Transcript Thomson (the electron)

Atoms: The Building Blocks
of Matter
Chapter 3
Objectives
– Recognize discoveries from Dalton (atomic
theory), Thomson (the electron),
Rutherford (the nucleus), and Bohr
(planetary model of atom) and understand
how these discoveries lead to the modern
theory.
– Describe Rutherford’s “gold foil”
experiment that led to the discovery of the
nuclear atom. Identify the major
components (protons, neutrons, and
electrons) of the nuclear atom and explain
how they interact.
Objectives
 Interpret and apply the laws of conservation of
mass, constant composition (definite
proportions), and multiple proportions.
 Describe how changes in the nucleus of an
atom during a nuclear reaction result in
emission of radiation.
History of the Atom
Not the history of the atom itself, but
the history of the idea of the atom.
Atom Definition
Atom
 Smallest particle of an
element that retains
the chemical identity of
that element
Democritus
• Greek Philosopher
• ~450 BC
• Thought atoms were tiny, individual,
indivisible atoms
• Used logic to formulate ideas
• One of the first to develop idea of atoms
http://images.search.yahoo.c
om/search/images/
Contributing Principles to Idea of
Atom
http://images.search.yahoo.com/search/images/
 Law of Definite
Composition
 A given compound
always contains the
same elements in the
same proportion by
mass
 Joseph Louis Proust
 1799
John Dalton’s Atomic Theory (1803)
www.english.upenn.edu/~jlynch/Frank/People/dalton.ht
ml - 2k
1. Elements composed of small
particles called atoms
2. All atoms of a given element
are the same, but different
from other elements
3. Atoms cannot be created or
destroyed in a chemical
reaction
4. Compounds are composed of
atoms combined in simple
whole number ratios
Studdy Buddy Review
 Describe the
contribution of each
towards the historical
development of the
atom:
– Proust
– Democritus
– Dalton
What is inside the atom?
J.J. Thomson (1897)
 Cathode Ray Tube
Experiments
Conclusions:
• Stream of negative particles
that have mass
• Named electrons
• Atoms are not indivisible
• Found ratio:
(electrical charge of
electron)
(mass of electron)
1.76 x 108 coulombs = 1
gram of electrons
Thomson’s Experiment
Voltage source
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+
Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
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+
Thomson’s Experiment
Voltage source
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Thomson’s Experiment
Voltage source
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Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field he found
that the moving pieces were negative
Robert Millikan (1909)
 Oil Drop
Experiment
Measured charge
of an electron
 Charge of one
electron =
-1.6x10-19C
http://webphysics.davidson.edu/Alumni/ToHaynie/OilDrop/oilappa.htm
THUS….
Mass of e- =
9.11x10-28g
Rutherford’s experiment
 English physicist Ernest Rutherford
(1911)
 Shot alpha particles at gold foil which can
be made a few atoms thick.
– alpha particles: positively charged helium
nuclei
– A form of radioactivity
 When an alpha particle hits a fluorescent
screen, it glows.
Lead
block
Uranium
Fluorescent
Screen
Gold Foil
What he got
How Rutherford explained results
 Atom is mostly empty
space.
 Small dense, positive
piece at center.
(NUCLEUS)
 Alpha particles
are deflected by it if
they get close
enough.
+
Credit for subatomic particles
 1897 Thomson discovered the electron
– Used cathode ray tube
 (1918) Rutherford named positive charged
particle the proton
– Goldstein (1886) first discovered positively charged particle
using cathode-ray tube with perforated cathode
 (1932) James Chadwick discovers neutrons
– Worked with cloud chambers to produced
neutrons and determined their masses
Subatomic particles
Name Symbol
Relative
mass
Charge (amu)
Actual
mass (g)
Electron
e-
-1
1/1840 9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
Studdy Buddy Review
 Name three subatomic
particles.
 Who is credited with
discovering each
particle?
 Describe the
Rutherford Gold Foil
Experiment.
Models of the Atom
Dalton Model of Atom
 Small, indivisible spheres
http://images.search.yahoo.com/search/images/
J.J. Thompson’s Model of Atom
 Plum Pudding Model,
1896
 Thought an atom was
like plum pudding
http://images.search.yahoo.com/search/images/
– Dough was positively
charged
– Raisins scattered
throughout the dough
were negatively
charged
– Didn’t know about
neutrons at this time
Rutherford’s Model of the Atom
 Rutherford Model,
1911
 Thought atom was
mostly empty space
– Nucleus in center is
dense, positively charge
– Electrons (negatively
charged) are in empty
space surrounding
nucleus
http://images.search.yahoo.com/search/images/
Bohr’s Model of the Atom
 Neils Bohr, 1913
 Similar to Rutherford’s
model
 Thought atom was
mostly empty space
– Nucleus in center is
dense, positively
charge
– Electrons move in orbits
around the nucleus
http://images.search.yahoo.com/search/images/
(Modern) Quantum Mechanical
Model of the Atom
 Heisenberg,
Schrodinger, many
others, ~1926
 Think atom is mostly
empty space
– Nucleus in center is
dense, positively
charge
– Electrons are around
the nucleus
– Cannot locate location
of electron at specific
time
http://particleadventure.org/particleadventure/frameless/modern_atom.html
Information about Atom from
Periodic Table
Atomic
Number
Avg Atomic Mass
Atomic Number and
Atomic Mass
 Chemical Symbol: abbreviation for element
name
 Atomic Number (Z): number of protons in
nucleus of atom (and electrons if neutral)
 Mass Number: number of protons and
number of neutrons in nucleus (whole
number)
Isotopes
 Isotopes: atoms with the same number of
protons but different number of neutrons
 Hyphen Notation:
– oxygen-16 and oxygen-17
 Nuclear Symbol:
16 O
8
17
8O
Average Atomic Mass
 Average Atomic Mass: weighted average
mass of atoms found in nature (decimal
number on periodic table)
 Can calculate average atomic mass of
elements if know percent abundance in
nature
 (WS Isotopes and Average Atomic Mass)
Ch. 25 Nuclear
Radioactivity
Objectives
 Describe how changes in the nucleus of an
atom during a nuclear reaction results in the
emission of radiation
• Describe alpha, beta, and gamma particles;
discuss the properties of alpha, beta, and
gamma radiation; and write balanced
nuclear reactions.
• Compare nuclear fission and nuclear fusion.
Objectives
• Explain the difference between stable and
unstable isotopes.
• Explain the concept of half-life of a
radioactive element, e.g., explain why the
half-life of C-14 has made carbon dating a
powerful tool in determining the age of very
old objects.
Radioactivity
Strong Nuclear Force
 Opposites attract, like charges repel
 So why do protons stay together in nucleus?
 Strong Nuclear Force holds nucleus
together and is stronger than electrostatic
repulsion between protons
– Only works over small diameter
– Neutrons help keep protons separated slightly
to reduce repulsion between protons
Mass Defect
 You’d expect the mass of an atom to be the
sum of the individual subatomic particles
4 He
2
2 (1.007276 amu) = 2.014552
2 (1.008665 amu) = 2.017330
2 (0.0005486 amu) = 0.001097
Total = 4.032979 amu
Actual mass helium atom = 4.00268 amu
 The difference between the calculated mass
and the actual mass is called mass defect.
Binding Energy
 In Einstein’s equation: E=mc2 the “lost”
mass can be converted into energy
 Binding energy: energy released when a
nucleus is formed from protons and
neutrons
 Could be considered as the amount of
energy to break apart the nucleus
 Associated with the strong nuclear force
holding particles together
Binding Energy per Nucleon
Radiation
 Stable nuclei have large binding energies
– High energy means it is hard for nucleus to
break apart
 Unstable nuclei can break apart and give off
particles
 Radiation: emission of energy as
electromagnetic waves or subatomic
particles
Discovery of Radiation
 Henri Becquerel (1896) experiment with
uranium found it was emitting particles
 Marie Curie (1898) discovered radioactive
element Polonium and Radium
Common Types of Radiation
 Alpha (a) 42 He)
– Helium nucleus
– Weak strength : can stop with
paper
 Beta (b) electron 0-1 e)
– Electron
– Medium strength: stop with
clothing
 mass #  4,
Atomic #  2
 Mass # stays
same, atomic # 
1
 Gamma (g)
– High energy
– High energy: stop with lead
 EM wave so
mass doesn’t
change
Other Types of Radiation
 Positron (0+1 e)
 mass # stays
the same,
 Atomic #  1
 Neutron (n) 10 n)
 Mass #  1,
atomic # stays
the same
Nuclear Equations

