Download Chapter 4 Section 4.3

Do Now:
If a student’s grade is weighted per the table
below, what would their grade be?
Tests
Classwork
Homework
Weight
50%
30%
20%
Average
80
95
85
YWBAT
• Explain what makes elements and isotopes
different from each other and how isotopes of
an element differ.
• Calculate the atomic mass of an element.
Distinguishing Among Atoms
• An atom is the smallest particle of an element
that still has the chemical properties of that
element.
• But what makes atoms of one element
different from another?
Atomic Number
• Atomic Number: the number of protons in the
nucleus of an atom of that element.
It’s like an
ID number
for an
element
Atomic Number
• Atoms are electrically neutral so
Protons = Electrons
Mass Number
• Mass Number: total number of protons and
neutrons in an atom
Mass Number = protons + neutrons
Mass Number
• You can also refer to atoms by using the mass
number and the name of the element.
Atoms
If you know the atomic number and mass
number, you can determine the atom’s
composition.
# Protons = Atomic Number
# Electrons = # Protons
# Neutrons = Mass Number – Atomic Number
Practice
How many protons, neutrons, and electrons are
in each of the atoms?
Isotopes
Isotopes are atoms of the same element that
have the same number of protons but different
numbers of neutrons.
Hydrogen has 3 isotopes
The correct way to represent an isotope is to
write the chemical name followed by the mass
number (protons + neutrons)
Hydrogen has 3 isotopes
1
0
1
1
1
1
1
2
1
The correct way to represent an isotope is to
write the chemical name followed by the mass
number (protons + neutrons)
Isotopes
Just because the number of neutrons is
changing does not change which element it is
because the atomic number does not change.
Isotopes
• In nature, most elements occur as a mixture of
two or more isotopes.
• Each isotope of an element has a fixed mass
and a natural percent abundance.
Average Atomic Mass
• The mass that is listed on the periodic table is
an average atomic mass.
• It is a weighted average of the atomic masses
of naturally occurring isotopes.
Calculating Average Atomic Mass
Chlorine-35 has an amu of 34.9689 with an
abundance of 75.771% while Chlorine-37 has an
amu of 36.9659 and an abundance of 24.229%
What is the average atomic mass of chlorine?
Carbon-12 (12.00 amu) makes up 98.93% of all
of the carbon atoms, while carbon-13 (13.003
amu) is about 1.07% abundant. What is the
average atomic mass of carbon?
Calculating Average Atomic Mass
Chlorine-35 has an amu of 34.9689 with an
abundance of 75.771% while Chlorine-37 has an
amu of 36.9659 and an abundance of 24.229%
What is the average atomic mass of chlorine?
35.453
Carbon-12 (12.00 amu) makes up 98.93% of all
of the carbon atoms, while carbon-13 (13.003
amu) is about 1.07% abundant. What is the
average atomic mass of carbon? 12.011
Review
How are the atoms of one element different
from the atoms of another element?
How are isotopes of the same element
different?
Electrons and the Structure of the
Atom
• Atoms of the same element have the same
number of protons, which is equal to an
atom’s atomic number.
• But atoms of the same element can have
different numbers of neutrons.
• Atoms of the same element with different
numbers of neutrons are isotopes.
Atomic Mass Units
• The actual masses of individual atoms are
inconveniently small.
• So instead, we compare the relative masses of
atoms using a reference isotope as a standard.
• The reference isotope chosen is carbon-12.
• The isotope of carbon has been assigned a
mass of exactly 12 atomic mass units.
Atomic Mass Unit (AMU)
• An atomic mass unit is defined as one-twelfth
of the mass of a carbon-12 atom.