Download Atomic Theory

Unit 4: Atomic Theory
Structure of the Atom
(& Radioactivity)
Early Atomic Theories
Models of the Atom
Date
100 BC
scientist
Democritus/Greeks
discovery________________
concept of the atom
1770
Antoine Lavosier
Law of conservation of mass
1800
Joseph Proust
Law of definite proportions
1803
John Dalton
Law of multiple proportions
Atomic Model I
1880
William Crookes
Cathode Rays (electrons)
1885
Goldstein
Canal Rays (protons)
1900
J.J. Thomson
Plum Pudding Model
Electron
Atomic Model II
1909
Ernest Rutherford
nucleus of atom
Atomic Model III
1913
Niels Bohr
Planetary Model
Atomic Model IV
1920Present
Schroedinger/Planck/
Modern or Wave Model
DeBroglie/Einstein/etc. Atomic Model V
Early Atomic Theories

Atomists and Democritus
 Greeks approx. 2,500 years ago
 Matter was made up of atoms
“atomos” or “Indivisible” particles
 Seashell experiment—broken into
smaller & smaller pieces
Early Atomic Theories

John Dalton



1766-1844; returned to theory of atoms
Atoms are like billiard balls (solid spheres)
which can’t be broken down further
4 major postulates
1.
2.
3.
4.
All elements are composed of atoms
Atoms of the same element are identical
Atoms can physically mix or chemically
combine in simple whole number ratios
Reactions occur when atoms separate, join,
or rearrange
Dalton’s Model of the Atom
No subatomic particles!
Early Atomic Theories

William Crookes
 Developed Crookes tube (a.k.a cathode
ray tube) in 1870’s
 First evidence for existence of electrons
because you could “see” electrons flow
and confirm their existence.
 Tube is precursor to today’s TV picture
tubes
Building the Atom – The Electron

J.J. Thomson
 Discovered electron in 1897
 Discovered positively charged particles
surrounded by electrons
 Found the ratio of the charge of an
electron to its mass to be 1/1837
Thomson’s Cathode Ray Tube
Experiment

J.J. Thomson
Video: Cathode Ray Tube Demo
Building the Atom – The Electron

J.J. Thomson


Cathode ray tube experiments – advancement
of Crookes tube
“plum-pudding model”
Thomson’s Model of the Atom
(Plum Pudding Model)
Millikan’s Oil Drop Experiment

Robert Millikan
 Oil drop experiment
 Determined the charge and mass of an
electron
Video: Millikan's Oil Drop Experiment
Building the Atom – The Nucleus

Ernest Rutherford
 Discovered nucleus (dense core of
atom) in 1911
 Famous Gold foil experiment

Quote from E.R.’s Lab Notebook

“It is about as incredible as if you had fired
a 15-inch shell at a piece of tissue paper
and it came back and hit you.” -ER
Rutherford’s Gold Foil Experiment
Video Clip: Rutherford Gold Foil Experiment
Rutherford’s Model of the Atom
Building the Atom – The Neutron

James Chadwick
 Discovered the neutron (no charge, but
same mass as proton)

Neutrons help disperse the strong
repulsion of positive charges
 Nucleus diameter = 10-5 nm
 Atom diameter = 10-1 nm
 If Nucleus = basketball -->
then Atom = 6 miles wide!
Building the Atom

Niels Bohr
 Improved on Rutherford’s work
 “Planetary model”- positive center is
surrounded by electrons in defined
orbits circling the center
Bohr Model of the Atom
(Planetary Model)
Bohr Model of the Atom Vocab.

Energy level – the location where an
electron is found at a set distance from
the nucleus dependent on the amount of
energy it has

Ground state – the typical energy level
where an electron is found; lowest energy

Excited state – an energy level higher
than the ground state for an electron;
temporary condition
Schrödinger Model
(Quantum Mechanical Model)

Quantum Mechanical Model



Erwin Schrödinger; Mathematical model
Electron locations are based on probability
Electrons are not particles, but waves!

Interactive Simulation: try it!
Defined:

Orbital – region where an electron is likely to
be found 90% of the time
Schrödinger Model of the Atom
(Quantum Mechanical Model)
Atomic Theory
 Atom
– the smallest particle of
matter that retains its properties.

