Atomic Theory
Bellringer

Suppose someone said they had a present for
you. When they gave it to you it was in a bag
that you couldn’t see through. What would you
do to figure out what was inside the bag without
opening it?
What is an atom?
 An
atom is the smallest particle into
which something can be divided.
 Building blocks of matter
 Over time, the theory of what an atom
looks like has kept changing as
scientists learn more.
Dalton Theory – late 1700’s

Dalton proposed the theory that:
All substances are made of atoms which cannot be
created, destroyed or divided
 Atoms of the same element are exactly alike
 Atoms of different elements are different
 Atoms join with other atoms to make new
substances

Thomson Theory – late 1800’s
Electrons Discovered!!


Based on his observations of electron beam
behavior, Thomson proposed that the particles
making up an electron beam have a negative
charge.
His model of the atom was that is was a solid
structure of positive charge with enough
electrons mixed in to make it neutral (no
charge).
Thomson’s Model

A solid mass with the electrons distributed
throughout it. Often called the “plum pudding”
model.
A more modern comparison
would be the “chocolate chip
ice cream” model.
Rutherford Model – Early 1900’s
Nucleus Discovered!


Based on the way that positive particles were
deflected in his gold foil experiments,
Rutherford concluded that there was something
extremely dense and positively charged in the
center of a particle.
His model of the atom was that of a positively
charged nucleus at the middle surrounded by
tiny electrons that constantly move.
Rutherford Model
Dense nucleus in center surrounded by
electrons that move constantly. Equal
number of protons and electrons to keep it
neutral.
Bohr’s Model



Bohr proposed that while electrons are traveling
around the nucleus, they do so on defined paths
called levels.
Each level can only hold a certain number of
electrons
Electrons can jump from one level to the next,
but cannot be in between
Bohr’s Model
Neutrons and
protons located
in the nucleus.
Electrons were
in specific orbits
around the
nucleus…..not
moving
randomly.
Electron Cloud Model

Current theory is that the electrons are located
in an “electron cloud” surrounding the nucleus.
While they still exist in levels, it is thought that
the path they travel is not quite as defined as
Bohr suggested.
Electron Cloud Model
(current theory for what an atom
looks like)
Evolution of Atomic Theory
Summary Questions
1.
2.
3.
In what part of an atom is most of the mass
located?
What are 2 differences between the atomic
theories as described by Thomson and
Rutherford?
What discovery demonstrated that atoms are
not the smallest particles?
Bellringer

Rewrite the following numbers in standard
form:
3x
-8
10
2x
22
10
How Small is an Atom

An atom is so small that their diameter is
three hundred-millionths of a centimeter!
0.00000003 cm

This means that a single penny contains twenty
thousand billion billion atoms
20,000,000,000,000,000,000,000
So What is Inside an Atom?
 Nucleus
made of protons and
neutrons
 Electrons
 Lots of empty space
Protons



Charge: Positive (+)
Mass: 1.7 x 10-24 grams
0.0000000000000000000000017
1 atomic mass unit (amu)
Location: Nucleus
Neutron



Charge: None
Mass: 1.7 x 10-24 grams
0.0000000000000000000000017
1 atomic mass unit (amu)
Location: Nucleus
Electron



Charge: Negative (-)
Mass: almost zero
Location: Electron clouds around nucleus
How Do Atoms of Different
Elements Differ

Each element has atoms that have a unique
combination of protons, neutrons and electrons.
Law of Electric Charge
 Opposite
charges are attracted to
each other while like (same)
charges repel each other (push
each other away)
Rules for Building an Atom



1. The atomic number of an atom is equal to the
number of protons in the atom.
2. All stable atoms have the same number of
protons and electrons…so atomic number
ALSO equals the number of electrons.
3. The atomic mass of an atom is equal to the
number of protons PLUS number of neutrons
in the nucleus of an atom.
How to Read Element Squares
Bellringer
 What
is the difference between
atomic number and atomic mass?
Answer

Atomic number is the number of protons in the
nucleus of an atom.
(Atomic # = protons)

Atomic mass is the sum of the number of
protons AND neutrons in the nucleus.
(Atomic mass = protons + neutrons)
Atomic mass is always the largest
number