Transcript Chapter 4 The structure of the atom

Chapter 4
The structure of the atom
AL-COS Objectives 1, 2,3,4,7, 10, 15, 20, 21, 22, 27and 28
You’ll learn to…
• Identify the experiments that led
to the development of the nuclear
model of atomic structure
• Describe the structure of the atom
and the subatomic particles that
comprise it
and…
You’ll also learn…
• Explain the relationship between
nuclear stability and radioactivity
• Write equations representing
nuclear decay
• Discuss the atomic models of
Democritus, Thomson, Rutherford,
and Bohr
Vocabulary
*atom
*atomic mass
*atomic mass unit (amu)
*atomic number
*electron
*isotope
*mass number
*neutron
*nucleus
*proton
Early Theories of Matter
 Democritus
( 460-370 BC)
A man ahead of his time…
• First to believe matter was not
infintely divisible
• Believed matter to be made of
small particles he called
“atomos”
• Believed these particles could
not be created or destroyed
Democritus’ Ideas
• Matter is composed of empty
space through which atoms
move
• Atoms are solid, homogeneous,
indestructible and indivisible
• Different kinds of atoms have
different sizes and shapes (give
matter its properties)
• Changes in matter are due to changes
in grouping of atoms
Aristotle
• One of most
influential
Greek
philosophers
Aristotle
• Believed matter was formed of
air, water, earth and fire
• Did not believe in the
“nothingness” of space
• All of his ideas were NOT based
on empirical (experimental)
science – just ideas
Aristotle
Given credit for
setting the
field of science
back nearly
2000 years!
John Dalton
(1766-1844)
• Revived the
ideas of
Democritus
• First atomic
model
• A teacher in
England
Dalton’s atomic theory
• Matter is made of small
particles called atoms
• All atoms of an element are
identical **** (isotopes?)
• Atoms of different elements
are different
As well as…
• Atoms cannot be created or
destroyed or divided into smaller
particles ****(nuclear fission?)
• Different atoms combine in simple
whole number ratios to form
compounds
• In chemical reactions, atoms are
separated, combined or rearranged.
The atom
• Smallest
particle of an
element that
retains the
properties of
that element
Relative size of an atom
How many atoms?
• World population =
6 000 000 000
• Number of atoms in a penny=
29 000 000 000 000 000 000 000
Subatomic particles and the
nuclear atom
• The electron
- Sir William
Crookes noticed
the flash of
light in a tube
Tube filled with a gas,
+ and a – end,
The cathode ray
Scientists were convinced this
cathode ray was really a stream of
charged particles -> electrons!
Further cathode tube
experiments were conducted by
J.J. Thomson (1856-1940)
• Was trying to determine the
charge/mass ratio of these
electrons
• Found the mass of this particles
was less than hydrogen’s
J.J. Thomson
• Identified the
first subatomic
particle – the
electron
• Atoms were
divisible into
smaller
particles
J.J. Thomson’s atomic model
•A spherical
atom composed
of evenly
distributed
positive charges
within which the
negatively
charged
electrons were
also found
Robert Millikan (1868-1953)
• American
physicist
• Determined the
charge of an
electron
• Calculated the
mass of a single
electron (e-)
Sir Ernest Rutherford
•Originally from New
Zealand
•Won a scholarship
competition to study at
Cambridge Univ. in
England
•Won the Nobel prize
in 1908 for discovering
radioactivity
Radioactivity
• 3 types
– Alpha particles
• Actually the nucleus of a helium
atom
– Beta particle
• A high speed electron
– Gamma particle
• A photon of electromagnetic
radiation
Radiation can be shielded by..
Designed an experiment to study
the atom
• Experiment was actually
conducted by his two graduate
students, Geiger (later
developed the Geiger counter)
and Marsden
• Rutherford was their
supervising professor
The gold foil experiment..
Used alpha particles for “bullets”
• Alpha particles
=
Helium nucleus
2 protons + 2
neutrons
What happened in the experiment
Observed that..
• Most of the alpha particles
went straight through
• Some were deflected away from
the foil
• Some hit something solid and
“bounced back”
What was learned?
• Most of an atom is empty space
• The nucleus has a positive charge
(He nucleus was also positive, which
caused the deflection)
• The nucleus, when hit straight on, is
dense and solid – not a plum
pudding!
Other subatomic particles
• Proton
– Discovered by Goldstein using
a cathode ray tube (1920’s)
• Neutron
– Discovered by James
Chadwick in 1932
Atomic number
• Henry Moseley
discovered that
atoms of
different
elements each
have a unique
positive charge
in their nucleus
Henry Moseley (1887-1915)
• Discovered atomic numbers
• Died at a young age in WWI
The atomic number
Mass number=
Protons + neutrons
12
6
Atomic number =
number of protons
in the nucleus
C
Symbol for element
Practice writing atomic notation
23
Na
11
What does this tell us?
Write atomic notation
• For an atom of
Fe
• For an atom of
U
And ……
56
238
Fe
26
U
92
Isotopes
• Same atomic number but a
different atomic mass
• The number of protons remains
the same, but the number of
neutrons vary.
Isotopes of hydrogen
1
2
H
1
Protium
3
H
1
deuterium
H
1
tritium
Calculating Atomic Mass
(Turn to pg. 102)
• 1. mass x %abundance (for each
isotope)
• 2. Add them together to get the
atomic mass
• 3. find that atomic mass on the
periodic table and that is the
element
VIOLA!
Example Problem: pg. 103
• 1.) 6X =(6.015)(0.075)= 0.451 amu
7X =(7.016)(0.925)= 6.490 amu
• 2.) 0.451 + 6.490 = 6.941
amu
• 3.) The element with an atomic
mass of 6.941 amu is Lithium, Li.
•
COOL!!!!!!!!!!!!
Look on pg. 104
• Let’s do # 15 and #16 together.
• Now, you do #17 on your own.
Pg. 104, #15
(10.013)(0.198) = 1.982 amu
(11.009)(0.802) = 8.829 amu
10.811 amu
Pg. 104, #16
Helium-4 is more abundant in
nature because the atomic mass
of naturally occurring helium is
closer to the mass of helium-4
than to the mass of helium –3.
Pg. 104, #17
• (23.985)(0.7899) = 18.946 amu
• (24.986)(0.1000) = 2.499 amu
• (25.982)(0.1101) = 2.861 amu
24.306 amu
The element with that atomic mass is
magnesium,
Mg.
Nuclear reactions
• The nucleus of the atoms
actually change
• Nuclear stability is based on
neutron/proton ratio
• Experience nuclear decay to
become more stable
Alpha decay
Nuclear equation becomes
238
234
U 
92
4
Th
90
+
He
2
Beta decay
Nuclear equation
14
0
N
7
15
e +
-1
O
8
Another example of Beta decay
Gamma radiation
γ
• Electromagentic radiation
• Has no charge
• Often found along with Beta or
alpha radiation
Gamma radiation
Fission of atoms
The end –
for now!