Download Honors Chemistry

Atoms and Elements
Mr. Matthew Totaro
Legacy High School
Honors Chemistry
Experiencing Atoms
• Atoms are incredibly small, yet they compose everything.
• Atoms are the pieces of elements.
• Properties of the atoms determine the properties of the
elements.
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Experiencing Atoms
• There are about 91 elements found in nature.
Over 20 have been made in laboratories.
• Each has its own, unique kind of atom.
They have different structures.
Therefore they have different properties.
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Early Theories of Matter
• The ancient Greeks
believed that all matter
was composed of only
four elements: earth,
air, water, and fire.
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Ancient Greek Theory of Matter
Four Elements of the Ancient Greeks
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First Atomic Theory
• Democritus (c.a. 460
BC) was the first
thinker ever to reason
that matter was not
infinitely divisible.
• He called the smallest
piece of matter
‘atomon’, which in
Greek means ‘not
cutable’.
Democritus
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The Ancient Greeks
• Aristotle (c.a. 380
BC) reasoned,
however, that matter
was infinitely divisible.
• Aristotle’s ideas
agreed more closely
with those of the
Christian church,
therefore, they were
accepted as fact for
over two millenia.
Aristotle
Enlightenment Thinking
• Robert Boyle (c.a.
Robert Boyle
1661) wrote a book
entitled The Sceptical
Chymist. In it, Boyle
argues that the four
classical elements of the
Greeks were actually
not elements at all.
• He also argued that
matter was composed of
indivisible particles or
atoms.
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The First Modern Atomic Theory
• After two hundred years
of experimental results,
the first modern atomic
theory was developed by
John Dalton (c.a. 1805).
• Dalton reasoned that
indivisible, spherical
particles made up all of
matter. He also argued
that these particles were
rearranged during a
chemical reaction.
John Dalton
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Dalton’s Original Theory of the
Atom and of Reactions
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Dalton’s Atomic Theory
1. Each Element is composed of tiny, indestructible
particles called atoms.
 Tiny, hard, indivisible, spheres.
2. All atoms of an element are identical.
 They have the same mass, volume, and other physical
and chemical properties.
 So, atoms of different elements are different.
 Every carbon atom is identical to every other carbon
atom.

They have the same chemical and physical properties.
 However, carbon atoms are different from sulfur atoms.

They have different chemical and physical properties.
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Dalton’s Atomic Theory
3. Atoms combine in simple, whole-number
ratios to form molecules of compounds.
 Because atoms are unbreakable, they must
combine as whole atoms.
 The nature of the atom determines the ratios
in which it combines.
 Each molecule of a compound contains the
exact same types and numbers of atoms.
 Chemical formulas
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Discovery of the Subatomic Particles
• J. J. Thomson (c.a.
1897) performed
experiments involving
cathode rays that
proved that atoms could
be broken down into
smaller particles.
• Thompson called the
particle that he
discovered an ‘electron’,
which means unit of
electricity in Greek.
J. J. Thomson
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Cathode Ray Tube (Discovery of the
Electron)
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Thomson’s Results
• the cathode rays are made of tiny particles
• these particles have a negative charge
because the beam always deflected toward the +
plate
• the amount of deflection was related to two factors,
the charge and mass of the particles
• every material tested contained these same particles
• Thomson called these particles ‘electrons’, because
they were the components of all electricity
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Some Notes on Charges
• There are two kinds of
charges, called positive and
negative.
• Opposite charges attract.
 + attracted to –.
• Like charges repel.
 + repels +.
 – repels –.
• To be neutral, something
must have no charge or
equal amounts of opposite
charges.
