Download Chapter 8

Chapter 8: Composition of the
Atom
• The Discovery of
Subatomic Particles
• Rutherford’s Model of the
Atom
• Atomic Number and
Isotopes
Chapter 8: Composition of the
Atom
Th Discovery
The
Di
off Subatomic
S b t i
Particles
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
Early Models of the Atom
• Democritus
– Suggested the existence of atoms
– Thought that atoms were indivisible and indestructible
– Problems
• Didn’t explain chemical behavior
• No
N experimental
i
t l supportt
• Dalton’s Model
– Studied ratios in which
elements combine in
chemical reactions
– Dalton’s Atomic Theory
• All elements are made of tiny indivisible particles called
atoms.
• Atoms of the same element are the same but are
different from those of other elements.
• Atoms of different elements can physically mix or can
chemically combine with each other in simple, wholenumber ratios to form compounds.
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
• Chemical reactions occur when atoms are separated,
joined, or rearranged. Atoms of one element never
change into other kinds of atoms in chemical reactions.
– Inaccuracies in Dalton’s Theory
• Atoms are divisible.
• Atoms of the same element aren’t entirely the same
always.
y
• Current theory of the atom
– Smallest particle of an element that still has all its
properties
– Extremely small and typically measured in angstroms (1 x
10-10 m)
• Electrons
– Negatively charged subatomic particle
– Extremely light compared to other subatomic particles
– Experiments
• J.J. Thomson
– Use of the cathode ray tube
– Attraction
Att
ti
and
d repulsion
l i
– Determined the mass of an electron is about 1/2000
the mass of a hydrogen atom
• Robert A. Millikan
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
– Calculated an accurate value for the mass of an
electron
– Discovered the electron carries exactly one unit of
negative charge
– Mass of an electron is 1/1840 of a hydrogen atom
• Protons and Neutrons
– Properties of atoms and charge
• Atoms have no net electric charge; they are neutral
• Electric charges are carried by matter
• Electric
El t i charges
h
always
l
exist
i t in
i whole-number
h l
b
multiples of a single basic unit
• Charges have to cancel each other out to be neutral
– Protons
• Positively charged subatomic particles
• Discovered by E.
E Goldstein in cathode ray tube
• 1840 times more massive than an electron
• Proton carries exactly one unit of positive charge
– Neutrons
N t
• Neutrally charged subatomic particles
• Discovered by James Chadwick
• As massive as a proton
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
• The Atomic Nucleus
– Original theory of uniform distribution
– Ernest Rutherford
• Atoms have a lot of empty space
• Core of atom is the nucleus composed of protons and
neutrons
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
• Discovery of the electron
– William Crookes
• Use of a partially evacuated tube containing low pressure
gas called a cathode ray
g
y tube
• Two electrodes (conductors of electricity) in the tube
– Anode (positive electrode)
– Cathode (negative electrode)
• Applied a voltage and noticed a green beam
• Applied a magnetic field and noticed a deflection of the
beam
• Conclusion: charge of the particle must be negative
C
k ’ C
th d R
b
Crookes’s
Cathode
Ray T
Tube
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
C
k ’ C
th d R
b
Crookes’s
Cathode
Ray T
Tube
– J.J. Thomson
• Use of a partially evacuated tube containing low pressure
gas called a cathode ray tube
• Coated the end with a fluorescent material that glowed
g
where particles struck
• Observations
– No magnetic field: straight-line
straight line path of beam
– Magnetic field: deflection of the beam
– Magnetic field and charged plates:
D fl ti
Deflection
off beam
b
and
d movementt toward
t
d positive
iti
charge
• Considerations
– Mass of the particle
» Heavier particles deflect less than lighter.
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
– Velocity of the particle
» Faster particles deflect less than slower particles.
– Electric charge of the particle
» More highly charged particles have more of a bent
path.
– Strength of the magnet
» Stronger magnets caused more bending.
bending
– Amount of charge on the plates
» Stronger charges on plates cause more bend, and
t
type
off charge
h
on plates
l t determines
d t
i
effects
ff t off
bend.
J
J Th
’ C
th d R
b
J.J.
Thomson’s
Cathode
Ray T
Tube
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
• Conclusions
– Negatively charged subatomic particle common to all
atoms
» Based on charge
g to mass ratio from experiments,
p
,
regardless of gas in tube
» Deflection pattern was always the same.
