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Chapter 4 Notes
The Structure of the Atom
4.1 Early Theories of Matter
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Remember atoms are made up of matter.
They are tiny units that determine the property of all
matter but the atoms themselves do not possess the
properties individually.
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Democritus (400 B.C.)
Aristotle (300 B.C.)
Dalton (1800’s)
Democritus and Aristotle
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Democritus (400 B.C.)
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Greek Philosopher
He hypothesized about atoms.
His theory explains some observations.
Unable to prove atoms exist.
Aristotle (300 B.C.)
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He rejected the ideas of Democritus because they conflicted
with his own ideas on nature.
There were many criticisms to Democritus’s ideas like:
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How can atoms move through empty space?
What held atoms together?
Eventually Democritus’s atomic “theory” was rejected.
Dalton (1800’s)
In 1700’s Chemistry was just starting to develop as a real
science, just beginning to gather reliable data.
 John Dalton, English School Teacher (1808)
 He proposed the atomic theory. Theory was widely
accepted and had much evidence to support it.
 Three points to his theory.
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1. Every element is made of tiny, unique particles called atoms
that can not be subdivided.
2. Atoms of the same element are exactly alike.
3. Atoms of different elements can join to form
molecules.
4.2 – Subatomic Particles and the
Nuclear Atom
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Electron, Thomson (late 1890’s) and Millikan (1909)
Nucleus, Rutherford (1911/1920)
Neutron, Chadwick (1932)
Properties of Subatomic Particles
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Charge, mass, location and symbol
Table 4-1 pg 97
The Discovery of the Electron
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In the late 1890’s, a British physicist, J.J. Thomson,
discovered the electron and the charge to mass ratio of
an electron.
Thomson’s experiments used a vacuum tube which is a
tube that has had all gases pumped out of it.
Cathode rays travel from a - plate (cathode) toward +
plates (anode).
The cathode rays bend toward the + plate and away from
a - plate indicating that they are
negatively charged.
The tube is a cathode ray tube or CRT.
CRT – Cathode Ray Tube
Cathode
- Charged
Plate
+ Charged Plate
Cathode Rays
Anode - Charged Plate
Spinner
+ Charged
Thomson's Cathode Ray Tube (CRT)
Zinc
Sulfide
Screen
Electron cont.
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Thomson's Results
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existence of electron.
determined approximate value for charge on electron.
atoms are composed of positive and negative particles
scattered throughout like a blueberry muffin.
Millikan
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He determined the actual charge of an electron.
His experimental setup and technique was so good
that the charge he measured almost 100 years ago
is within 1% of the currently accepted value.
Discovery of the Nucleus and Protons
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In 1909, Rutherford carried out experiments that
revealed an arrangement far different from Thomson’s
model of the atom.
The experimenters set up a lead-shielded box containing
radioactive polonium, which emitted a beam of positively
charged subatomic particles through a small hole.
The sheet of gold foil was surrounded by a screen
coated with zinc sulfide, which glows when
struck by the positively charged particles
of the beam.
The Gold Foil Experiment
Gold Foil Expt. continued
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What exactly was happening in the experiment?
Gold Foil Expt. continued
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To explain the results of the experiment, Rutherford’s team proposed a
new model of the atom.
Because most of the particles passed through the foil, they concluded that
the atom is nearly all empty space.
Because so few particles were deflected, they proposed that the atom has a
small, dense, positively charged central core, called a nucleus.
Chadwick and the Discovery of the
Neutron
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Confirmed the existence of the neutron in the early
1930's.
Neutrons have no charge -- they're neutral.
Neutrons have a mass that is close to the mass of the
proton.
Copy Table 4-1 on pg 97 of your textbook.
Section 4.3 – How Atoms Differ
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Atomic Number (Z) is equal to the number of protons of
each atom of a particular element.
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Carbon Z=6
Potassium Z=19
Silver Z=47
Determining the Number of Neutrons in
an Atom
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Mass Number (A) equals the number of protons and
neutrons in the nucleus of an atom.
mass = protons + neutrons
or
number of neutrons = A – Z
Example:
 Determine the number of neutrons in the following
isotopes: carbon–14, chlorine-37 and magnesium-25
Isotopes
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Isotopes are atoms of the same element that have a
different mass because they have a different # of
neutrons.
So same # of P but diff # of N.
Isotopic notation is used to identify the different isotopes
of an element. For example, hydrogen has 3 isotopes
hydrogen-1, hydrogen-2 and hydrogen-3 or 1H, 2H or 3H
Ions
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All atoms are neutral because they have the same number
of protons and electrons. Ions, though, are atoms of an
element where the # of P do not equal # of e- so
therefore have an overall charge.