Smallest individual unit of an element
 One
atom of hydrogen is different from one
atom of carbon.
 Subatomic
particles – the
component parts of an atom: proton,
neutron, and electron
Atomic Theory

Ion - atom with the same number of
protons but a different number of
electrons i.e. an atom with a charge!

If the atom has a (+) charge it has more
protons than electrons.

If the atom has a (-) charge it has more
electrons than protons.
Subatomic Particles
Subatomic
Particle
Mass and
Abbreviation
Proton
p+
Mass =1 amu
+1
Nucleus
----
Neutron
n
Mass =1 amu
0
Nucleus
Chadwick
in 1932
-1
Electron
cloud
(outside
nucleus)
Thomson
in 1897
Electron
eMass ≈ zero amu
Charge Location Discoverer
Atomic Symbols
Atomic symbol – the letter or
letters that represent an element.
13
Al
26.981
Atomic number
Atom symbol
Atomic mass or weight
Atomic Number
Atomic number =
the number of protons in the nucleus.
(same for every atom of that element)
Atomic number
13
Al
Atom symbol
26.981
Atomic mass or weight
Mass Number
Mass Number = # protons + # neutrons
A
Boron atom can have:
5 p + 5 n = 10 amu
Mass
number
Atomic
number
Named as boron-10
Calculations w/ Subatomic Particles
 Atomic
number = # of protons
 Mass number = # of protons + # of
neutrons
 (For
a neutral atom):
# of protons = # of electrons
 (For
a charged ion):
Charge = #p+ - #e-
Isotope Notation

Isotope (Isotopic Notation)
Mass #
Atomic #
Atomic
Symbol
Example: Uranium-238
Z
A
X
Example

Example

Sample Problem
Write the atomic symbols for the following:

The isotope of carbon with a mass of 13

The nuclear symbol when A = 92 and the
number of neutrons = 146.
31
Isotopes

Isotope – atoms of the same element
with different numbers of neutrons
(different mass numbers)


Example: Carbon-12
Carbon-14
Atomic mass – weighted average of the
masses of all the isotopes of an element
Atomic Mass
The weighted average is the
addition of the contributions from
each isotope.
Isotopic Abundance is the
percent or fraction of each isotope
found in nature.
33
Most Abundant Isotope
Usually can round atomic mass on the
periodic table to nearest whole number
(but not always!!)
13
Al
26.981
Atomic number
Atom symbol
Atomic mass or weight
34
Example: Determine the average atomic
mass of magnesium which has three isotopes
with the following masses: 23.98 amu
(78.6%), 24.98 amu (10.1%), 25.98 amu
(11.3%).
1)
Multiply the mass number of the isotope by the
decimal value of the percent for that isotope.
2)
Add the relative masses of all of the isotopes to
get the atomic mass of the element.
35
Example: Determine the average atomic
mass of magnesium which has three isotopes
with the following masses: 23.98 amu
(78.6%), 24.98 amu (10.1%), 25.98 amu
(11.3%).
36
Now You Try!
If 90% of the beryllium in the world has a mass
number of 9 and only 10% has a mass number
of 10, what is the atomic mass of beryllium?
Radioactivity - Vocabulary

Radioactivity - the spontaneous emission
of radiation from a substance

Radiation - rays and/or particles emitted
from radioactive material

Nuclear reactions - reactions involving
changes in an atom’s nucleus
Radioactivity
 Radioactive

isotopes are unstable
These isotopes decay over time by
emitting particles and are transformed
into other elements
 Particles
emitted:
Alpha (α) particles: helium nuclei
 Beta (β) particles: High speed electrons
 Gamma (γ) rays: high energy light

39
Types of Radiation – α particles

Alpha radiation - stream of high energy
alpha particles

Alpha particles consist of 2 protons and 2
neutrons and are identical to helium-4 nucleus.

Symbol:

Not much penetrating power, travel a few
centimeters, stopped by paper, no health
hazard
4He 2+
2
Types of Radiation – β particles

Beta radiation - High speed electrons


To form beta radiation a neutron splits into
a proton and an electron
The proton stays in nucleus and the
electron propels out at high speed.

Symbol:
0e-1
0β
-1

100 times more penetrating then alpha radiation,
pass through clothing to damage skin
Types of Radiation –  Radiation

Gamma radiation


Similar to X rays
Doesn’t consist of particles (instead, high
energy light)
 Symbol:
0

0

Penetrates deeply into solid material, body
tissue, stopped by Pb or concrete, dangerous
End of Unit 4!