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The Structure of the Atom
Or
Modern Atomic Theory
Dalton’s Atomic Theory
revised Democritus’s
ideas based on his
research
matter is made of tiny
particles called atoms
atoms of the same
element are identical
(size, mass, chemical
properties)
English school teacher
(1766-1844)
Dalton’s Atomic Theory
(continued)
atoms of one element differ from atoms of
another element
atoms cannot be created, destroyed or
divided
atoms combine to form compounds
atoms can combine, separate or rearrange
in chemical reactions
Structure of
the Atom
atom – the smallest particle of an
element that can exist that retains
the properties of the element.
The size of an atom
a single copper penny contains:
– 29,000,000,000,000,000,000 atoms
in scientific notation that number is
– 2.9 x1019 atoms of copper
atoms can be seen with a special kind of
microscope
Questions remained
what is the shape of an atom?
are atoms made of smaller particles?
is an atom homogenous or heterogenous?
Subatomic particles and the
atomic nucleus
Static Electricity
have you ever been shocked when you
touched a doorknob
static electricity made scientists in the
1800s look for a relationship between
matter and electric charge
Experiments with Electricity
how would electricity
behave in the
absence of matter (in
a vacuum)
they connected
electrodes to the ends
of vacuum tubes
allowing electricity to
travel through the
tube
Cathode Rays
streams of negatively charged particles
could travel through the tubes
similar cathode rays were found in all
types of matter
JJ Thompson
researched the ratio
of a cathode ray
particle’s mass and
charge
placed electrical and
magnetic fields
around cathode ray
tubes and measured
changes in the
particle’s path
1856-1940
JJ Thompson
discovered the mass of a cathode ray
particle was smaller than the smallest
known atom (hydrogen)
this was the electron
based on the mass, atoms must contain
other particles
This was called the Cathode Ray Tube
Experiment
Lingering Questions
How can atoms be
neutral, if they contain
negative particles?
Thompson proposed
his plum pudding
model
cookie dough
– dough is positively
charged sphere
– chips are negatively
charged particles in it
Ernest Rutherford
studied positively
charged alpha
particles
shot alpha particles at
gold foil (a few atoms
thick) to see if the
path would deflect
An alpha particle is a
helium nucleus
Gold Foil
Experiment
1871-1937
Rutherford’s Results
he thought the alpha particles would
go straight through or deviate a little
some particles bounced almost
straight back
they must have hit something BIG
Rutherford’s Experiments
Showed
an atom is mostly empty space
there is a tiny dense region (nucleus) in
the center of an atom
the nucleus contains most of the atom’s
mass
positive charges are found in the nucleus
(Be sure to write these down. The
results/analysis/conclusions are the most
important parts of an experiment.)
More Rutherford Discoveries
later he concluded the atomic nucleus
contains protons
the charge of a proton is equal (but
opposite) to that of an electron
Nuclear Atomic Model
Atomic Particles
Particle
Charge
Mass (kg)
Location
Electron
-1
9.109 x 10-31
Electron
cloud
Proton
+1
1.673 x 10-27
Nucleus
Neutron
0
1.675 x 10-27
Nucleus
Subatomic Particles
Neutrons
Protons Electrons
Nucleus
Orbitals
Modern View
The atom is mostly
empty space
Two regions
Nucleus- protons and
neutrons
Electron cloudregion where you
might find an electron
How atoms differ
Chapter 4.3
How atoms differ
There are over 110 different
kinds of atoms.
What makes atoms of one
element different than
another?
Henry Moseley
(1887-1915)
discovered atoms of each element have a
unique positive charge
the number of protons determines the
element (e.g. every atom with 6 protons is
carbon)
corresponds to the number of protons
corresponds to the atomic number
Atomic Number
elements are listed in order of increasing
atomic number in periodic table
the number of protons is the atomic
number (e.g. every atom with 6 protons
is carbon)
since an atom has no net charge:
Atomic # = # of protons = # of electrons
Isotopes
atoms of the same element always have
the same number of protons
but they can have different numbers of
neutrons
most elements are a mixture of isotopes
isotopes are atoms of the same element
with a different number of neutrons and a
different mass
How atoms differ
Chapter 4.3B
atom
the smallest piece of an element that has
all the properties of that element
electron
small negatively charged particle found in
all atoms in the electron cloud. It has a
charge of -1
proton
subatomic particle found in an atom’s
nucleus. It has a charge of +1
neutron
subatomic particle found in an atom’s
nucleus it has no charge (it is neutral)
Atomic Particles
Particle
Charge
Mass (g)
Location
Electron
-1
9.11 x 10-28
Electron
cloud
Proton
+1
1.67 x 10-24
Nucleus
Neutron
0
1.67 x 10-24
Nucleus
atomic number
the number of protons in an atom. It
identifies the atom
Remember: APE
Atomic number = # Protons = # Electrons
(since atoms are neutral)
mass number
number written after an elements name,
representing the sum of its protons and
neutrons
isotope
atoms of the same element with the same
# of protons, but different # of neutrons
Mass Number
the mass of an atom is called the mass
number
to calculate the number of neutrons in an
isotope:
mass number - atomic number = number
of neutrons
Isotope Notation
To distinguish between two isotopes
the mass number is given along with
the elements name
Potassium-39
Potassium-40
Potassium-41
Symbols of isotopes
Contain the symbol of the element, the
mass number and the atomic number
Mass
number
Atomic
number
X
Messy Math
the mass of subatomic particles are so
small, they are difficult to work with
scientists adopted an atomic standard
the mass of a carbon-12 atom is said to be
12 atomic mass units (amu)
protons and neutrons have an
approximate mass of 1 amu
Why aren’t atomic masses
whole numbers?
atomic mass reported on the periodic table
are a weighted average of the isotopes of
that element
Chlorine is a mixture of 2 isotopes
75.770% chlorine-35 (atomic mass = 34.969)
24.230% chlorine-37 (atomic mass = 36.766)
(0.75770)(34.969) + (0.24230)(36.966) =
26.496 +8.9569 = 35.4529  35.453 amu
atomic mass unit
1/12th the mass of a carbon-12 atom
atomic mass
weighted average mass of the isotopes of
that element
Measuring Matter
Chapter 11.1
Common Counting Units
eggs
– dozen (12)
paper
– ream (500 sheets)
shoes
– pair (2)
pencils
– gross (144)
Chemist’s Counting Units
need a counting unit for atoms and
molecules
– since they are extremely small
mole = counting unit to measure the
amount of a substance
1 mole = the number of particles in exactly
12 grams of pure carbon-12
Avogadro’s Number
1 mole = 6.0221367 x 1023 particles
rounded to 6.02 x 1023 particles
particles can be
– atoms (elements)
– molecules
– formula units
How much is a mole?
Converting from moles to
particles
use conversion factors
how many roses are in 3.5 dozen?
3.5 dozen  12 roses = 42 roses
dozen
mole conversions
how many particles are in 3.50 moles of
sucrose?
3.50 moles  6.02 x 1023 particles
mole
21.1 x 1023  2.11 x 1024 particles
particle to mole conversions
how many moles are in 4.50 x 1024 atoms
of Zn?
4.50 x 1024 atoms 
1 mole
6.02 x 1023 atoms
7.48 moles Zn
mole
SI base unit to measure the amount of a
substance (element or compound)
molar mass
mass in grams of any pure substance; for
elements, the molar mass is the atomic
mass
Avogadro’s number
the number 6.0221367 x 1023, represents
particles in a mole