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Chapter 4: Atomic
Structure
What is an “atom”?
• The word atom comes from the Greek and
means “indivisible”.
• The smallest particle that retains the identity
of the element in a chemical reaction.
Early Atomic Theories
1. Democritus
NO experimental evidence, but the
first one to come up with the IDEA
of an atom (~400 BC).
• Matter is composed of tiny
particles called atoms, which are
indivisible, indestructible
fundamental units of matter
• The universe is made of:
a) ATOMS b) EMPTY SPACE
Early Atomic Theories
2. Aristotle & Plato (~400 BC)
-opposed Democritus; wrong, but
more popular… root of the
Medieval understanding of
humors (e.g. phlegmatic,
sanguine, etc.) and the practice
of bleeding…
•Earth-cool, heavy
•Wind-light
•Fire-hot
•Water-wet
The 4 elements of “Hyle”
Contributors to
Dalton’s Atomic Theory
1. Antoine Lavoisier (France 1782)
-Law of Conservation of Mass:
In a chemical reaction, mass is
conserved.
2. Joseph Proust (France 1799)
-Law of Definite Proportions:
The elements that comprise a
compound are always in a certain
proportion by mass.
John Dalton (England 1766-1844)
• Formulated
1st modern Atomic Theory
• School teacher
• Studied the ratios in which
elements combine in chemical
reactions
Dalton’s Atomic Theory
1. All matter is made of tiny indivisible particles called
atoms, and atoms are the building blocks of elements
2. All atoms of the an element are the same; Different elements have
different atoms.
3. LAW OF MASS CONSERVATION: Atoms are not created or destroyed in
chemical reactions (just rearranged when bonds are broken or formed).
The total mass of the universe if ALWAYS the same….
4. LAW OF DEFINITE PROPORTIONS: Two or more different atoms bond
together in SIMPLE WHOLE NUMBER RATIOS to form a specific
compound.
-The ratio of elements (or the mass ratio of elements) in a particular
compound is ALWAYS the same.
-If the ratio changes, it’s a different compound.
7
MASS CONSERVATION Problem:
• You heat 2.35 grams of metallic
mercury in air, which produces
2.73 grams of a red-orange
residue. Assume that the
chemical change is the reaction
of the metals with oxygen in air.
What is the mass of the oxygen
that reacts?
When you strongly heat the red-orange
residue, it decomposes to give back the
mercury and release the oxygen, which
you collect. What is the mass of oxygen
you collect?
MASS CONSERVATION Problem:
• You place 1.85 grams of wood in a vessel with 9.45 grams of
air and seal the vessel. Then you heat the vessel strongly so
that the wood burns. In burning, the wood yields ash and
gases. After the experiment, you weight the ash and find that
its mass is 0.28 grams. What is the mass of the gases in the
vessel at the end of the experiment?
Law of Definite Proportions
A specific compound ALWAYS has the same ratio of elements
ex: H2O
ex: H2O2
1.
Can be expressed in terms of an ATOMIC or MOLE RATIO:
–
–
2.
Water is always 2 H: 1 O
Hydrogen peroxide is always 2 H: 2 O (or reduced, 1 H :1 O)
Can be expressed in terms of a MASS RATIO:
– Water is always 2 g H: 16 g O (or reduced, 1 g H : 8 g O)
– Hydrogen peroxide is always 2 g H: 32 g O (or reduced, 1 g H: 16 g O)
3.
Can be expressed in terms of a MASS PERCENTAGE:
–
–
Water is always 11% H & 88.9% O by mass
Hydrogen peroxide is always 5.9% H & 94.1% O
2.0 g H/ 18.0g H2O x 100% = 11 % H
16.0 g O/ 18.0g H2O x 100% = 88.9 % O
2.0 g H/ 34.0g H2O2 x 100% = 5.9 % H
32.0 g O/ 34.0g H2O2 x 100% = 92.1 % O
DEFINITE PROPORTION Problem:
• Aluminum metal reacts with bromine, a red-brown
liquid with a noxious odor. The reaction is vigorous
and produces aluminum bromide, a white crystalline
substance. A sample of 27.0g of aluminum yields
266.7 g of aluminum bromide. How many grams of
bromine react with 15.0 g of aluminum?
Dalton’s Atomic Model
Atoms have no net electrical charge.