– Robert Millikan
• Used an apparatus to measure the mass and specific
charge of an electron
• Conclusions
– Electron charge: 1.602 x 10–19 coulombs
– Electron mass: 9.10953 x 10–28 grams
• Discovery of the proton
– J.J. Thomson
• Use of a partially evacuated tube containing low pressure
hydrogen gas in a cathode ray tube (as he did to discover
electrons)
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles --
-- The Discovery of Subatomic Particles --
• Observations
– Second beam (aside from electrons) moving toward
negatively charged cathode
– Deflection was greater
g
• Conclusions
– Hydrogen atoms ionized into electrons and positively
charged hydrogen ions
– Since hydrogen contains only one electron, there had
to be a positively-charged particle with equal and
opposite charge to the electron.
electron
– Hydrogen ions are protons, common to all atoms.
– Development of the “plum-pudding” model of the
atom.
atom
» Electrons present in a positively-charged
“pudding” of protons.
» Revision to Dalton’s model of the atom
First atom (left):
( f ) boron atom (5
( protons balancing 5 electrons))
Second atom (upper right): B+ (neutral boron atom losing 1
electron)
Chapter 8: Composition of the Atom
-- The Discovery of Subatomic Particles -– E. Goldstein
• Similar equipment as that used by
Thomson
• Observations
– Second beam (aside from
electrons) moving toward
negatively
g
y charged
g cathode
– Deflection was greater
• Conclusions
– Referred to these “rays”
rays as
“canal rays” and later dubbed
them as protons
– 1840 times more massive than an
electron (based on deflection)
Chapter 8: Composition of the
Atom
R th f d’ Model
Rutherford’s
M d l off the
th Atom
At
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- Rutherford’s
Rutherford s Model of the Atom --
-- Rutherford
Rutherford’s
s Model of the Atom --
• Radiation
– Energy given off from a source
and traveling through space
– Originally discovered by Henri
Becquerel (1896)
• Use of photographic plates
and uranium (a radioactive
element)
• Strong images of uranium
material remaining on
plates
– Led to the concept of
radioactivity
• Radioactivity
– Spontaneous emission of radiation from the nucleus of an
atom
– Processes by which unstable atomic nuclei achieve stability
– Later discovered by
y Marie and Pierre Curie in radium and
polonium
• Types of radiation
– Alpha radiation
• Alpha particles (nucleus of a helium atom – 2 protons and
2 neutrons) emitted from a radioactive source
• Also identified as helium ions
• One-tenth the speed of light (c = 3.0 x 108 m/s)
• Cannot penetrate paper or clothes
– Beta radiation
• Fast moving beta particles (electrons formed when a
neutron decomposes into a proton and electron)
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- Rutherford
Rutherford’s
s Model of the Atom --
-- Rutherford
Rutherford’s
s Model of the Atom --
• Approaches the speed of light
• Greater energy and penetration because of speed
• Cannot penetrate a few millimeters of aluminum
– Gamma radiation
• High energy electromagnetic radiation emitted by certain
radioactive nuclei
• More energy than X-rays
• Travels at the speed of light because of no mass and no
electric charge
• Cannot penetrate several centimeters of lead or even
thicker amounts of concrete
• Discovery of the nucleus
– Ernest Rutherford
• Bombarded gold foil with alpha particles and masked the
source with a lead block to allow particles only to emit out
through the one opening
• Fluorescent screen placed around target with sensors
around
• Observations
– Most particles went through the foil.
– A handful of particles
were deflected back.
p
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- Rutherford’s
Rutherford s Model of the Atom --
-- Rutherford’s
Rutherford s Model of the Atom --
• Conclusions
– Deflections caused by a small,
dense center of positive
charge
– More than 99.9% of the mass
of the atom located in what he
called the nucleus
– Electrons moving around the
nucleus
– Slight deflections caused by
charges repelling
– “A nucleus is to an atom what
a pea is to a football field.”
Rutherford’s Model of the Atom
Chapter 8: Composition of the Atom
-- Atomic Number and Isotopes --
Chapter 8: Composition of the
Atom
At i N
Atomic
Number
b and
d Nucleus
N l
• Discovery of the neutron
– Irene and Frederic JoliotCurie
• Bombarded beryllium
with alpha particles
• Radiation struck
material containing
hydrogen and gave off
protons but emitted
high, neutral energy
(assumed to be gamma
rays)
– James Chadwick
• Repeated the
experiment
i
t off the
th
Joliot-Curies but also
used a cloud chamber
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- Atomic Number and Isotopes --
-- Atomic Number and Isotopes --
• Observations
– Mass present to what was colliding with paraffin
– Electrically neutral particle that was not a gamma ray
• Conclusions
– A neutron has approximately the same mass as a
proton.
– A neutron has no charge.