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Thomson’s Plum Pudding Atom
• the structure of the atom contains
many negatively charged electrons
• these electrons are held in the atom
by their attraction for a positively
charged electric field within the atom
 there had to be a source of positive
charge because the atom is neutral
 Thomson assumed there were no
positively charged pieces because none
showed up in the cathode ray experiment
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Predictions of the Plum Pudding Atom
• the mass of the atom is due to the mass of the
electrons within it
electrons are the only particles in Plum Pudding
atoms
• the atom is mostly empty space
cannot have a bunch of negatively charged
particles near each other as they would repel
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Radioactivity
• in the late 1800s, Henri Becquerel and Marie Curie
discovered that certain elements would constantly
emit small, energetic particles and rays
• these energetic particles could penetrate matter
• Ernest Rutherford discovered that there were three
different kinds of emissions
 alpha, a, particles with a mass 4x H atom and + charge
 beta, b, particles with a mass ~1/2000th H atom and –
charge
 gamma, g, rays that are energy rays, not particles
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Radioactivity
• Three types of radiation were discovered by
Ernest Rutherford:
  particles
  particles
  rays
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Discovery of the Nucleus
• Ernest Rutherford
(c.a. 1908) utilized
radioactivity in the gold
foil experiment to prove
that the plum-pudding
model of the atom was
wrong.
• In so doing, he
discovered the atomic
nucleus and the proton.
Ernest Rutherford
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The Gold-Foil Experiment
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Rutherford’s Results
• Over 98% of the  particles went straight
through
• About 2% of the  particles went through
but were deflected by large angles
• About 0.01% of the  particles bounced
off the gold foil
“...as if you fired a 15” cannon shell at a piece
of tissue paper and it came back and hit you.”
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Rutherford’s Conclusions
• Atoms were mostly empty space
because almost all the particles went straight
through
• Atoms contain a dense particle that was small in
volume compared to the atom but large in mass
because of the few particles that bounced back
• This dense particle was positively charged
because of the large deflections of some of the
particles
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Plum Pudding
Atom
•
•
•
•
•
•
•
•
•
•
a few of the
 particles
do not go through
•
•
•
•
•
•
•
•
•
•
•
•
if atom was like
a plum pudding,
all the  particles
should go
straight through
Nuclear Atom
.
.
.
most  particles
go straight through
some  particles
go through, but are deflected
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Rutherford’s Interpretation –
the Nuclear Model
1) The atom contains a tiny dense center called the
nucleus
 the amount of space taken by the nucleus is only about
1/10 trillionth the volume of the atom
2) The nucleus has essentially the entire mass of the
atom
 the electrons weigh so little they give practically no mass
to the atom
3) The nucleus is positively charged
 the amount of positive charge balances the negative
charge of the electrons
4) The electrons are dispersed in the empty space of
the atom surrounding the nucleus (electron cloud)
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Structure of the Atom
• Rutherford proposed that the nucleus
had a particle that had the same amount
of charge as an electron but opposite
sign
 based on measurements of the nuclear
charge of the elements
• these particles are called protons
 charge = +1.60 x 1019 C
 mass = 1.67262 x 10-24 g
• since protons and electrons have the
same amount of charge, for the atom to
be neutral there must be equal numbers
of protons and electrons
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The Nuclear Atomic Model
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Rutherford’s Model of
the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
“If the atom is the Cowboys Stadium, then the
nucleus is a marble on the 50-yard line.”
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2.2
Atomic Theory from 1800-1911
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Some Problems
• How could beryllium have 4 protons stuck
together in the nucleus?
shouldn’t they repel each other?
• If a beryllium atom has 4 protons, then it
should weigh 4 amu; but it actually weighs
9.01 amu! Where is the extra mass coming
from?
each proton weighs 1 amu
remember, the electron’s mass is only about
0.00055 amu and Be has only 4 electrons – it
can’t account for the extra 5 amu of mass
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There Must Be Something Else There!
• to answer these questions, Rutherford
proposed that there was another particle in
the nucleus – it is called a neutron
• neutrons have no charge and a mass of 1
amu
mass = 1.67493 x 10-24 g
slightly heavier than a proton
no charge
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The Sub-Atomic Particles
Subatomic
Mass
Mass
Location Charge Symbol
Particle
g
amu
in atom
Proton
1.67262 1.00727
nucleus
+1
p, p+, H+
empty
-1
e, e-
0
n, n0
x 10-24
Electron
0.00091 0.00055
x 10-24
Neutron
1.67493 1.00866
space
nucleus
x 10-24
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Elements
• each element has a unique number of
protons in its nucleus
the number of protons define the element
• the number of protons in the nucleus of an
atom is called the atomic number
the elements are arranged on the Periodic
Table in order of their atomic numbers
• Each element also has a corresponding
mass number.
