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ATOMIC STRUCTURE
 Democritus
believed that all matter
consisted of small particles that could not
be divided.
 He called these particles atoms from the
Greek word atamos meaning “uncut” or
“indivisible”
 He thought there were different types of
atoms for example atoms in liquids were
round and smooth.
Aristotle did not think
there was a limit to the
number of times matter
could be divided.
 He thought that all
substances were built
from four elements:
Element-matter that is
made of one type of
atom.
Ex. Hydrogen is only
made of hydrogen
atoms

 John
Dalton, English
teacher that studied
the behavior of gases
in the air.
 Based on the way
gases exert pressure,
he correctly
concluded that a gas
consists of individual
particles.
 Dalton
measured the masses of elements
that combine when compounds form.
 He noticed that the ratio of masses is
always the same, they have a fixed
composition.
 He proposed the theory that all matter is
made up of individual particles called
atoms, which cannot be divided.
1.
2.
3.
4.
All elements are composed of atoms.
All atoms of the same element have the
same mass, and atoms of different
elements have different masses.
Compounds contain atoms of more than
one element.
In a particular compound, atoms of
different elements always combine in
the same way.
 Dalton
used solid
spheres to represent
atoms….. Incorrect!
 “All atoms of the same
element have the same
mass” - what about
isotopes?
 Matter is composed of
indivisible particles –
atoms can be divided
(nuclear reaction)
 “different
elements have different
masses” - Correct!
 “elements always combine in the same
way.” - Correct!
 Theory
was widely accepted just revised
once new discoveries were made.
 Objects
such as amber and
glass can attract and repel
other objects…remember
that opposites _________.
 JJ
Thomson used an electric
current to learn more about
atoms.
 He discovered first subatomic
particles – electronsnegatively charged
particles!!!! (add to notes)
He hypothesized that a
stream of charged
particles that
interacted with air in
the tube would cause
air to glow.
 He observed that the
beam was repelled by
the negatively charged
plate- Anode and
attracted by the
positively charged
plate.- cathode

 Thomson
concluded that the particles in
the beam had a - charge because they
were attracted to the + plate.
 This changed the way scientists thought
about atoms (remember they thought an
atom was a solid ball of matter)
 Thomson’s experiments provided the
first evidence that atoms are made of
even smaller particles, he revised
Dalton’s model.
 In
his model, negative charges were
evenly scattered throughout an atom.
 “Plum pudding” model, chocolate chip
ice cream model.
 1. Who said there
• A. Aristotle
• B. Democritus
• C. Dalton
• D. Thomson
 2. Negative
charges evenly scattered
throughout.
•
•
•
•
were only 4 elements?
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
 3. Matter
is made up of atoms which are
solid spheres.
•
•
•
•
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
 4. Termed
small particles atoms from Greek
word atamos.
•
•
•
•
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
 His
student, Ernest
Marsden, set up gold foil
experiment to find out
what happens to particles
as they pass through a
thin sheet of gold.
 Rutherford predicted that
most particles would
travel in a straight path
from their source.
 During
the gold foil
experiment, a beam
was aimed at the
gold, which would
produce a flash of
light when it struck
the screen.
 By observing the
flash, he could figure PROBLEM!!!!!!!!!!!
•More particles were deflected
out the path of the
than expected!
particle.
•Some even bounced straight
back!
 Rutherford
concluded that the positive
charge of an atom-proton is not evenly
spread throughout the atom.
 It is concentrated in a very small, central
area that Rutherford called the nucleus.
 The nucleus is a dense, positively
charged mass located in the center of the
atom.
 According
to Rutherford’s model, all of an
atom’s positive charge is concentrated in
its nucleus.
 The particles that were reflected back
came close to the nucleus and those that
went through passed through space
surrounding nucleus.
 If
an atom had the
same volume as a
football stadium, its
nucleus would have
the volume of a
marble!!!
 Total volume of an
atom is about a
trillion times that of
the nucleus.
