The Structure of the Atom
The Building Block of Matter
Atoms are the smallest particles of an
element that retains the chemical
properties of that element

Parts:


Nucleus – small region in
the center of the atom.
The nucleus contains
protons and neutrons
Electron Cloud – large
region, compared to
nucleus, where electrons
are found.
Size of nucleus vs. electron cloud
If an atom was the size of a sports stadium, the
nucleus would be the size of a marble!
Subatomic Particles - Proton

Is found inside the nucleus

Has a positive (+) charge

Has a relative mass of 1

Determines the atomic number
Subatomic Particles - Neutron

Is found inside the nucleus

Has neutral (0) charge

Has a relative mass of 1

Determines the isotope
Subatomic Particles - Electron

Found outside the nucleus





Bohr model – electrons are in specific energy
levels
Electron cloud model – electrons are in a random
cloud surrounding the nucleus
Has a negative (-) charge
Has a relative mass of 0
Determines the ion
Subatomic Particles Summary
Subatomic Particles:



Protons : + charge, relative mass = 1.007 atomic
mass units (amu); round to 1
Neutrons: = charge, relative mass = 1.009
atomic mass units (amu); round to 1
Electrons: - charge, relative mass = 0.0005
atomic mass units (amu); round to 0 (not
factored in when figuring total mass of an atom)
What is the structure of an atom?
Bohr Model
Schrödinger Model
“Planetary Model”
“Electron Cloud Model”
How are protons, neutrons and electrons
related?

# protons = atomic #

# electrons = # protons in a neutral atom

# protons + # neutrons = atomic mass
Atomic Number


Number of protons in the nucleus (also the
number of electrons)
Typically the number on top of the symbol on
the Periodic Table.
13
Al
26.981
Atomic Number
Atomic Mass


Number of protons + number of neutrons in an
atom.
When solving for number of neutrons: round mass
number properly, subtract atomic number from
mass number.

For Al: atomic mass – atomic number = # of neutrons
27-13=14 neutrons
13
Al
26.981
Atomic Mass
Let’s practice! (Round to whole #s)
Element
Symbol
#P
#E
#N
Atomic #
Li
3
3
4
3
7
Bromine
Br
35
35
45
35
80
Potassium
K
19
19
20
19
39
Sodium
Na
11
11
12
11
23
Ar
18
18
22
18
40
14
14
14
28
Lithium
Argon
Silicon
Si
14
Atomic
Mass
Element Symbols


Hyphen Notation:
Element symbol – Mass # (rounded)
Example: Helium  He-4
Nuclear Symbol:
Superscript Mass Number, Subscript
Atomic Number, Symbol
Example:
4
2
He
Isotopes




Atoms with the same number of protons &
electrons but a different number of neutrons.
They are the same element, but have
different masses.
All isotopes are used to calculate atomic
mass (reason why weight is a decimal).
Most elements consist of a mixture of
isotopes.
Average Atomic Mass



An element can exist in a number of forms, called
isotopes. Isotopes are forms of the same atom that
vary in mass.
For example, there are two different types (isotopes)
of copper atoms. One type of copper atoms (copper63) weighs in at 62.93 amu, the other (copper-65)
has a mass of 64.94 amu.
The lighter isotope is more common with 69.09% of
the naturally occurring copper having a mass of
62.93 amu per atom. The remainder of the atoms,
30.91 %, have a mass of 64.94 amu.
Average Atomic Mass

To find the AVERAGE ATOMIC MASS of an
atom, we take into account all of the isotopes
that exist and the percentage of each
type. The calculation of the average atomic
mass is a WEIGHTED AVERAGE.

Average atomic mass = Σ (mass of isotope ×
relative abundance)
Average Atomic Mass

The bottom line is that to find the average
atomic mass of copper, we insert the
information about copper’s isotopes into the
formula and solve. There are two isotopes, so
we will be adding the contributions of 2
isotopes. (That’s where the Σ sign comes
in.) The relative abundance is simply the
percentage of the isotope, but in decimal
format. 60.90% corresponds to a relative
abundance of 0.6090.
Average Atomic Mass

Let’s look at our copper again: One type of copper atoms weighs in
at 62.93 amu, the other has a mass of 64.94 amu.

The lighter isotope is more common with 69.09% of the naturally
occurring copper having a mass of 62.93 amu per atom. The remainder
of the atoms, 30.91 %, have a mass of 64.94 amu.

Average atomic mass of copper = (62.93 amu
× 0.6909) + (64.94 amu × 0.3091)= 63.55 amu

From the calculation, we know that an AVERAGE atom of copper has a
mass of 63.55 amu. Notice that in this problem, we would predict that
the average is closer to the weight of the lighter isotope. This is
because the lighter form of copper is more abundant.
Let’s Practice
Isotope Name
Silver-107
106.90509
Isotope
Percentage
51.86
Silver-109
108.90470
48.14


Isotope Mass
What is the percentage for Silver-109?
Find the average atomic mass of an atom of Ag.
Average atomic mass = (106.90509 x .5186) +
(108.90470 + .4814)
Average atomic mass = 107.88