Atomic
Structure
What does the
atom look like???
Chapter 3 Part I
3-1 Early Models of the Atom
• Democritus: c. 400 BC matter is composed of
tiny indivisible particles called atoms.
• An atom is the smallest
part of an element that
retains the chemical
properties of that
element.
Antoine Laurent Lavoisier: A.D. 1780
• Law of Conservation of
Matter states that matter is
neither created nor destroyed.
• Lavoisier is known as the
Father of Chemistry.
• In 1771, at age 28, Lavoisier
married the 13-year-old
Marie-Anne Pierrette Paulze.
• Over time, she proved to be a
scientific colleague to her husband.
• She translated documents and
chemistry books from English.
• She created many sketches and
carved engravings of the laboratory
instruments he used.
• She also edited and published
Lavoisier’s memoirs after his death.
• She hosted parties at which
eminent scientists discussed ideas
and problems related to chemistry.
Engravings of Lavoisier’s Equipment by his wife
Lavoisier was Guillotined May 8, 1794
• An appeal to spare his life so
that he could continue his
experiments was cut short by a
judge saying: "The Republic needs
neither scientists nor chemists; the
course of justice cannot be delayed.”
• One and a half years following
his death, Lavoisier was
exonerated by the French
government.
• When his private belongings
were delivered to his widow, a
brief note was included reading
"To the widow of Lavoisier,
who was falsely convicted."
Joseph Louis Proust: A.D. 1799
•Law of Constant Composition states
that compounds always have the same
elements in the same
proportion by mass.
Ex) the ratio of H:O
in water is always 2:16.
John Dalton A.D.1766-1844
• Some of the original
chemical symbols
from his book:
John Dalton: A.D. 1803
• Four Part Atomic Theory:
1. All elements are made of atoms.
2. All atoms in an element are identical.
3. Atoms are neither created nor destroyed in
chemical reactions.
4. Compounds have definite proportions
of elements.
His model of the atom was like “Billiard Balls”
Benjamin Franklin: 1706-1790
Ben’s lightning
rod in the
Franklin Institute
In 1752 Benjamin Franklin
•
Was the first person to suggest the structure
of the atom was related to electricity.
•
He found that there were two types of charge
and that:
negative and negative: repel
negative and positive: attract
positive and positive: repel
Cathode Rays and Electrons
•
Cathode Ray Tube: An evacuated glass tube
where a beam of electrons flows from the
cathode (negative electrode) to the anode
(positive electrode.)
Effect of a magnet on a cathode ray
Cathode Ray Tube
J.J. Thomson: 1897
• English Physicist who
said a cathode ray is
made of electrons, they
have mass (9.1 x 10 g) and
are negatively charged
particles. Thus he
is credited with
“discovering”
electrons.
-28
Henri Becquerel: 1896
• Becquerel
“discovered”
radioactivity
using
pitchblende, a
uranium ore (a
rock that has a
metal in it.)
A rock, a drawer and a tired chemist
on a Friday afternoon…
Radioactivity
• Radioactivity is the
spontaneous emission of
matter & energy from a
sample. It exposed the film in
Becquerel’s drawer and left
an image.
Marie & Pierre Curie: 1903
•
•
•
•
Shared the Nobel
Prize in Chemistry
with Becquerel.
They discovered 2
new radioactive
elements, Radium and
Polonium.
Marie called Radium
the “Radiant”
element and
Polonium she named
for her native Poland.
How do you think
Marie & Pierre
died???
1859-1906
1867-1934
What happened to The Curies?
• Pierre was run over by a
horse-drawn wagon in Paris
and killed in 1906. Marie was
left alone with two daughters,
aged 2 and 9.
• In 1911 she went on to win a
second Nobel Prize in
chemistry.
• In 1934, Marie died of
leukemia brought on by
radiation exposure one year
before her daughter and sonin-law won the Nobel Prize
for Chemistry.
Ernest Rutherford: 1903
• Rutherford studies under
Thomson.
• He found that there was 3
types of natural radiation
or radioactive decay.
α - Alpha Particles
β - Beta Particles
γ - Gamma Rays
high energy X-rays
Rutherford’s 1st Experiment 1903
What can we tell about α, β, & γ?
Rutherford’s Conclusions:
• α is drawn slightly
toward the negative
plate so it must be
large & positive.
• β is drawn a lot
toward the positive
plate so it must be
small & negative.
• γ is not deflected at
all so it is neutral –
and has no mass.
View of the atoms in the
Gold Foil Experiment
• Rutherford's Gold Foil Experiment
Rutherford’s Gold Foil Experiment 1909
• This experiment showed the atom has a
small, central positive nucleus and that most
of the atom is empty space.
Positive Protons are compacted in
the Nucleus
J.J. Thomson: 1897
• Thought the atom looked like “plum
pudding” with negative charges
distributed throughout:
Rutherford: 1909
• After his Gold Foil Experiment, Rutherford
modifies his model of the atom to contain a
small dense positive nucleus (protons) with
electrons outside.
Neils Bohr: 1913
• Thought the atom was
like the solar system
(planetary model).
• It was eventually shown to
be inaccurate and too
simplistic.
• He wins the Nobel
Prize for this model
in 1922.
• What happened
to his medal?
