Atomic Theory
First person to purpose
that matter was not
infinitely divisible
“atomos”
•Rejected
atomic theory
• did not believe in
“nothingness” of space
 Modern
atomic theory –
19th century
 Conservation of mass –
separation, combination,
or rearrangement of atoms
 Was all of his theory
accurate? NO!!
Dalton’s Atomic Theory (1808)
 All matter is composed of extremely small
particles called atoms
 Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass, and other
properties
 Atoms cannot be subdivided, created, or
destroyed
 Atoms of different elements combine in simple
whole-number ratios to form chemical compounds
 In chemical reactions, atoms are combined,
separated, or rearranged
 The
smallest particle of an element that still
retains the properties of the element
 How small is an atom?


World population: 6300000000
Atoms in a penny: 29000000000000000000000
 Scanning
tunneling microscope – allows
individual atoms to be seen

Discovered Cathode
Ray Tube
William Crookes

Used Cathode Ray
tube to discover
Electron
JJ Thomson
Discovery of the Electron
1897: J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged particle.
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
Some Modern
Cathode Ray Tubes
 Cathode
ray deflected in a magnetic field,
indicated charged particles
 Deflected towards positively charged plate,
indicating particles must have negative
charge
 Altering gas, altering material had no effect
on results, so particles must be in all matter
 Called…. ELECTRONS!!!!
 First subatomic particles!
 Meant Dalton was… wrong!!!
1909 – Robert Millikan determines the mass
of the electron.
 Mass of the electron was much smaller than
that of the hydrogen atom, the smallest
known atom
 Meant atoms were divisible into subatomic
particles
 Mass = 9.1 x 10-28 = 1/1840 mass of hydrogen
 Charge = -1

Conclusions from the Study of
the Electron
 Electrons are negative.
 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



Atom breakable!!
Atom has structure
Electrons suspended in a positively charged
electric field



must have positive charge to balance negative charge of
electrons and make the atom neutral
mass of atom due to electrons
atom mostly “empty” space

compared size of electron to size of atom
 Cookie
dough model
Rutherford’s Gold Foil Experiment
 Alpha particles are positively charged
 Particles were fired at a thin sheet of gold
foil
 Particle hits on the detecting screen (film)
are recorded
Rutherford’s Findings
 Most of the particles passed right through
 A few particles were deflected
 GREATLY Deflected particles were repulsed
by positive charge of nucleus
Conclusions: Nuclear Model

The atom contains a tiny dense center
called the nucleus



The nucleus is essentially the entire mass of
the atom
The nucleus is positively charged


the volume is about 1/10 trillionth the volume of
the atom
the amount of positive charge of the nucleus
balances the negative charge of the electrons
The electrons move around in the empty
space of the atom surrounding the nucleus
 Rutherford


Subatomic particle in nucleus
protons :+1 charge (equal, opposite of electrons)
 Chadwick


- Protons
– Neutrons
Subatomic particle in nucleus
Mass nearly equal to a proton, but carries no
electrical charge
Atoms
are
Actually,
Atoms
can
Mr.
be
What!?!
Dalton,
divided
weinto
have
indivisible!
electrons,
proved that
protons,
part
of
and
your
neutrons.
theory
wrong.
 Atom
– electrically neutral particle composed of
protons, neutrons, electrons

Spherical shape
 Atoms

Nucleus – 99.7% of mass



consist of two regions
Very small, dense region in the center.
Contains protons & neutrons.
Electrons Cloud



Mainly empty space surrounding nucleus
Very large compared to the nucleus.
Contains electrons.
 Subatomic

particles
Protons, neutrons, and electrons
Atomic Particles
Particle Charge
Mass (kg)
Electron
-1
9.109 x 10-31
Proton
Neutron
+1
1.673 x 10-27
Location
Electron
cloud
Nucleus
0
1.675 x 10-27
Nucleus
Atomic Number
Atomic number (Z) of an element
is the number of protons in the
nucleus of each atom of that
element. Identifies the atom.
Element
Carbon
Phosphorus
Gold
# of
protons
6
15
Atomic #
(Z)
6
15
79
79
Isotopes
Elements occur in
nature as
mixtures of
isotopes.
Isotopes are atoms
of the same element
that differ in the
number of neutrons
Isotopes…Again
(must be on the test)
Isotopes are atoms of the same element having
different masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
Hydrogen-3
(tritium)
1
1
2
Nucleus
Mass Number
Mass number is the number of
protons and neutrons in the nucleus
of an isotope.
Mass # = p+ + n0
Nuclide
p+
n0
e-
Mass #
Oxygen - 18
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Atomic Masses
Atomic mass is the average of all the
naturally isotopes of that element.
On Periodic Table Carbon = 12.0125 amu
Isotope
Symbol
nucleus
% in nature
Carbon12
12C
98.89%
Carbon13
13C
Carbon14
14C
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
1.11%
<0.01%
 P.
104 # 15-17
He
Mass #
3
Atomic #
2
Atomic
Symbol
How many protons, electrons, and neutrons?
2 protons, 2 electrons, 1 neutron
Mass # - Atomic # = # Neutrons
Uranium-235, Helium-3, or Carbon-14
Name Mass #
of atom
How many proton,
electrons, neutrons?
92 protons, 143
neutrons, 92
electrons
Convert these hyphen notation to nuclear
symbols.
Uranium-235, Helium-3, or Carbon-14
U
235
3
92
2
He
14
6
C