Their charge is 0 (neutral).
Dalton’s  Modern Atomic Theory
Dalton’s theory:
1. INDIVISIBLE atom
2. IDENTICAL atoms for an
element
3. LAW of MASS
CONSERVATION
Modern refinement:
1.
Atoms can be further divided into
their sub-atomic particles
(electrons, protons, & neutrons)
2.
ISOTOPES (atoms of an element
can have different number of
neutrons/ be heavier)
3. Nuclear chemistry 
Law of Conserviation of
Mass & ENERGY; atoms of one
element can be transmuted into
atoms of another element in a
nuclear reaction
How did we go from
Dalton’s model…
to
a model with
subatomic particles?
Subatomic
Structure
What’s inside an atom & how is it arranged?
Discoveries that led to a refinement of Dalton’s
simple atomic theory….
(OMG—there’s something inside of an atom!)
Discovery of the electron
THE
CATHODE
RAY
TUBE
J(oseph) J(ohn) Thompson, discoverer of the eNobel Prize, 1906
J. J. Thomson - English physicist.
-Made a piece of equipment called a cathode ray tube (CRT).
-A CRT is a vacuum tube - all the air has been pumped out.
Application of the CRT
QUESTIONS TO RESOLVE:
1. How did he detect this ray?
2. What is the charge of this ray?
3. Where does the ray originate?
4. Is the ray pure energy or is it
composed of matter (something
with mass and volume)?
Cathode Rays—old school computer monitors & TVs
Thomson’s Experiment
Voltage source
-
+
Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
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
Cathode Ray
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
Deflection of Cathode Ray
Thompson 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 concluded that these
particles were part of
every atom.
-the cathode ray was made of negatively charged particles – electrons.
-able to estimate that the mass of the electron was equal to about 1/1840 of the mass
of a hydrogen atom.
Mass of e- = 9.11 x 10-28 g
Discovery of the electron’s charge
The mass and charge of each droplet were measured; careful analysis of the data
showed that the charge was always an integral multiple of a smallest charge, e.
Electron charge, e = the basic quantity of
charge.
• Electric charges always exist in whole
number multiples of a single basic unit,
the electron, 1e, 2e, 3e, etc.
• A particle with a positive charge must be
present in the atom to balance each
negatively charge electron.
New discovery/ evidence  New MODEL
If atoms are neutral, and they contain negative electrons, there must
also be a positive part to the atom
MODEL: Thompson’s Plum Pudding Model
(1st attempt at imagining what could sync up with the data):
•
Said the atom was like
plum pudding
•
A bunch of positive stuff,
with the electrons able to be
removed
More like cookie dough…
Discovery of the NUCLEAR atom
THE GOLD FOIL
EXPERIMENT
Ernest Rutherford, English physicist, 1910
-proves Thompson’s model erroneous,
refines atomic model
Lead
block
Rutherford’s experiment
Gold Foil
-Wanted to see how big
atoms were
• Used radioactive uranium,
which releases alpha
particles (i.e. positively
charged helium nuclei)
• Shot them at gold foil
which can be made a few
atoms thick
Uranium
Fluorescent
Screen
• When the alpha particles
hit a fluorescent screen, it
glows.
He Expected….
• The alpha particles to
pass through without
changing direction
very much
Closeup view
of gold atom:
• In the plum pudding
model, the positive
charge & mass is
spread out evenly,
and would not be
enough to stop the
alpha particles
He Expected….
What really happenned….
Rutherford
• Expected
• Found
Rutherford’s
Conclusions
• The atom is mostely
empty space.
• All of the positive
charge, and most of
the mass of an atom
are concentrated in a
small core, called the
nucleus.
How it explains the experimental
results:
+
Another view:
Experimental evidence: Rutherford’s
Gold Foil Expt.
THE ATOM ISN’T DENSE. IT’S GOT A POSITIVELY CHARGED NUCLEUS (and lots
of empty space),
PROTON = +1.602 x 10-19 Coulombs
New discovery/ evidence  New MODEL
Model: The “Nuclear” atomic model
• Atom is mostly empty space
• Small dense, positive piece
at center “NUCLEUS”
• Alpha particles are deflected
by it if they get close enough
Location/ Size
of Subatomic Particles
An atom is mostly EMPTY SPACE.
