Atomic Structure
Who are these men?
In this lesson, we’ll learn
about the men whose
quests for knowledge
about the fundamental
nature of the universe
helped define our views.
Democritus 460 BC
► Greek
Philosopher
► Suggested world was made of
two things – empty space and
“atomos”
 Atomos – Greek word for
uncuttable
►2
Main ideas
 Atoms are the smallest possible
particle of matter
 There are different types of
atoms for each material
2
John Dalton’s Atomic Theory
1804
1. All matter is made of atoms.
2. Atoms of one element are all
the same.
3. Atoms cannot be broken down
into smaller parts
4. Compounds form by
combining atoms
Dalton’s Early Atomic Model
► “Billiard
Ball” model
► he envisioned atoms as solid, hard spheres,
like billiard(pool) balls, so he used
wooden balls to model them
J.J. Thomson
1897
Discovered the electron
He was the first scientist to show
the atom was made of even smaller
things
JJ Thomson
► Used
the Cathode ray tube to discover
electrons
Thomson’s Experiment
Voltage source
-
+
Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
►By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
- Adding an electric field cause the beam to
move toward the positive plate.
 Thomson concluded the beam was made of
negative moving pieces.

Thomson’s “Plum Pudding” Atom Model
Democritus, Dalton, Thomson Video
Eugen Goldstein 1850-1930
Using a cathode ray tube he discovered
canal rays which are beams of positively
charged particles.
He is credited with the discovery of
protons in an atom.
Canal Rays discovery
Millikan’s Oil Drop Experiment
• Determined the charge on an electron
• Used Thomson’s charge to mass ratio to
calculate the mass of an electron
Millikan’s Oil Drop
Experiment
Millikan’s Oil Drop
Experiment – another
video
Ernest Rutherford - 1913
• discovered the
nucleus of a
gold atom with
his “gold foil”
experiment
Using J.J Thomson’s Plum Pudding
atomic model, Rutherford predicted the
alpha particles would pass straight
though the gold foil. That’s not what
happened.
Gold Foil Experiment Results
most alpha particles go straight through
the gold foil
A few alpha particles are sharply deflected
Rutherford’s Conclusion
►The
atom is mostly
empty space.
►There is a small,
dense center with a
positive charge.
►Rutherford
discovered the
nucleus in atoms
Rutherford’s Contribution to the
Atomic Theory
►The atom is
mostly empty
space.
►The nucleus is
a small, dense
core with a
positive charge.
Gold Foil video
Rutherford’s Atomic Model
Bohr Model
In
1913, the
Danish scientist
Niels Bohr proposed
an improvement. In
his model, he
placed each
electron in a
specific energy
level.
SIR JAMES CHADWICK
DISCOVERED
THE
NEUTRON
IN
1935
Bohr Model
According
to Bohr’s
atomic model,
electrons move in
definite orbits
around the
nucleus, much like
planets circle the
sun. These orbits,
or energy levels,
are located at
certain distances
from the nucleus.
Electron Cloud Model
 De
Broglie and,
Schrodinger~1926
 Think
atom is mostly
empty space
http://particleadventure.org/particleadventure/frameless/modern_atom.html
Nucleus in center is
dense, positively charge
Only a probability of
finding electrons in
certain areas around the
nucleus
Timeline on the Brief History of an Atom
Year
Philosopher/
Scientist
Contribution
450 BC
Empedocles
Asserted that all things are composed of four primal
elements: earth, air, fire, and water
400 BC
Democritus
Believed that all matter is made up of small particles called
atoms, which cannot be divided into smaller units
Around
380- 320 BC
Aristotle
He thought that all matter was continuous and can be
further divided into smaller and smaller pieces infinitely
1799
Joseph Proust
Proposed the law of definite proportions
1808
John Dalton
Formulated the atomic theory and proposed the law of
multiple proportions
1869
Dmitri Mendeleev
Arranged the known elements in the Periodic Table based
on their atomic mass
1890s
Antoine Becquerel, Marie
Curie
Observed that radioactive materials cause the atoms to
break down spontaneously
1895
Wilhelm Rontgen
Discovered X-rays
1897
John Joseph Thomson
1904
Discovered electrons
Suggested the plum pudding model- negative electron in a
positive structure
1908-1917
Robert A. Millikan
Found that the charge of an electron is equal to -1.6022 x 1019 C
1910-1911
Ernest Rutherford
Observed that atoms are mostly empty space
1913
Niels Bohr
Came up with a model that shows electrons move in orbits
Henry Moseley
Developed applications of X-ray spectra to study atomic
structure
1919
Ernest Rutherford
Developed protons
1932
James Chadwick
Discovered neutrons
Structure of the Atom
► Atom
– smallest particle of an element that can
exist alone
 Two regions of an atom
►Nucleus
 Center of atom
 Protons and neutrons
►Electron
“cloud”
 Area surrounding nucleus
containing electrons
Structure of the Atom
► Proton
– Positive charge (+), 1 atomic
mass unit (amu); found in the nucleus
 amu -Approximate mass of a proton or a
neutron
► Neutron
– Neutral charge (0), 1 amu;
found in the nucleus
► Electron – Negative charge (-), mass is
VERY small
Counting Atoms
► Atomic
Number
 Number of protons in nucleus
 The number of protons determines identity
of the element!!
► Mass
Number (Atomic Mass)
 Number of protons + neutrons
 Units are g/mol
Counting Atoms
► Isotopes
 Atoms of the same element with varying
number of neutrons
 Different isotopes have different mass numbers
because the number of neutrons is different
Composition of the atom video
Start at 3:25
Counting Atoms
 Nuclear Symbol Notation
Atoms
► Protons
have a positive (+) charge and
electrons have a negative (-) charge
► In
a neutral atom, the number of protons
equals the number of electrons, so the
overall charge is zero (0)
 Example/ Helium, with an atomic number of 2,
has 2 protons and 2 electrons when stable
Ions
► In
a neutral atom
 Atomic number = # of protons = #of electrons
► Sometimes
atoms will gain or lose electrons
and form IONS
► Because an electron has a negative charge:
 When an atom GAINS electrons it becomes
NEGATIVE
 When an atom LOSES electrons it becomes
POSITIVE
Ions
Cation = a positive ion
Anion = a negative ion
Let’s Practice
► Aluminum





