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Take Good Notes! There is a quiz at the
end…and you can use your notes.
What is an atom?
 An
atom is
the smallest
physical
particle of an
element that
still retains the
properties of
that element.
First scientist to
suggest atoms:
Democritus

“Everything that is
must be eternal.”

Democritus
believed that there
were basic things
that always exist, but
can be rearranged;
he called these
“atomos”.

At sea level, one cubic
centimeter of air (size of
a sugar cube, or marble)
will have 45 billion atoms
within it.

◦ 45,000,000,000,000,00
0,000
How many atoms would
it take to fill a universe?

 All
atoms are identical.
 Chemical
reactions occur when
atoms of one element are
changed into atoms of another
element.

Law of Conservation of Mass

Law of Constant Composition

Law of Multiple Proportions
Cr2O3
CrO3
What does an atom look like?
One century after Dalton,
Viennese scientist Ernst Mach
wrote that “atoms cannot be
perceived by the senses… they
are things of thought.”

Many scientists that made
contributions to atomic
structure – J.J. Thomson,
Robert Millikan, Chadwick,
Ernest Rutherford, and Neils
Bohr, Schrodinger.

1.
2.
3.
All elements are composed
of tiny particles called
atoms.
Atoms of the same element
are identical. Atoms of
different elements are
always different.
Atoms of different elements
can chemically combine in
fixed ratios to form
compounds.
4.
Chemical reactions occur when atoms are
joined, separated, or rearranged. Atoms of
one element are NEVER changed into another
during a chemical reaction.

Atoms are solid and indivisible.
Oil Drop
Experiment


1908
Measured the
charge on an
electron.


Electrons
◦ J.J. Thomson in
1897
◦ Particles deflected
in Cathode Ray
tube
◦ Negative one
charge
◦ Very small mass
1/2000 that of
the lightest atom
Thompson passed electric current through
gases in a glass tube. The result was a glowing
beam flowing from the negative side to the
positive side.

Glowing ray (cathode ray) was attracted to a
positive magnet on the side of the tube.


Beam must be negatively charged.
Thompson
hypothesized that all
atoms had such negative
charges within. (plum
pudding model)

Later, scientists proved
that atoms contained
both positive and
negative charges (which
balance each other out).

Born in New
Zealand.
 Terrible at
mathematics.
 Diligent problem
solver.
 First to discover
the power of the
atom, power to
make bombs that
could “make this old
world vanish in
smoke.”

◦ Ernest Rutherford 1911
◦ Gold Foil Experiment using
alpha particles
◦ nucleus
Shot radioactive particles
through a sheet of gold
foil.

Most went straight
through, but others were
knocked off course or
bounced back.

“Like shooting a fifteeninch shell at a sheet of
paper and having it bounce
off.”

Nucleus – compact
center of the atom
where protons and
neutrons are located

Nucleus has very
little of the atom’s
volume, but contains
most of the atom’s
mass.

Chadwick found that the
mass of an atom is
greater than the mass of
protons plus electrons.

Neutrons – portion of an
atom with no charge

Protons, neutrons, and
electrons are called
subatomic particles.


Proton



Mass equal to H
Positive one charge
Found in the nucleus

Neutron

Mass equal to H
 No charge
 Found in the nucleus
Electron
 Mass 1/2000 of H
 Negative one charge
 “orbits” the nucleus

Electrons orbit the nucleus in orbits, like a solar
system.

Planetary Model
How do atoms differ?
Atoms of different
elements have different
structures.

Elements differ
because their atoms
contain different
numbers of protons!

Atomic Number –
number of protons in
the atom

Electrons have negligible
mass compared to protons
and neutrons.

Mass of a neutron is
approximately equal to
mass of a proton.

Mass Number - number
of protons plus number of
neutrons in an atom.

 Beryllium
– 5 neutrons
◦ Mass Number – ?
 Sodium
– 12 neutrons
◦ Mass Number – ?
 Gold
– 118 neutrons
◦ Mass Number = ?
Atoms of the same
element with
different number of
neutrons

Chemically
equivalent

Most elements
occur as a mixture
of isotopes

Mass of a proton is
more than 1800 times
the mass of an electron

Spectrometer used to
find masses of atoms

Masses of subatomic
particles too small to
work with, so a
reference isotope is
used

Mass Spectrometer
 Charged atoms fly towards
a magnet.
 Where they hit the wall
(detection screen)
determines their mass.
 How many hit a certain
spot determines the
abundance (percent) of each
isotope.
magnet
WALL
One amu (atomic
mass unit) is
defined as 1/12
the mass of a
Carbon-12 atom

1 amu is approx.
equal to the mass
of a proton or
neutron

Atomic masses found on
periodic table are averages of all
isotopes of that element

Based on abundance (weighted
mass)

To calculate the atomic
mass of an element, multiply
the mass of each isotope by
its natural abundance
(expressed as a decimal),
and then add the products.



Chlorine is a mixture of Cl-35 and Cl-37.
75.77% Cl-35 (35)
24.23% Cl-37 (37)
0.7577 x 35 amu = 26.50 amu
 0.2423 x 37 amu = 8.957 amu

Added together… 35.45 amu
+
 Carbon-12
 Carbon-13
 Atomic
Mass?
98.89%
1.11%
 Silicon-28
 Silicon-29
 Silicon-30
 Atomic
Mass?
93.37%
4.63%
2.00%
1. What is an atom?
2. What is the symbol for…
a) Calcium?
b) Phosphorus?
c) Potassium?
d) Copper?
e) Silver?
f) Tin?
g) Mercury?
3. What is the atomic number of…
a)
b)
c)
d)
Lithium?
Lead?
Gold?
Neon?
4. Describe Dalton’s atomic
model.
5. Name the four premises of
Dalton’s atomic theory.
6. Explain the difference between atoms
and molecules.
7. Describe Thompson’s experiments
and his atomic model.
8. Describe Rutherford’s experiments
and his atomic model.
9. What is an atomic number?
10. Compare and contrast
protons, neutrons, and
electrons.
11. Chlorine-35?
a) Mass Number?
b)Number of Protons?
c) Number of Electrons?
d)Number of neutrons?
1. Atoms are the tiniest particle that retains an
elements properties.
2. a) Ca
3. a)
3
b) P c) K d) Cu e) Ag f) Sn g)Hg
b) 82 c) 79 d) 10
4. A solid mass, not divisible.
5. Name the four premises of Dalton’s atomic theory.
1. All elements are composed of tiny particles
called atoms.
2. Atoms of the same element are identical. Atoms
of different elements are always different.
3. Atoms of different elements can chemically
combine in fixed ratios to form compounds.
4. Chemical reactions occur when atoms are
joined, separated, or rearranged. Atoms of one
element are NEVER changed into another during a
chemical reaction.
6. Atoms – smallest part of an element that
retains properties.
Molecules - a group of atoms.
7. Thompson’s experiment – cathode ray tube
found electron negative
His atomic model – Plum pudding model
8. Rutherford’s experiment – gold foil experiment
His atomic model – planatary model
9. Atomic number - the number of protons
10. Proton



Mass equal to H
Positive one charge
Found in the
nucleus
Neutron
Mass equal to H
 No charge
 Found in the
nucleus
Electron
 Mass 1/2000 of H
 Negative one
charge
 “orbits” the nucleus

11. Chlorine-35?
a)
b)
c)
d)
Mass # = 35
# of Protons = 17
# of Electrons = 17
# of neutrons = 18