• Atomic Number
– Number of protons in an atom of an element
– Same as the number of electrons in a neutral atom (since
protons cancel out electron charges in a neutral atom)
• Mass Number
– Total number of protons and neutrons in an atom
– Mass of an atom does not come from electrons because of
how light
g they
y are
– # of neutrons = Mass number - atomic number
Summary of Subatomic Particles
Relative charge
Relative mass (amu)
Actual mass (g)
Electron (e–)
Proton (p+)
Neutron (n0)
1–
1+
0
0.000549
1.007
1.009
9.11 x 10–28
1.673 x 10–24
1.675 x 10–24
Also known as oxygen-16 (“element” - mass number)
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- Atomic Number and Isotopes --
-- Atomic Number and Isotopes --
• Isotopes
– Atoms with the same number of protons but different
numbers of neutrons
– Have different mass numbers because of the same number of
protons and electrons but differing neutrons
– Chemically
Ch i ll alike
lik because
b
off protons and
d electrons
l
– Some very stable while other extremely unstable because of
proton-to-neutron ratio
– Isotopes of hydrogen
• Atomic Mass
– Possible to predict using a mass spectrometer
– Atomic mass units
• Abbreviated “amu”
• 1/12 the mass of a carbon-12 atom
– Relative abundance
• Each isotope of an element has a fixed mass and occurs
in a certain percentage in nature, like how nitrogen makes
up a certain percentage of air, oxygen another percentage,
etc.
• Of approximately 1500 known isotopes, only 264 are
stable and therefore have larger relative abundances.
– Calculating (average) atomic mass
• Weighted average of masses
• Average atomic mass = (% abundance1)(mass1) +
(% abundance2)(mass2) + …
Chapter 8: Composition of the Atom
Chapter 8: Composition of the Atom
-- Atomic Number and Isotopes --
-- Atomic Number and Isotopes --
Natural Percent Abundance of Oxygen
Average atomic mass ((“weighted
weighted average
):
average”):
= 99.759%  15.995 + 0.037  16.995 + 0.204  17.999
= 0.99759  15.995 + 0.00037  16.995 + 0.00204  17.999
= 15.956 + 0.0063 + 0.0367
= 15.999 amu
Chapter 8: Composition of the Atom
-- Atomic Number and Isotopes -• Atomic mass inferences
– Average atomic masses are never whole numbers
– Example: average atomic mass of copper
• Average atomic mass of copper is 63.546 among
isotopes
p of copper-63
pp
and copper-65
pp
• Since 63 is closer to 63.546 than 65, copper-63 must be
more abundant than copper-65
Chapter 9: Nuclear Chemistry
• Exploring Radioactivity
• Using Nuclear Reactions
for Research
• Nuclear Reactions for
Energy
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity --
Chapter 9: Nuclear Chemistry
E l i Radioactivity
Exploring
R di
ti it
• Nuclide
– General name for nucleus of atom
– Neutrons are the only things that change in nuclides of
atoms of the same element
• Types of radiation
– Alpha radiation ()
• Alpha particles emitted by a radioactive source
– Positively charged particle emitted from certain
radioactive nuclei
– Consists of two protons and two neutrons and is
identical to the nucleus of a helium atom
4
2
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -204
82
Al h particle
ti l
He  Alpha
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -• Examples
4
Pb 

 
 200
80 Hg  2 He
• Atomic numbers must add up, and mass numbers must
add up.
up The new atomic number identifies the element as
mercury (Hg).
• Properties of alpha radiation
– Involves a helium nucleus
– Charge: 2+ (no electrons to balance protons)
– Mass: 4 amu
– Penetrating power: 0.05 mm body tissue
– Shielding: paper, clothing
– Can be dangerous when ingested
Parent
nuclide
Daughter
nuclide
• Parent and daughter nuclides
– Parent nuclide: initial nucleus
– Daugher nuclide: resulting nucleus (aside from
emitted particle)
» Less energetic than parent nuclide
» Formed for stability
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -– Beta radiation ()
• Beta particles emitted by a radioactive source
– Fast-moving electron emitted from certain radioactive
nuclei
– Formed when a neutron decomposes into a proton and
an electron
0
1
14
6
C
C

 
 147 N 
0
1
e
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity --
 Positron
15
8
-- Exploring Radioactivity -– Penetrating power: 4 mm body tissue
– Shielding: metal foil or thin pieces of wood
– Process used in carbon-14 dating
• Examples
e  Beta particle
• Properties of beta radiation
– Involves an electron
– Charge: 1- (charge of an electron)
– Mass: 1/1837 amu
8
5
Chapter 9: Nuclear Chemistry
B
84 Be 
O
157 N 
0
1
e
0
1
e
– Done when too few of neutrons are present in the atom
– Charge: 1+ (charge of a proton)
– Mass: 1/1837 amu
• Positron emission
– Some nuclei have too few neutrons for a ratio of
stability and need to be balanced by converting
protons to neutrons by emitting positrons (particle that
has the same mass as an electron but that has a
positive charge).