238

14

9
92 U 
6 C 
234
14
7
90
Th + _________
N + _________
12 C +
Be
+
_________

4
6
Answers: alpha, beta, alpha
1
0
n
Study Buddy Review




What force holds the nucleus together?
What is binding energy?
What happens when a nucleus is unstable
What is an alpha particle? Beta particle?
Gamma radiation?
Nuclear Decay and Half Life
Decay
 Radioactive decay: spontaneous emission
of radiation from nucleus of atom
 Transmutation: change in the identity of an
element due to the emission of particles
from the nucleus
Half-Life
 Half-life: time required for half of a sample
of an element to decay into another
element
 Known as rate of radioactive decay
 Different for each isotope
A = Ao(½)n
Half Life of Some Radioactive
Isotopes
Half life of Potassium-40
Half-Life Problem
 The half life of polonium-210 is 138.4 days.
How many milligrams of polonium-210
remain after 415.2 days if you start with 2.0
mg of the isotope?
 Answer: 0.25 mg
Nuclear Fission and Fusion
Fusion
 Energy of our sun and other stars is
produced from nuclear fusion reactions
 Fusion: light massed nuclei combine to
form a heavier, more stable nucleus
 Produces a lot of energy, also nuclear
waste
4 11 H

4
0 b + ENERGY
He
+
2
2
-1
Fission
 Nuclear power plants create energy from
fission reactions
 nuclear fission: a heavy nucleus splits into a
more stable nuclei of intermediate mass
– energy produced
– nuclear power plants
– Nuclear waste produced
235
92
U + 10 n  9336 Kr + 14056 Ba + 3 10 n + ENERGY
Study Buddy Review
 What is half-life?
 What is radioactive decay?
 Compare and contrast fusion and fission.