Wave (electron cloud) Model:
1924 to Present
• Using Quantum Mechanics, the electron can
be found in a probability region.
Evolution of the Atomic Model
Progression of the Atomic Model
James Chadwick: 1932
• Studied under Rutherford.
• He determined that the atom
also contained a neutron which
had approximately the same
mass as a proton, but had a
neutral charge (it was actually
a proton and an electron
together – so its mass was
slightly larger than a proton.)
• Chadwick won the Nobel Prize
for his work in 1935.
• This discovery made it possible
to develop the Nuclear Bomb.
Therefore: 
• There are 3 subatomic particles: protons, neutrons
and electrons. These are measured in “atomic
mass units” (amu) as their mass is so small.
Subatomic
Particle
Mass (amu)
Location
Charge
( p+ )
1 (1.673 x 10
-27
kg)
In the nucleus
+
Neutron ( n0 )
1 (1.675 x 10
-27
kg)
In the nucleus
0
Electron ( e- )
0 (9.1x 10
Outside the nucleus
-
Proton
-31
kg)
John Moseley: 1914
•
•
•
Worked under Rutherford.
He determined that each
atom has a different
number of protons (Atomic
Number). Each atom is
electrically neutral and
therefore has an equal
number of electrons.
Killed by a sniper at
Gallipoli in WWI…what
else might he have done?
Atomic Number and Mass Number
• Atomic Number = the number of protons
In a neutral atom, the number of protons
equal the number of electrons. Ex) carbon = 6
• Mass Number equal to the total number of
protons + neutrons in the nucleus of an atom.
Ex) carbon-12 has 6 protons and 6 neutrons
Ions
• Formed when an atom gains or loses an electron
a. Charge = # of protons - # of electrons
Ex) Mg +2 = lost 2 electrons
# of protons: 12 # of electrons: 10 Charge: +2
Ex) N-3 = gained 3 electrons
# of protons: 7 # of electrons: 10 Charge: -3
Isotopes
Atoms that have the same number of protons
but a different number of neutrons (mass.)
Isotopic Notation
Shorthand way of representing an isotope of an
element.
Ex) 37
17
Cl
top number is the mass number (#p + #n)
bottom number is the atomic number (#p)
May also be written: chlorine-37 or Cl-37
The actual average atomic mass for all chlorine isotopes is 35.45 amu
Isotopes of Hydrogen
a. hydrogen (hydrogen – 1)
b. deuterium (hydrogen – 2)
c. tritium
(hydrogen – 3)
1p+
1p+
1p+
0n0
1n0
2n0
1
1
2
1
3
1
H
H
H
Two Isotopes of Sodium
Isotopes of Carbon
Isotope
Protons
Neutrons
Mass
Number
Electrons
Isotopic
Notation
6
12
6
C
13
6
13
6
C
14
6
Carbon-12
6
6
12
Carbon-13
6
7
Carbon-14
6
8
14
6
C
Atomic Mass of Li = 6.94 amu
• How are the 2 isotopes of
Li the same?
same # of protons
• How are the 2 isotopes of
Li different?
Li-6 has 1 fewer neutron
Atomic Mass:
• The mass of an atom expressed in amu (atomic mass units.)
• One amu is equal to 1/12 the mass of a carbon-12 atom.
Average Atomic Mass:
• The weighted average of all an element’s isotopes.
• This is the number shown in the box on the Periodic Table.
• It is calculated by: (mass1 x %1) + (mass2 x %2) + …
Weighted Average Grade Example:
Straight Class
Weighted Class
Ex) carbon
Ex) hydrogen
93% Tests
90% HW
70% Participation
84.3% Average
x 70% = 65.1
x 20% = 18
x 10% = 7
Weighted
Average: 90.1%
C-12
C-13
C-14
? Straight
Average
13???
Actual Average
Atomic Mass =
12.011 amu
H-1
H-2
H-3
? Straight
Average
2???
Actual Average
Atomic Mass =
1.0079 amu
Isotopes of Hydrogen
The Atom Hydrogen
Proton
Electron
Hydrogen has one proton, one electron and NO neutrons
The Atom Helium
Proton
Electron
Neutron
Helium has two electrons, two protons and two neutrons
The Atom Lithium
Protons
Electrons
Neutrons
Lithium has three electrons, three protons and four neutrons
The Atom Beryllium
Protons
Electrons
Neutrons
Beryllium has four electrons, four protons and five neutrons.
The Atom Boron
Protons
Electrons
Neutrons
Boron has five electrons, five protons and six neutrons.
The Atom Carbon
Protons
Electrons
Neutrons
Carbon has six electrons, six protons and six neutrons.
The Atom Nitrogen
Protons
Electrons
Neutrons
Nitrogen has seven electrons, seven protons and seven neutrons.
The Atom Oxygen
Protons
Electrons
Neutrons
Oxygen has eight electrons, eight protons and eight neutrons.
The Atom Fluorine
Protons
Electrons
Neutrons
Fluorine has nine electrons, nine protons and ten neutrons.
The Atom Neon
Protons
Electrons
Neutrons
Neon has ten electrons, ten protons and ten neutrons.
The Atom Sodium
Protons
Electrons
Neutrons
Sodium has eleven electrons, eleven protons and twelve neutrons.