Atomic Theory
Mr. Montero
Chemistry
Dr. Michael M. Krop High School
Early Atomic Theory
• Material World is made
up of tiny indivisible
particles.
• A-tomos (Not divisible)
• There was one unique
atom for every substance
• No proof
Democritus (460 – 370 BC)
Early Atomic Theory
Democritus is WRONG!!!
Matter is continuous. The essence
of matter is called hyle. However,
I offer no proof.
Everyone believed me but it turns
out I WAS WRONG.
Aristotle (384 – 322 BC)
The Law of Conservation of Mass
Mass is neither created nor
destroyed during chemical
or physical reactions.
Total mass of reactants
=
Total mass of products
Antoine Lavoisier (1734-1794)
The Law of Definite Proportions
In a pure compound, the
elements combine in
definite proportions to
each other
9 g H2O = 1 g H + 8 g O
18 g H2O = 2 g H + 16 g O
Water is Oxygen and Hydrogen
combined in a 8:1 mass ratio
Joseph Louis Proust (1754 -1826)
Dalton’s Atomic Theory
• Elements are composed of
very small indivisible
particles called Atoms
• Atoms of a given element
are identical to each other.
Atoms of different elements
are different from each
other.
John Dalton (1766-1844)
Dalton’s Atomic Theory
• Compounds are formed when
atoms of more than one element
combine; a given compound
always has the same relative
number and kind of atoms.
• Atoms of an element are not
changed during a chemical
reaction. Reactions involve
rearranging of atoms.
John Dalton (1766-1844)
Rearrangement of Atoms in a
Chemical Reaction
Combustion of methane
STM image of the surface of
gallium arsenide (GaAs).
Gallium atoms (blue spheres), Arsenic atoms (red spheres).
DEFINITION OF THE ATOM
Basic building block of matter.
Smallest particle of an element that
retains the chemical identity of the
element.
Is the atom truly indivisible?
Is the atom indivisible like a solid ball?
NO!!!
Atoms are composed of
electrically charged
particles called
(Subatomic Particles)
Law of Electrostatic attraction
Like charges repel each other.
Unlike charges attract each other.
Cathode Rays
What is traveling from the Cathode (-) to the Anode (+)?
Why is the ray bent by a magnet?
Discovery of the Electron
Cathode Rays are actually
particles (since light does
not bend) with a
“negative” charge.
ELECTRONS!
J.J. Thomson (1856-1940)
Thomson’s Atomic Model
Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it was called
the “plum pudding” model. (Think of a Chocolate Chip Cookie)
Mass of the Electron
• Millikan finds the mass of
the electron in 1909
• The mass of the electron is
9.11 x 10-31 kg
• That is 2000 times lighter
than Hydrogen
Robert Millikan (1868-1953)
Millikan’s Oil Drop Experiment
The Oil Drop Apparatus
Millikan’s Oil Drop Experiment
Conclusions from the study of the
Electron
• Cathode rays have identical properties regardless of
the element used to produce them. All elements
must contain identically charged electrons.
• Atoms are neutral, so there must be positive
particles in the atom to balance the negative charge
of the electrons
• Electrons have so little mass that atoms must
contain other particles that account for most of the
mass
Radioactivity
I discovered that Uranium
gives of Radioactivity. It
spontaneously emits
radiation.
I suggested to Marie and
Pierre Curie to work on this
phenomenon
Henri Becquerel (1852-1908)
Radioactivity
We are the most
famous couple in
Science
We worked on
Radioactivity. An
element was named
after us (Curium Cm)
Pierre Curie (1859-1906)
Marie Curie (1867-1934)
Radioactive Particles α, β, γ
• Alpha particles (α) are the nucleus of Helium (2 protons, 2
neutrons) +2 charge
• Beta particles (β) are high-speed electrons. -1 charge.
• Gamma Rays (γ) are high energy radiation, not particles. Zero
Charge
Rutherford’s Gold Foil Experiment
Rutherford’s Gold Foil Experiment
• Most Particles pass right
through
• Some particles are
slightly deflected
• Very seldom a particle will
bounce right back
• This experiment demolishes J.J.
Thomson’s Plum-Pudding Model
Rutherford’s Findings
“Like Howitzer shells
bouncing off of tissue
paper!”
Ernest Rutherford (1871-1937)
• The nucleus is Dense
• The nucleus is Small
• The nucleus is positively
charged
How dense is the Nucleus?
• A pea with the mass of 250 million tons
• The mass of all cars in the US in an object
that would easily fit in a teaspoon
How small is the nucleus?
• If the diameter of the atom is the size of a
football field how big is the nucleus?
The size of a Cherry!
Other Subatomic Particles
• Protons were discovered by Rutherford in
1919
• Neutrons were discovered by Chadwick in
1932
Subatomic Particles
Particle
Charge
Mass (kg)
Location
10-31
Electron
cloud
Electron
-1
9.109 x
Proton
+1
1.673 x 10-27
Nucleus
Neutron
0
1.675 x 10-27
Nucleus
How big is an atom?
The diameter of a US Penny is 19 mm. The
diameter of a copper atom is 2.6 Å. How many
atoms of copper would fit side by side in a straight
line across the diameter of a penny?
1 Å = 10-10 m
Answer: 7.3 x 107 Cu atoms
Atomic Definitions
Atomic Number (Z): The number of protons in
an atom
Mass Number (A): The number of nucleons
(protons and neutrons)
Atomic Mass: Mass of the atom (expressed in
atomic mass units or “u”)
Atomic Symbols
Isotopes
• Dalton predicted that all atoms of the
same element are identical. This is not
true.
• Isotopes are atoms that have the same
number of protons but different number of
neutrons (different mass number).
Isotopes of Carbon
Symbol
Number of
Protons
Number of
Electrons
Number of
Neutrons
11C
6
6
5
12C
6
6
6
13C
6
6
7
14C
6
6
8
Heavy Ice
D2O is water that
has Deuterium (2H)
instead of Protium (1H )
Deuterium is heavier
than hydrogen since
it contains an extra
neutron.
Atomic Mass Unit (amu)
• We need a more convenient unit to measure the
mass of atoms.
• Atomic mass of Carbon-12 is assigned an exact
value of 12 amu
• 1 amu = 1.66054 x 10-24g
Particle
Proton
Neutron
Electron
Mass (amu)
1.0073
1.0087
5.486 x 10-4
Adding the mass of protons and
neutrons
The mass of one 12C atom is EXACLTY 12
amu.
12C
has 6 protons and 6 neutrons.
6 x (1.0073) + 6 x (1.0087) = 12.096 u
Where is the mistake????
Measuring the mass of isotopes
Mass Spectrometer
Calculating Atomic Mass
• Same atoms have different masses (Isotopes)
• In nature some isotopes are more abundant
than others.
• It is possible to take an average of the atomic
masses of all existing isotopes.
• The average atomic mass is the number
reported in the periodic table
Calculating Atomic Mass
• The atomic mass depends on the relative
abundance of the isotope. Therefore, it is a weighted
average
Finding the atomic mass of Carbon. Naturally occurring
Carbon is Composed of 98.892% 12C and 1.108% 13C.
Their masses are 12 u (exact) and 13.00335 u
respectively. What is the average atomic mass?
(0.98892)(12 u) + (0.01108)(13.00335 u) = 12.011 u