Chapter 3.2 Atomic Structure Preview
Objectives
The Structure of the Atom
Properties of Subatomic Particles
Discovery of the Electron
Discovery of the Atomic Nucleus
Gold Foil Experiment
Gold Foil Experiment on the Atomic
Level
• Composition of the Atomic Nucleus
• The Sizes of Atoms
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•
Summarize the observed properties of
cathode rays that led to the discovery
of the electron.
•
Summarize the experiment carried out
by Rutherford and his co-workers that
led to the discovery of the nucleus.
•
List the properties of protons, neutrons,
and electrons.
•
Define atom.
• An
atom is the smallest particle of an
element that retains the chemical
properties of that element.
• The
nucleus is a very small region located
at the center of an atom.
• The
nucleus is made up of at least one
positively charged particle called a
proton and usually one or more neutral
particles called neutrons.
•Surrounding
the nucleus is a
region occupied by negatively
charged particles called
electrons.
•Protons,
neutrons, and electrons
are often referred to as
subatomic particles.
Visual Concept
What particle did
Thomson discover?
 J.J. Thomson discovered
that atoms are made of
smaller negativelycharged particles
called electrons.
 Thomson’s discovery
was the result of doing
experiments with
“cathode ray tubes”

Stream of electrons is attracted to positively
charged plate here.
“What are these particles? Are they atoms,
or molecules, or matter in a still finer state of
subdivision?” quote by Thomson
 Attracted
to the positive electrode
 Not visible but could make things
“glow”
 Traveled in a straight line
 Could be bent by electric or magnetic
fields
 A plate in it’s path acquired a negative
charge
 Same regardless of material


Thomson did not know
how the electrons in an
atom were arranged. He
believed they were mixed
throughout an atom.
He proposed that the atom
was a sphere of positively
charged material.
• Spread throughout the atom were the
negatively charged electrons similar to
plums in a pudding or chocolate chips in
ice cream.
Charge
and Mass of the Electron
Joseph John Thomson’s cathode-ray tube
experiments measured the charge-tomass ratio of an electron.
Robert A. Millikan’s oil drop experiment
measured the charge of an electron.
With this information, scientists were able
to determine the mass of an electron.
Visual Concept
 Awarded
the Nobel
Prize in Chemistry for
his discovery of alpha
particles, positively
charged particles
emitted from
radioactive elements
 Was a student of J.J.
Thomson but
disagreed with the
“Plum Pudding
Model”
• Ernest
Rutherford and his associates
Hans Geiger and Ernest Marsden
wanted to investigate the atom more.
• The results of their gold foil
experiment led to the discovery of a
very densely packed bundle of
matter with a positive electric
charge.
• Rutherford called this positive bundle
of matter the nucleus.

Most of the particles traveled straight through the gold foil

What was the surprising behavior of a few of the particles?

A few of the particles were deflected and some even bounced back
• Except
for the nucleus of the simplest type of
hydrogen atom, all atomic nuclei are made of
protons and neutrons.
•A
proton has a positive charge equal in magnitude
to the negative charge of an electron.
• Atoms
are electrically neutral because they
contain equal numbers of protons and electrons.
•A
neutron is electrically neutral.
•The
nuclei of atoms of different
elements differ in their number of
protons and therefore in the amount of
positive charge they possess.
•Thus,
the number of protons determines
that atom’s identity.
Forces
in the Nucleus
When two protons are extremely close to
each other, there is a strong attraction
between them.
A similar attraction exists when neutrons are
very close to each other or when protons
and neutrons are very close together.
The short-range proton-neutron, protonproton, and neutron-neutron forces that
hold the nuclear particles together are
referred to as nuclear forces.
• The
radius of an atom is the distance
from the center of the nucleus to the
outer portion of its electron cloud.
• Because
atomic radii are so small,
they are expressed using a unit that
is more convenient for the sizes of
atoms.
• This
unit is the picometer, pm.
The diameter of a pinhead is 100,000 times smaller
than the diameter of a stadium. Likewise the diameter
of the nucleus of an atom is 100,000 times smaller
than the diameter of an atom
•
Summarize the observed properties of
cathode rays that led to the discovery
of the electron.
•
Summarize the experiment carried out
by Rutherford and his co-workers that
led to the discovery of the nucleus.
•
List the properties of protons, neutrons,
and electrons.
•
Define atom.