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Atomic Structure and Nuclear Chemistry
How Atoms Differ*
*This is not our modern view of the atom
How Atoms Differ
• Explain the role of atomic number in
determining the identity of an atom.
• Define an isotope and explain why atomic
masses are not whole numbers.
• Calculate the number of electrons,
protons, and neutrons in an atom given its
mass number and atomic number.
Symbols
Periodic Chart Symbols contain
the symbol of the element
element name
the atomic mass
the atomic number
Symbols of Elements
Elements are symbolized by one or two letters.
Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
Atomic Mass
The mass of an atom in atomic mass units (amu)
is the total number of protons and neutrons in
the atom.
A-Z Notation
Contain the symbol of the element, the
mass number and the atomic number
Mass
Number
A
Atomic
Number
Z
X
A-Z Notation
Find the
number of protons
number of neutrons
number of electrons
atomic number
mass number
19
9
F
A-Z Notation
Find the
number of protons
number of neutrons
number of electrons
atomic number
mass number
80
35
Br
A-Z Notation
if an element has an atomic
number of 34 and a mass number
of 78 what is the
number of protons
number of neutrons
number of electrons
complete symbol
A-Z Notation
if an element has 91 protons and
140 neutrons what is the
atomic number
mass number
number of electrons
complete symbol
A-Z Notation
if an element has 78 electrons and
117 neutrons what is the
atomic number
mass number
number of protons
complete symbol
Isotopes
Dalton was wrong.
Atoms of the same element can have different
numbers of neutrons
different mass numbers
called isotopes
Isotopes:
• Atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
Naming Isotopes
Put the mass number after the name of
the element
carbon - 12
carbon - 14
uranium - 235
Atomic Mass
Atomic and
molecular masses
can be measured
with great accuracy
with a mass
spectrometer.
Atomic Mass
How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
Isotopes of oxygen: oxygen-16, oxygen-17, oxygen18
More concerned with average atomic mass.
Based on abundance or percentage of each
element’s isotopes in nature.
Don’t use grams to define average atomic
mass, because the mass is too small.
Measuring Atomic Mass
Unit is the Atomic Mass Unit (amu)
One twelfth the mass of a carbon-12 atom.
Equal to roughly the mass of one proton.
Hydrogen is 1.00794 amu or 1/12 the mass of a
carbon atom.
Each isotope has its own atomic mass; we
can calculate the average atomic mass of an
element from the percent abundance of each
of the element’s isotopes.
What does that mean?
Calculating Averages
If you have five rocks, four with a mass of 50 g,
and one with a mass of 60 g. What is the average
mass of the rocks or the average mass of one
rock?
You would find the total mass and divide by the total
number of rocks.
Total mass = (4 x 50g) + (1 x 60g) = 260 g
Average mass = 260 g/5 or 52g per rock
Calculating Averages
• You can also think about averages in terms of
percentages or abundance.
• Since you have 5 rocks, then 1 rock is 20% of the total.
• Therefore the average mass of one rock can be
calculated by summing the percentages of each set of
rocks multiplied by their respected masses = ∑(%mass)
• 80% of the rocks weighed 50g and 20% weighed 60g
• Average mass = (0.8 x 50g) + (0.2 x 60g) = 52g
• Average = (% abundance as decimal x mass) + (%
abundance as decimal x mass) + (% abundance as
decimal x mass) + …..
Calculating the Atomic Weight of an
Element from Isotopic Abundances
Naturally occurring chlorine is 75.78% 35Cl, which has an atomic mass
of 34.969 amu, and 24.22% 37Cl, which has an atomic mass of 36.966
amu. Calculate the average atomic mass (that is, the atomic weight) of
chlorine.
Solution The average atomic mass is found by multiplying the
abundance of each isotope by its atomic mass and summing these
products. Because 75.78% = 0.7578 and 24.22% = 0.2422, we have
This answer makes sense: The average atomic mass of Cl is between
the masses of the two isotopes and is closer to the value of 35Cl, which
is the more abundant isotope.
Practice Exercise
Three isotopes of silicon occur in nature:
28Si (92.23%), which has an atomic mass of
27.97693 amu; 29Si (4.68%), which has an
atomic mass of 28.97649 amu; and 30Si
(3.09%), which has an atomic mass of
29.97377 amu. Calculate the atomic weight
of silicon.
Answer: 28.09 amu
Practice Exercise
Boron has two naturally occurring
isotopes: boron-10 (abundance = 19.8%,
mass = 10.013 amu), boron-11
(abundance = 80.2%, mass = 11.009
amu). Calculate the atomic mass of
boron.
Answer: 10.8 amu
Atomic Mass
Is not a whole number, because it is an
average.
It is the decimal number under the
element symbol on the periodic table.
Review Key Ideas
• Explain the role of atomic number in
determining the identity of an atom.
• Define an isotope and explain why atomic
masses are not whole numbers.
• Calculate the number of electrons,
protons, and neutrons in an atom given its
mass number and atomic number.