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Ch 5: Atomic Structure
warmup
All matter is composed of very small particles called atoms.
In middle school science you learned about the atom.
1. Draw a picture of a typical atom. Identify and label the
location of the nucleus, electron cloud, electrons,
neutrons, protons.
Objective 5.1-5.2
I
can summarize Dalton's
theories.
 I can describe properties of
protons, neutrons and electrons,
who discovered them and where
they are found in an atom.
Section 5.1
What is an atom? Describe it’s size
 Definition: The

atom is the smallest particle of an element
that retains the characteristics of that element.
A typical atom is 0.000000001 meters across -- that's one
billionth of a meter
Video: Just how small is an atom 5:20
What is an atom?
A.
B.
C.
An atom is the smallest particle that can exist
An atom is the smallest particle that still has
characteristics of an element
An atom is a solid sphere, like a marble only really
really small
Section 5.1
Dalton’s 4 Postulates (1808)
1.All
elements are composed of tiny
indivisible particles called atoms
2.Atoms of the same element are identical.
Atoms of different elements are different
from another element.
3.Atoms of different elements can
chemically combine to form compounds
4.Any chemical reaction is simply a
John Dalton 1776-1844
re-arrangement of atoms, but the atoms “Father of Modern Chemistry”
are not changed into different elements.
Section 5.2
Structure of the atom


Electrons
Discovered by
J.J. Thomson in 1897.
Sir Joseph John Thomson (18561940)
Nobel Prize for Physics (1906)
•Discovered the electron
•First to discover evidence for isotopes
of stable elements.
Section 5.2
Structure of the atom


Thomson created and used a Cathode Ray Tube to study the
existence of electrons.
He passed an electric current through a sealed vacuum.
Negatively charged electrons are repelled by the negatively
charge cathode, and are attracted to the positively charged
anode causing a glowing cathode ray to appear
• Thomson - determined
the presence of these
electrons in all gases
and concluded that
electrons were part of
all atoms
Section 5.2
The Atomic Nucleus

In Thomson’s “Plum Pudding Model”: Scientists
assumed the negative electrons were evenly
distributed throughout a positive atom.
Which of Dalton’s four theories did J.J.
Thomson disprove in his famous CRT
experiment?
A.
B.
C.
D.
All elements are composed of tiny indivisible
particles called atoms
Atoms of the same element are identical. Atoms
of different elements are different from another
element.
Atoms of different elements can chemically
combine to form compounds
Any chemical reaction is simply a
re-arrangement of atoms, but the atoms are not
changed into different elements.
Section 5.2
Structure of the atom



Robert Millikan - experimented to
find the relative electrical charge of
an electron to be –1
Millikan discovered electron mass
to be 1/1840 of the mass of a
hydrogen proton
Electrons in an atom determine its
chemical properties
Robert Millikan 1869-1953
Nobel Prize for Physics 1923
first measured the electron
charge and mass
Section 5.2
Structure of the atom





Protons
Since atoms are electrically neutral, there
must be a particle that neutralizes the
negative charge of an electron
Protons were discovered by Eugen
Goldstein in 1886 who observed additional
rays in a cathode ray tube that traveled in
the opposite direction of the cathode ray.
Protons have a electrical charge of +1 and a
mass of 1 amu.
The number of protons in an atom
determines the type of element
Eugen Goldstein
1850-1930
discoverer of the proton
Section 5.2
Structure of the atom
Neutrons
 James Chadwick discovered
the existence of neutrons in 1932
using beryllium, alpha rays and
known atomic masses.
 Neutrons hold the protons together and
thus contribute to the stability of the atomic
nucleus.
 Neutrons have a mass of 1 and
no electric charge.

James Chadwick 1891-1974
Nobel Prize for Physics 1935
It has a very small mass, and a negative
charge. What is it and where is it
located in the atom?
A.
B.
C.
D.
A Neutron is located outside the nucleus
A Proton located in the nucleus
An Electron is located in the nucleus
An Electron is located in the electron cloud outside the
nucleus.
Section 5.2
Structure of the atom

In 1909 Ernest Rutherford disproves the
plum pudding model by bombarding gold
foil with alpha particles(+).
Ernest Rutherford
Nobel Prize for
Chemistry 1908
Section 5.2
Structure of the atom

According to the Thomson’s
popular “Plum Pudding”
atomic theory of the time, the
alpha particles should pass
through the foil with only a
slight deflection
Gold Atom
 Instead, Rutherford observed that almost all
particles went straight thru, but a few of the
particles were deflected at odd angles or
sometimes directly backwards!!
 This result was completely unexpected.
Section 5.2
Rutherford’s planetary model




Rutherford uses this information to
propose a new atomic theory.
The atom is mostly made up of
empty space.
The positive charge of the atom, and
most of the mass, is concentrated in
a small area
This small, dense area is known as
the nucleus, and is surrounded by
orbiting electrons.
Section 5.2
Rutherford’s Experiment
a = Alpha Particle
2 protons + 2 neutrons

http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#

Animations
 Cathode Ray Tube
 Alpha Particle Scattering …Rutherford’s experiment
In his famous gold foil
experiment, Rutherford shot
alpha particles into gold.
What result did he prove?
A.
B.
C.
D.
Atoms have a large diffuse nucleus, like a pudding.
Atoms are made up of mostly empty space
Atoms have a small dense positively charged nucleus
The nucleus is actually much smaller and more dense
than anyone knew
Atom Facts
Electron
Proton
Neutron
Charge
-1
+1
0
Mass
1/1840
1 amu
1 amu
(almost negligible)
Location in Atom
electron cloud
nucleus
nucleus
Discoverer
J.J. Thomson
Goldstein
James
Chadwick
Function
determines
chemical properties
determines an
elements identity
holds nucleus
together
Atom Facts
Electron
Proton
Neutron
Charge
-1
+1
0
Mass
1/1840
1 amu
1 amu
(almost negligible)
Location in Atom
electron cloud
nucleus
nucleus
Discoverer
J.J. Thomson
Goldstein
James
Chadwick
Function
determines
chemical properties
determines an
elements identity
holds nucleus
together