(1,000,000,000,000)
QUICK REVIEW – USE CARDS
 1. One
of the first people to state that
matter is made up of atoms was
•
•
•
•
A. Aristotle
B. Democritus
C. Dalton
D. Thomson
 2. Thomson’s
model of an atom is best
described as a(n) ____
a. solar system
b. solid sphere
c. plum pudding
 3. Who
provided the first evidence that atoms
contain subatomic particles?
• A. Aristotle
B. Democritus
• C. Dalton
D. Thomson
4. Who discovered electrons?
• A. Aristotle
B. Democritus
• C. Dalton
D. Thomson
5. Which scientist experimented with gold foil?
• A. Aristotle
B. Democritus
• C. Rutherford
D. Thomson
 6. What
was one incorrect concept in
Dalton’s model?
A. used solid spheres to represent atoms
B. States that all atoms of the same
element have the same mass
C. Matter is composed of indivisible
particles
D. All of the above
Niels
Bohr was a Danish
physicist who expanded
Rutherford’s Atomic Model
Bohr focused his
arrangement around the
positioning of electrons
 In
Bohr’s model, electrons move with
constant speed in fixed orbits around
the nucleus, much like planets orbit a
sun.
 The possible energies that electrons
have in an atom are called energy
levels.
 If an atom gains or loses energy, the
energy of an electron can change.
 Scientists
can measure the energy gained
when electrons absorb energy and move
to a high level
 Example:
• Heat produced by an explosion causes some
electrons to move to higher energy levels. When
the electrons move back to lower energy levels,
they emit some energy in the form of light. (Not
all energy is lost, it has to go “somewhere” and in
this case it is released in the form of light)
 Building
from Bohr’s Model scientists now
know that electrons move in a much more
unpredictable manner than Bohr
proposed.
 An Electron Cloud is a visual model of
the most likely locations for electrons in
an atom.
 Scientists use the electron cloud model to
describe the possible locations of
electrons around the nucleus.
Electro
ns
move so
quickly
they are
a blur
THE STRUCTURE OF AN ATOM
PARTICLE SYMBOL CHARGE RELATIVE LOCATION
MASS
PROTON
p+
+
1
Nucleus
e-
1/1836
n
1
ELECTRON
NEUTRON*
Electron
cloud –
orbiting
nucleus
Nucleus
* James Chadwick concluded that neutrons are neutral because a
charged object did not deflect the paths of the particle.
 Atoms
of an element
always have the same
number of protons.
 Atomic # = number of
protons
 Since electrons =
protons, atomic # also
tells you the # of
electrons
 For example: Sulfur has
an atomic # of 16 so it
has 16 protons and 16
electrons.
 How
many protons
does Carbon have?
6
 How many
electrons does
Carbon have?
6
 Mass
number is the
sum of the protons
and neutrons in the
nucleus.
 To find the # of
neutrons = Mass #
minus # of protons
 For example:
 Sulfur has a mass # of
32.06
 Subtract # of protons
 32.06-16 = 16 neutrons
 How
many neutrons
does Lithium have?
 Mass
# of 7-3 protons
= 4 neutrons
 Every
atom of the same element has the
same number of protons and electrons. For
example all atoms of Carbon have 6
electrons and 6 protons.
 Every atom of an element does not have the
same number of neutrons.
 Isotopes are atoms of the same element that
have different numbers of neutrons and
therefore different mass numbers.
 Isotopes
of an element have the same atomic
number but different mass numbers because
they have different numbers of neutrons.
 For example Carbon 14 is a radioactive
isotope. Usually, carbon has a mass number
of 12, but Carbon 14 has a mass number of
14 due to the 2 extra neutrons.
 6 protons + 8 neutrons = mass number of14
 How many neutrons are in Carbon 13?
 Mass of 13-6 protons = 7 neutrons
ISOTOPE
ATOMIC #
# OF
NEUTRONS
Nitrogen-15
7
8
Iodine-131
53
78