoms of the same element differ?
What is an Isotope?
• Isotopes are atoms of the same element that
have the same number of protons but a different
number of neutrons.
• All elements consist of naturally occurring
isotopes and artificially produced isotopes
How are isotopes of an element
similar? How are they different?
The isotopes of an element have:
• Identical Chemical Properties (this is because they have
the same numbers of protons and electrons and
subatomic particles are responsible for chemical
• Different Physical Properties (different mass and
different number of neutrons)
How do we represent specific isotopes?
• Nuclear symbols are used to represent specific
• To write a nuclear symbol:
1.) the symbol of the element is written first
2.) the mass number is written as a
superscript to the left of the symbol
3.) the atomic number is written as a
subscript to the left. Study the illustration below
• Given a nuclear symbol, what does the top
left number represent?
– What does the bottom left number represent?
Can we write isotopes in a different way?
• You can also use the mass number and the
name of the element to designate the atom or
– This is called hyphen notation
• For example, two isotopes of carbon are carbon-12 and
– The nuclear symbols for these two isotopes would be:
• What does the number after the hyphen
What do the numbers used when
writing isotopes represent?
• The mass number
• total number of protons and neutrons in a
specific nucleus of an atom.
• The atomic number
• always refers to the total number of
protons in an atom.
How do I figure out the number of
neutrons when writing isotopes?
• To determine the number of neutrons in a
– subtract the atomic number from the mass number
• See the formula below
Mass Number (neutrons + protons)
Atomic Number (number of protons)
(number of neutrons)
• Boron consists of two isotopes, B-10 and B-11
1.) Determine the number of neutrons in each of the
10 – 5 = 5 neutrons
11 – 5 = 6 neutrons
2.) Using the periodic table, write the nuclear symbol
for each isotope given in hyphen notation
What does the atomic weight of an element
• To identify isotopes the mass number is placed after
the element’s name
– Ex: chlorine-35 potassium-37
• The atomic weight of an element depends on the
abundance of its isotopes.
– If you know the mass of the isotopes and the percent (fractional)
abundance of the isotopes, you can calculate the element's
How do I calculate the atomic mass of an element based
on percent natural abundance and isotopic masses?
• Example: Chlorine has 2 naturally occurring isotopes
• To calculate the average atomic weight:
– add the mass of each isotope multiplied by its percent abundance
• This is the solution for chlorine:
(34.968852) * (0.7577) + (36.965303) * (0.2423) = 35.45 amu
Complete based on the
29 protons, mass 65
Barium - 138
17 protons, mass 36
Tin - 120
16 protons, mass 30