Nucleus = the eye of the head on a dime;
Electrons = 1/1000 nucleus = a pin head?
Atom size = football stadium
Discovery of the neutron
AND, shortly after,
a student of
Rutherford
discovers the
NEUTRON!
Chadwick-discover of the neutron (from study of
radioactive substances), won a Nobel Prize for his
discovery… paved the road for nuclear fission, etc.
Nuclear atom, refined
• Useful for discussion of isotopes
The atom is mostly
empty space
Two regions
1. Nucleus- protons
and neutrons
2. Electron cloudregion where you
might find an
electron
Subatomic Particles
Particle Symbol Relative Mass
Mass (g)
Charge Relative to
the Proton
Electron e-
1(-1e)
1/1840
9.11 x 10-28 g
p+
1+
(+1e)
1
1.67 x 10-24g
0
1
1.67 x 10-24g
Proton
Neutron n0
Scanning Tunneling Microscope
Scanning Tunneling Microscope
Review
1.
Compare/ contrast Thompson and Rutherford’s atomic models.
2.
Which of these could be the charge of an object?
A.
B.
C.
D.
3.
0.80 x 10-19 C
2.0 x 10-19 C
3.2 x 10-19 C
4.0 x 10-19 C
e = 1.602 x 10-19 C
The magnitude of the charge on an electron was determined in the
_______.
A.
B.
C.
D.
cathode ray tube, by J.J. Thompson
Millikan oil drop experiment
Dalton atomic theory
atomic theory of matter
4.
Which one of the following is not true concerning cathode rays?
A.
B.
C.
D.
E.
5.
The gold foil experiment performed in Rutherford’s lab ________.
A.
B.
C.
D.
E.
6.
They originate from the negative electrode
They travel in straight lines in the absence of electric or magnetic fields
They impart a negative charge to metals exposed to them
They are made up of electrons
The characteristics of cathode rays depend on the material from which they are
emitted
confirmed the plum-pudding model of the atom
led to the discovery of the atomic nucleus
was the basis for Thomson’s model of the atom
utilized the deflection of beta particles by gold foil
proved the law of multiple proportions
In the Rutherford nuclear-atom model:
A.
B.
C.
D.
the heavy subatomic particles reside in the nucleus
the principal subatomic particles all have essentially the same mass
the light subatomic particles reside in the nucleus
mass is spread essentially uniformly throughout the atom
The Discovery of the Neutron
• Chadwick bombarded alpha
particles(helium nuclei) at Beryllium.
• Neutrons were emitted and in turn hit
parafin and ejected protons from the
parafin.
Discovery of the Neutron
Neutrons
• Neutrons have mass similar to protons.
• No electrical charge.
Atomic Number, Mass Number, Atomic
Mass and Isotopes
Atomic Number (Z): is the number of protons
in the nucleus of the atom.
Z=#p
• The number of protons (atomic number)
determine the identity of an element.
Atoms have no overall electrical charge so,
an atom must have as many
electrons
as there are
protons
in its nucleus.
The atomic number of an element also equals
the number of electrons
in a neutral atom of that element.
Ions
• Positively charge atom (cation)
– Atom lost electrons.
• Negatively charged atom (anion)
– Object gained electrons.
Ex: Sodium
• What is the atomic number of Sodium?
• How many protons does sodium have?
• How many electrons does sodium have?
Mass Number (A):
• The sum of the protons and neutrons in the
nucleus.
A=#p + #n
Notation
Other ways to write elements:
Mass Number
Mass Number
36
17Cl
Atomic Number
Cl-36
Nucleons:
protons and neutrons
Isotopes
of an element have
different mass numbers
because
they have
different numbers of neutrons,
but they have
the same atomic number.
Example:
Isotopes of Carbon and Hydrogen
Isotopes of Hydrogen
protium deuteriumtritium
H
H
Isotopes of Carbon
H
Atomic Mass Unit
is a unit used to compare
the masses of atoms
and has the symbol
u
or
amu.
1 amu or u
is approximately equal to
the mass of
a single
proton or neutron.
Carbon-12
Chemists have defined
the carbon-12 atom
as having a mass of
12 atomic mass units.
1 u = 1/12 the mass of a Carbon-12 atom.
Atomic Mass
is the weighted average
mass
of all the naturally occurring
isotopes
of that element.