HW for Next Class:
 Read pages 28-29
 Worksheet: 5.2 (Structure of Nuclear Atom)
 Quiz: Next time
CHAPTER 4
ATOMIC STRUCTURE
and the PERIODIC TABLE
J. J. Thomson
English
Physicist
1897 – discovered electrons:
negatively charged subatomic
particles
Experiments using flow of
electric current through gases
J. J. Thomson (1856-1940)
CATHODE RAY TUBE
CATHODE RAY TUBE
 Thomson found that cathode rays were
attracted to metal plates with a positive
electrical charge.
 Plates with a negative charge repel the ray.
 Knowing that like charges repel and
opposite charges attract, he determined that
the ray was made of negatively charged
particle – electrons.
Deflection of Cathode Ray
CATHODE RAY
 Thomson showed that the production of the
cathode ray was not dependent on the type
of gas in the tube, or the type of metal used
for the electrodes.
 He conclude that these particles were part of
every atom.
 Determined the mass of the electron as
1/2000 the mass of a Hydrogen atom.
HW for Next Class:
 Chapter 2: Read pgs. 26-28
 Worksheet: 5.1 Atoms
 Quiz: Mon. Tues. Wed. (Nov. 1, 2 or 3)
 Retest – sign up sheet (below 80) –
Thursday October 28 or Friday October 29
ATOMIC STRUCTURE
Democritus – teacher, 4th Century BC
Atomist school of thought
Matter is composed of tiny particles
called atoms
These atoms are invisible,
indestructible fundamental units of
matter
Democritus
John Dalton (1766-1844)
English school teacher
Studied the ratios in which
elements combine in chemical
reactions
formulated Dalton’s Atomic
Theory
John Dalton (1766-1844)
Dalton’s Atomic Theory
All elements are composed of
submicroscopic, indivisible particles
called atoms
2. Atoms of the same element are
identical. The atoms of any one
element are different from those of
any other element.
1.
Dalton’s Atomic Theory
3. Atoms of different elements can
physically
mix together, or can chemically combine
with one another in simple WHOLE
number ratios to form compounds.
4. Chemical reactions occur when atoms are
separated, joined or rearranged. Atoms of
one element are never changed into atoms of
another element as a result of a chemical
reaction.
THE ATOM
Smallest particle of an element
that retains the properties of that
element
How small are they?
– If you placed 100,000,000 Cu atoms
side by side they would form a line
only 1 cm long.
Atomic Theory
Much of Dalton’s theory still
holds today
Revision in Dalton’s theory:
atoms are NOT indivisible
HW
 Read Chapter 2 pages: 29-31
 Worksheet: Atomic number and mass
number
 Test: November 17, 18 and 19 (Wed. Thur.
Friday)
Take away the electrons and
what is left?
Consider these four ideas about
matter and electric charge…
1. Atoms have no net electric
charge.
2. Electric charges are carried
by particles of matter
Electric charges always
exist in whole number
multiples of a single basic
unit.
3.
4.
When a number of negative
particles combine with an equal
number of positively charged
particles, the resulting particle has no
net charge.
Protons
 A particle with one positive charge must be
present to balance each negatively charge
electron.
– Evidence of these particles provided by E.
Goldstein in 1886.
– He observed a ray traveling in the opposite
direction of the cathode ray – canal ray.
– Composed of positive particles – Protons.
Is that all?
In 1932 James Chadwick (English
physicist 1891-1974) confirmed the
existence of another sub-atomic
particle – the neutron.
Neutrons have mass similar to protons,
but no charge
The Atomic Nucleus
How is the atom put together?
– Difficult to study because of small
size.
– Originally it was believed that the
positive and negative charges were
evenly distributed throughout the
atom.
Ernest Rutherford (1871-1937)
 University of Manchester, England
 Tested theory of atomic structure
 Bombarded thin gold foil with a beam of
‘alpha’ particles.
 If the positive charge was evenly spread
out, the beam should have easily passed
through.
Ernest Rutherford (1871-1937)
Rutherford's Experiment
Rutherford and coworkers aimed a
beam of alpha particles at a sheet
of gold foil surrounded by a
florescent screen.
Rutherford's Experiment
Most particles passed through with no deflection, while some
were highly deflected
Rutherford concluded that most particles passed through
because the atom is mostly empty space.
Rutherford’s Experiment
 Based on his results, Rutherford concluded
that all of the positive charge, and most of
the mass of an atom are concentrated in a
small core.
 This region would have a positive charge
capable of causing the deflection observed.
 This region is called the nucleus.