Transcript chem_Atomic Structure

Matter and substance
Breaking up a substance
/ an element
S3 “Atomic Structure”
Defining the Atom
Democritus’s ideas about
atoms.
Defining the Atom

The Greek philosopher Democritus (460
B.C. – 370 B.C.) was among the first to
suggest the existence of atoms
(from the Greek word
“atomos”)

He believed that atoms were indivisible and
indestructible
Defining the Atom
Dalton’s atomic theory.
(Dalton 1766 – 1844)
Dalton’s Atomic Theory
John Dalton
(1766 – 1844)
Sizing up the Atom
Elements are able to be subdivided into
smaller and smaller particles – these are the
atoms, and they still have properties of that
element.
If you could line up 100,000,000 copper atoms in a single file, they would be
approximately 1 cm long.
Structure of the Nuclear Atom
 OBJECTIVES:
Identify
three types of
subatomic particles.
Structure of the Nuclear Atom

One change to Dalton’s atomic theory is that atoms are
divisible into subatomic particles:

E.g. in Nuclear fission

Electrons, protons, and neutrons are examples of
these fundamental particles
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively
charged particle: the electron
Modern Cathode Ray Tubes
Television
Computer Monitor
Cathode ray tubes pass electricity
through a gas that is contained at a
very low pressure.
Cathode ray are negatively
charged – can be deflected by magnets or charged plates
Cathode ray are negatively
charged – can be deflected by magnets or charged plates
Mass of the Electron
Mass of the
electron is
9.11 x 10-28 g
The oil drop apparatus
1916 – Robert Millikan determines the mass
of the electron: 1/1840 the mass of a
hydrogen atom; has one unit of negative
charge
Conclusions from the Study
of the Electron:
a) Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
b) Atoms are neutral, so there must be positive
particles in the atom to balance the negative
charge of the electrons
c) Electrons have so little mass that atoms
must contain other particles that account for
most of the mass
Conclusions from the Study
of the Electron:
 Eugen Goldstein in 1886 observed what
is now called the “proton” - particles
with a positive charge, and a relative
mass of 1 (or 1840 times that of an
electron)
 1932 – James Chadwick confirmed the
existence of the “neutron” – a particle
with no charge, but a mass nearly equal
to a proton
Subatomic Particles
Particle Charge
Electron
(e-)
Proton
(p+)
Neutron
(n)
-1
Mass (g) /
Relative mass
9.11 x 10-28
1/1840
(negligible)
Location
Electron
cloud
1.67 x 10-24
+1
1
Nucleus
1.67 x 10-24
0
1
Nucleus
Proposed models of atomic
structure….????
Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like
plums embedded in a positively charged
“pudding,” thus it was called the “plum pudding”
model.
the “plum pudding” model of
JJ Thomson
Ernest Rutherford’s
Gold Foil Experiment - 1911
Rutherford was a student of Thomson and decided to see
if there was more evidence for the 'plum pudding
/blueberry muffin' theory.
His idea: to bombard the atom with dense alpha particles
alpha particle = He atom with no electrons
- this was like shooting bullets at a blueberry muffin
Ernest Rutherford’s
Gold Foil Experiment - 1911
this was like shooting bullets at a blueberry muffin…. So
what did Rutherford expect?
Surprising result of
Rutherford’s
Gold Foil Experiment - 1911
http://www.youtube.com/watch
?v=Q8RuO2ekNGw
Rutherford’s Findings
Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected

“Like gun shells bouncing off of
tissue paper!”
Rutherford’s Findings and
deductions
Findings
Deductions
Most of the particles
passed right through
an atom was much more than just empty
space and scattered electrons
A few particles were
deflected,
Since alpha particles are relatively heavy,
positively charged particles, the fact that
the occasional particle would be deflected
by either a small or large extent, an atom
must have a positively charged center
that contains most of its mass (which
Rutherford termed as the nucleus).
very few were greatly
deflected
Rutherford’s Findings and
deductions
An illustration that compares the plum
pudding model of an atom to what Rutherford
observed in his experiment.
The top structure shows how the alpha
particles would have passed through
the gold foil atoms if the plum
pudding model was correct in its
assumptions.
The bottom structure shows what
Rutherford and his collogues observed
and is the true depiction of an atom's
structure.
The nucleus as described by Rutherford:
a) The nucleus is small
b) The nucleus is dense
c) The nucleus is positively charged
Modern understanding of an
atom
a) The nucleus is small
b) The nucleus is
dense – contain most
of the mass of an
atom
c) The nucleus is
positively charged
d) The majority of the
volume an atom is
just empty space
Evolution of atomic model……
Changing definition of an atom
Dalton’s Atomic Theory
The atom is the smallest unit of an element that
cannot be divide any more (indivisible).
Changing definition of an atom
Dalton’s Atomic Theory
The atom is the smallest unit of an element
that cannot be divide any more
(indivisible). – proven wrong
Modern definition
The atom is the smallest unit of an element and
that still retains the properties of an element. 