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -– Gamma radiation ()
• Gamma rays emitted by a radioactive source
– High energy rays
– Have no mass and no electrical charge
• Often emitted along with alpha or beta radiation by the
nuclei of decaying radioactive atoms

 Gamma ray
4
Th
226
88 Ra  2 He + 
230
90
• Properties of gamma radiation
– Involves rays having no charge or mass
– Charge: 0 (no charge in and of itself)
– Mass: 0 amu
Chapter 9: Nuclear Chemistry
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -– Penetrating power: through entire body
– Shielding: lead,
lead concrete (incomplete shielding)
– Most dangerous of all radiation
– Usually accompanied by alpha or beta particles
– Similar in properties to X-rays
• Ionizing vs. nonionizing radiation
– Ionizing radiation
• Has enough energy to change atoms and molecules into
ions
• Examples
– Alpha radiation
– Beta radiation
– Gamma radiation
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -– Nonionizing radiation
• Does not have enough
g energy
gy to change
g atoms and
molecules into ions
• Examples
– Microwaves
– Light waves
– Radio waves
• Sources
S
off radiation
di ti
– The sun (cosmic radiation)
– Food
– Building materials
– Background radiation (natural radiation)
• Carbon-14
• Hydrogen-3 (tritium)
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -• Effects of ionizing radiation
– Genetic mutations/annihilations
• Sensitivity of DNA
• Carcinogenic consequences
– Threshold for radiation
• Below: minimal damage
• Above: radiation sickness
– Hair loss
– Deformities
– Cellular degradation/mutation
-- Exploring Radioactivity -• Examples
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -• Examples
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -• Mass conversion
– Antimatter
• Antiparticles of matter than can annihilate each other
– Every particle has an antiparticle.
– When a particle meets an antiparticle
antiparticle, they disappear
in a process known as pair annihilation.
– Their mass-energy is used to create photons or other
particles.
ti l
– “Mass-energy equivalence”
– Extremely
y short existence because it is annihilated
when it comes into contact with ordinary matter
– Four basic forces of the universe
• Gravitational force
• Electromagnetic force
Chapter 9: Nuclear Chemistry
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity --
-- Exploring Radioactivity --
• Weak nuclear force
• Strong nuclear force
– Short range force keeping protons and neutrons
together
– Gets extremely
y weak as p
protons and neutrons g
get
farther apart
– When strong force does not exist, radioactivity occurs.
– Elementary
El
t
particles
ti l
• Leptons (particles involved in the weak and
electromagnetic forces)
– Electron
– Muon
– Tau
– Neutrino
• Hadrons (particles that take part in the nuclear forces,
including weak and electromagnetic forces)
– Mesons
M
((particles
ti l that
th t decay
d
into
i t photons
h t
and
d leptons)
l t
)
» Pion
» Kaon
» Eta
– Baryons (particles that decay into protons and/or other
thi
things)
)
» Proton
» Neutron
» Lambda
» Sigma
» Xi
» Omega
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -Table of Elementary Particles
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -– Albert Einstein
• Mass is lost in nuclear reactions.
• Nuclear reactions cause mass to be converted into
energy.
• Equation:
q
E = mc2
– E: energy (J)
– m: mass (kg)
– c: speed of light (3.0  108 m/s)
– Mass is derived by how much mass is lost in the
nuclear reaction.
• Half-life
– The time required for one-half of the atoms of a radioisotope
to emit radiation and decay to products
Chapter 9: Nuclear Chemistry
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity --
-- Exploring Radioactivity --
– Symbol: T
– A=A0(0.5)t/T where A is the final amount, A0 is the initial
amount, t is the time, and T is the half-life in the same
units
– Example
• A sample initially contains 50.0 g of cobalt-60. After 2.00
years, the sample contains 38.4 g of cobalt-60. Calculate
th half-life
the
h lf lif or cobalt-60.
b lt 60
– A0 = 50.0 g
– A = 38.4 g
– t = 2 years
Chapter 9: Nuclear Chemistry
-- Exploring Radioactivity -• Decay chain
– Chain of decay in which unstable isotopes decay into stable
iisotopes
t