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Unit 3 The Atomic Theory
Pre-Assessment –
Draw a colorful model of what you think the
structure of the atom looks like. This is your model
from past science classes and what you know the
structure of the atom to be.
Origin of the term “Atom”
• The word “atom” comes from the Greek
word atomos.
• The word atom means
o
Unable to be cut, something that cannot be
divided further.
Ancient Theories of the Atom
Democritus’ (450 BC)
“all matter that makes
up the world around us
is actually composed of
tiny invisible particles.
(atomos)”
Antoine Lavoisier
• Elements as a simple substance
cannot be broken down. (1777)
o
From this he devised a theory of the
formation of chemical compounds
from elements.
• Law of Conservation of Matter – Matter
is neither created nor destroyed in a
chemical reaction.
Law of Conservation of Matter:
*was beheaded at the
height of the French
Revolution.
18th Century Theories:Proust
• A given compound
always contains the
same elements in the
same proportions by
mass.
• Law of Constant
Composition:
(1799) Joseph Louis
Proust found that a
given compound
always contains the
same elements in the
same proportions by
mass.
Modern Theories of the Atom
• John Dalton (1805) used what was known about
the atom to establish the Atomic Theory of
Matter
Dalton’s Theory 4 postulates
• #1 – Each element is composed of extremely
small particles called atoms.
Dalton’s Theory
• #2 – All atoms of
a given element
are identical , but
they differ from
those of any other
element.
Dalton’s Theory
• #3 – Atoms are
neither created
nor destroyed in
any chemical
reaction.
Dalton’s Theory
• #4 – A given compound always has
the same relative numbers and
kinds of atoms.
Unit 3 The Atom Theory
Wednesday 10/21 Intro the Atom with video and
outlining Chapter 3.
Thursday 10/22 Guiding Readings on JJ Thomson
and Ernest Rutherford for
Performance Task
Friday 10/23
Discussion on JJ Thomson and
Ernest Rutherford
Monday 10/26
Performance Task #1 on Gold Foil
Experiment
Wednesday 10/28 Subatomic Particles
What was the view of the model before the
Cathode Ray Tube Experiment?
JJ Thomson’s Model
The Plum Pudding Model
Diffuse cloud of (+) charge with (-) charges
throughout.
J.J. Thomson (IN 1896)
He discovered the first subatomic particle –
the electron
• http://highered.mcgrawhill.com/sites/0072512644/student_view0/chapte
r2/animations_center.html#
The stream of electrons was attracted to the + pole
of the magnet and repelled by the (-) pole of the
magnet.
Using the charge to mass ratio he was able to
calculate the mass to being 2000x lighter than a
hydrogen atom (smallest atom known).
Summarize JJ’s Work
Because the particles in the cathode ray tube were
much lighter than the smallest known element he
concluded that these are electrons that are (-)
charged.
He knew that positive charges also had to be
present so he came up with the Plum Pudding
Model.
Robert Millikan - 1909
• He measured the charge of an electron.
http://highered.mcgrawhill.com/sites/0072512644/student_view0/chapte
r2/animations_center.html#
New Zealand Scientist
Ernest Rutherford(18711937)
Had performed
experiments to find that
atoms were made of
positively charged
particles moving around
a very tiny positively
charged nucleus.
Rutherford’s Gold Foil
experiment
He did an famous experiment whereby he shot a
stream of alpha particles at a thin piece of gold
(Au) foil expecting most of the particles to be
deflected-instead most of them passed directly
through the foil and onto the photographic film
If JJ Thomson’s
model was correct
Rutherford’s Model
Rutherford experiment
animation
http://www.mhhe.com/physsci/che
mistry/essentialchemistry/flash/rut
her14.swf
Rutherford’s Atomic Structure Model
+ =positively
charged
particles
- = electrons
Comparison of Atomic Models
Dalton’s
Model
Thomson’s Model Rutherford’s Model
• YOU WILL NEED:
–NOTEBOOK
–PERIODIC TABLE
• Protons (positive) are
bigger than electrons. The
part of an atom that gives
an element its identity.
• Neutrons (neutral) are
slightly bigger in mass than
a proton.
• Electrons (negative)
charge, have a mass of
close to zero-negligible
Atomic Mass Units (amu): roughly equal to the mass
of a proton or neutron.
**the mass of an atom is measured in amu’s
1 amu = 1.66x10-24g
Particle
Proton
Location
Inside
nucleus
Neutron Inside
nucleus
Electron Outside
nucleus
Charge(C)
Mass (g)
Mass
(amu)
1.602 x 10-19
1.673x10-24 1.0073
=1
0
1.675x10-24 1.0087
=1
-1.602 x 10-14 9.109x10-28 0.0006
=0
• Every element has a unique atomic number
which is the number of protons in the atom.
• Elements atomic number is just above the
chemical symbol on periodic table.
• Atoms are electrically neutral-means
number of protons always equal number of
electrons.
Electron Number
• An Element’s atomic number also indicates
number of electron in its atoms.
Ions
•Ions are atoms that have
lost or gained one or
more electrons giving
them a positive or
negative charge!
If an ion gains an electron it
has a negative charge.
If an ion loses an electron it has a
positive charge.
For example: Na (Sodium), has
originally 11 electrons but when an
electron is lost it becomes a positive
ion. Na+
A neutral magnesium atom (atomic
number=12) has 12 protons/electrons.
If it loses 2 electrons it becomes an ion
with a charge of 2+.
Number of protons 12
Number of electrons - 10
Charge of Ion 2+
Ion
19. Cu²+
Protons
Electrons
29
27
20. As3-
33
36
Isotopes
• Isotopes are atoms that have the same
number of protons but different numbers of
neutrons
• Most elements in the first two rows of the
periodic table have at least 2 isotopes with
one being more common than the other
• In nature, elements are almost always
found as a mixture of isotopes
Differences between
isotopes?
• Isotopes react in the same way as others of
the same element
Atomic mass: weighted average of the
masses of the existing isotopes of an
element.
Atomic Number/
number of protons
Atomic mass
• What was Dalton’s 2nd Postulate?
• Was it correct?
Hmmm
Isotopes of Hydrogen
• The most common isotope of hydrogen has no
neutrons at all
• There's also a hydrogen isotope called deuterium,
with one neutron, and another, tritium, with two
neutrons.
• To identify an isotope more specifically, chemists
add a number after the elements name.
ex. Carbon-11 Carbon-12 Carbon-14
• This number is called the isotope’s mass number
and is the sum of the isotope’s number of protons
and neutrons.
• What is the atomic mass on your Periodic Table?
• Which of the 3 isotopes of Carbon is the most
abundant?
Complete Chemical Symbols
Mass number
141 2+
Ba
56
Charge
Atomic number
Number of protons = Atomic #
Number of neutrons = Mass # - Atomic #
Charge = Atomic # – Number of electrons
Complete the Table
Chemical
Symbol
No. of
Protons
No. of
Electrons
No. of
Neutrons
46
45
50
14
29
Atom or
Ion?
90 2+
38Sr
Atom
WARM UP
• What is the atomic mass unit of a proton?
Neutron? Electron?
• What is the atomic mass of an element represent?
Complete the Table
Chemical
Symbol
No. of
Protons
No. of
Electrons
No. of
Neutrons
46
45
50
14
29
Atom or
Ion?
90 2+
38Sr
Atom