Protons + neutrons.
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Elements
• Each element has a unique name and symbol.
 The symbol is either one or two letters
 One capital letter or one capital letter + one lower case letter.
H = Hydrogen = “water-former”
Br = Bromine = ‘stench’
Liquid Bromine
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The Periodic Table of Elements
Atomic number
Element symbol
Atomic
mass
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Elemental & Isotope Notation
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
Atomic Number
A
ZX
238
92
Element Symbol
U
Isotope Notation = Uranium-238
2.3
Example: How many protons, electrons,
and neutrons are in an atom of 52
24 Cr ?
Given:
Find:
Concept Plan:
52
24 Cr
therefore A = 52, Z = 24
# p+, # e-, # n0
symbol
symbol
Relationships:
Solution:
Check:
atomic
number
atomic & mass
numbers
# p+
# e-
# n0
in neutral atom, # p+ = # emass number = # p+ + # n0
Z = 24 = # p+
# e- = # p+ = 24
A = Z + # n0
52 = 24 + # n0
28 = # n0
for most stable isotopes, n0 > p+
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Isotopes:
• Isotopes = atoms of the same element with different
masses.
• Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
Carbon-11
Carbon-12
Carbon-13
Carbon-14
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Neon
Number of
protons
Symbol
Number of
neutrons
A, mass
number
Percent
natural
abundance
Ne-20 or
20 Ne
10
10
10
20
90.48%
Ne-21 or
21 Ne
10
10
11
21
0.27%
Ne-22 or
22 Ne
10
10
12
22
9.25%
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Mass Spectrometer
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Tro: Chemistry: A Molecular Approach, 2/e
Mass Spectrum
• A mass spectrum is a
graph that gives the
relative mass and relative
abundance of each
particle
• Relative mass of the
particle is plotted in the
x-axis
• Relative abundance of
the particle is plotted in
the y-axis
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Atomic Mass
Atomic mass is the mass of an atom in atomic
mass units (amu)
By definition:
1 atom 12C “weighs” 12 amu
1 amu = 1.6606 x 10-24 g
On this scale
1H
= 1.008 amu
16O
= 16.00 amu
3.1
Average Atomic Mass
• Because in the real world we use large
amounts of atoms and molecules, we use
average masses in calculations.
• Average atomic mass is calculated from
the isotopes of an element weighted by
their relative abundances.
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Natural lithium is:
6Li
(6.015 amu) = 7.42%
7Li
(7.016 amu) = 92.58%
Average atomic mass of lithium:
(7.42 x 6.015) + (92.58 x 7.016) / 100 = 6.941 amu
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Charged Atoms
• The number of protons determines the
element.
All sodium atoms have 11 protons in the nucleus.
• In a chemical change (aka: a reaction), the
number of protons in the nucleus of the atom
doesn’t change, however the number of
electrons may.
• Atoms in a compound that gain or lose
electrons and have a charge, these are called
ions.
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Types of Ions
Cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
Cl-
17 protons
18 electrons
Anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
2.5
Ions of Hydrogen
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Ions
A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
Nitrate
Polyatomic
Ion
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Practice—Fill in the Table.
Ion
p+
e-
-
Cl
K+
2-
S
Sr
2+
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Practice—Fill in the Table,
Continued.
+
-
p
e
1-
17
18
1+
19
18
S2-
16
18
38
36
Ion
Cl
K
Sr
2+
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Example —Find the Number of Protons
and Electrons in Ca2+.
Review
• What is the atomic number of boron, B?
• What is the atomic mass of silicon, Si?
• How many protons does a chlorine atom have?
• How many electrons does a neutral neon atom have?
• Will an atom with 6 protons, 6 neutrons and 6
electrons be electrically neutral?
• Will an atom with 27 protons, 32 neutrons, and 27
electrons be electrically neutral?
• Will an Na atom with 10 electrons be electrically
neutral?
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