(Al) (no periodic table)
Protons = 13
Electrons =
Neutrons = 14
Atomic Number =
Atomic Mass =
Let’s Practice
► Aluminum





(Al) (no periodic table)
Protons = 13
Electrons = 13
Neutrons = 14
Atomic Number = 13
Atomic Mass = 27
Let’s Practice w/ nuclear symbol
notation
► Nuclear
Symbol notation
(no periodic table)





Protons =
Electrons =
Neutrons =
Atomic Number =
Atomic Mass =
Ag
108
47
Let’s Practice w/ nuclear symbol
notation
► Nuclear
Symbol notation
(no periodic table)





Protons = 47
Electrons = 47
Neutrons = 61
Atomic Number =47
Atomic Mass = 108
Ag
108
47
Let’s Practice w/ isotopes
► use





the periodic table
Protons =
Electrons =
Neutrons =
Atomic Number =
Atomic Mass =
Uranium-235
Let’s Practice with Ions
Use the periodic table
 Charge =
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
39
K
1+
1. Which of the following is not a sub-atomic
particle?
A.
B.
C.
D.
Proton
Isotope
Neutron
Electron
2. The element Cobalt has a relative atomic
mass of 59 and an atomic number of 27. Which
of these is a true statement about each neutral
cobalt atom?
A. It contains 59 neutrons.
B. It contains 27 electrons.
C. It contains 32 protons.
D. It contains equal numbers of neutrons and
electrons.
3. The Periodic Table displays iron as
shown below. This indicates that Fe atoms:
56
Fe
26
A. contain 56 neutrons.
B. contain 30 electrons.
C.contain 26 protons.
D.contains more protons than neutrons.