Positive ions are called CATIONS
Negative ions are called ANIONS
You can determine the charge using the formula:
ion charge = #P - #eIf charge = 0 then it is not an ion, it is an atom.
Relative Atomic Mass
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A relative scale of atomic masses where one atom has
been chosen as the standard and assigned a relative mass
value that all other atoms are compared to.
The standard that scientist use is the carbon-12 atom. It
has been assigned a mass of exactly 12 atomic mass units
(amu).
One atomic mass unit is exactly 1/12 the mass of a carbon12 atom.
The mass of both a proton and a neutron are almost
equal to 1 amu.
Average Atomic Mass
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Most elements occur naturally as a mixture of isotopes.
The percentage of an elements isotopes is taken into
account when calculating the elements average atomic
mass.
The average atomic mass is the weighted average of the
atomic masses of the naturally occuring isotopes of an
element.
Example #1: Bromine consists of 50% 79Br and 50%
81Br, calculate the avg atomic mass of bromine.
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Ar = [ (50 x 79) + (50 x 81) ] /100 = 80
So the relative atomic mass of bromine is 80 amu
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Example #2: Chlorine consists of 75% chlorine-35
and 25% chlorine-37.
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Think of the data based on 100 atoms, so 75 have a mass of 35
and 25 atoms have a mass of 37.
The average mass = [ (75 x 35) + (25 x 37) ] / 100 = 35.5
So the relative atomic mass of chlorine is 35.5 amu
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Do problem #____ on pg ____ of your textbook.
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Ions
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While atoms are neutral, ions are not because:
(# of protons) ≠ (# of electrons)
An ion is an atom or a combination of atoms with a
charge.
To determine the charge:
charge = (# protons) – (# electrons).
2 types of ions
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Cations = positive ions (formed from metal elements)
Anions = negative ions (formed from non-metal elements)
Ions cont.
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The charge is written to the upper right corner of the atomic
symbol or symbols. The sign of the charge is written after the
number. If the charge is 1+ or 1-, the 1 can be dropped.
Ex:
Li+ or Li+1 or Li1+
Practice Problem:
Determine the number of protons, neutrons and
electrons in the following:
Answers
The Sodium-23 ion has 11 protons, 12 neutrons and 10
electrons.
The Sulfur-32 ion has 16 protons, 16 neutrons and 18
electrons.
Aluminum-27 ion has 13 protons, 14 neutrons and 10
electrons.
Potassium-39 atom has 19 protons, 20 neutrons and 19
electrons.
Oxygen-16 ion has 8 protons, 8 neutrons and 10 electrons.
4.4 – Unstable Nuclei and Radioactive
Decay
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Radioactivity
Types of Radioactivity
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Alpha
Beta
Gamma
Characteristics of Radiation Types
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Table 4-3 pg 107
Radioactivity
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Nuclear reactions involve changes in the nucleus of an
atom because the neutron-to-proton ratio of an atom’s
nucleus determines is unstable.
This is different from a chemical reaction involves changes
in the electrons surrounding an atom.
Unstable nuclei undergo radioactive decay, emitting
radiation in the process.
There are three main types of radiation: alpha, beta and
gamma.
Alpha Particles
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Represented by α (alpha) or
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They are equivalent to the nuclei of a He atom
(+) Charged, heavy and slow moving particles
Limited penetrating power, stopped by sheet of paper
Equation Example:
Beta Particles
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Represented by
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(beta) or -10e
(-) Charged, little mass, 100x faster than alpha
Basically high-speed electrons
Intermediate Penetrating Power, Stopped by
Aluminum Sheet
Equation Example
Gamma Particles
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Represented by
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(gamma)
Electromagnetic wave, no charge (neutral) or mass
Great speed, high-energy
High penetrating power, stopped by lead sheet
Equation Example
Radioactivity
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Alpha, Beta and Gamma Particles
Magnets
Pb Sheet
α
γ
S
S
Radiation Source
in Lead Casing
β
Paper Sheet
Al Sheet
Three Types of Radiation Recognized By Rutherford
Practice Problems
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Complete the following nuclear reactions and identify the type:
Practice Problems
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Answers 1-4:
He
β
Alpha Beta Gamma
Alpha Beta Gamma
230
89
Ac
Alpha Beta Gamma
216
84
Po
Alpha Beta Gamma
Practice Problems
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Answers 5-8:
216
Alpha Beta Gamma
85
At
212
Alpha Beta Gamma
82
Pb
210
Alpha Beta Gamma
84
Po
7
Alpha Beta Gamma
3
Li