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Chapter 2
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CHAPTER 2 – Building Blocks of Materials
Matter is everything that is around us that we interact
with – “anything that occupies space and has mass”.
A classification scheme that you have likely seen….
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Matter can generally be found in one of three forms:
Solids – rigid, occupying a defined volume
Liquids – fluid, occupying a defined volume
Gases – fluid, occupying the space available
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We can explain the states of matter using “The KineticMolecular Model of Matter” or “Kinetic-Molecular Theory”.
Indeed, it was the development of KMT that lead to the
conclusion that atoms and molecules existed. The theory
explains many different physical phenomena.
We will treat this in more detail in Chapter 11, but for now
it is important to understand two fundamental concepts:
1) Matter exists in discrete units – either atoms, molecules,
or ions.
2) Temperature is a measure of how fast atoms,
molecules, or ions are moving.
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Homogeneous and Heterogeneous mixtures are
classified based on whether or not we can “see” the
distinct components. For example, salt in water is a
homogeneous mixture as it is impossible to “see” the salt
in the water.
Pepper in water is a heterogeneous mixture as the
pepper floats and is easy to see. Indeed, pepper itself is
a heterogeneous material made with many different
types of material.
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Pure substances are composed of a single type of atom,
molecule, or ionic compound.
We often speak of “pure water” or “pure gold” but these
are actually only relative terms. It is virtually impossible to
have a “pure” substance as there are almost always
impurities present.
This has to do with the size of molecules. Consider that a
single millilitre of water contains 3.34 x1022 molecules. To
be pure, that would mean that all of these molecules
must be water and that there is nothing else present.
Not likely. Instead, we speak of things being “pure” when
we can no longer detect the impurities present.
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Q2.1 The ages of civilization can be classified as “Stone”,
“Bronze”, “Iron”, and “Silicon”. Which of these ages
constitutes the use of “pure” substances?
a)
b)
c)
d)
e)
Stone
Bronze
Iron
Silicon
None – a trick question!
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Q2.2 In the “Silicon age”, we rely heavily on computers
and transistors to carry on many functions. These are
made from silicon. Which of the following steps in the
process would employ a pure substance?
a)
b)
c)
d)
e)
Silica mineral
Silicon extracted from minerals
Silicon wafers
Transistors
Micro-chips
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Levels of Operation – Levels of Understanding
“Observable level” – this is the level that we operate at in
the laboratory and the world around us. We observe
properties such as taste, smell, colour, and feel. Chemists
add a whole layer of observation beyond these using
spectroscopy.
“Molecular level” – this is the level that nature operates at
in that everything is made up of atoms and molecules .
“Symbolic level” – this is the level that we use to connect
the two other levels. We use symbolic representations to
explain how molecular features lead to observable
properties. We use symbolic representations when we
discuss observable properties with other scientists.
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At both the symbolic and molecular levels, chemists
employ “atoms” as the basic building block.
Literally, “atoms” means “not cuttable” . It is a term that
originates in ancient Greece with a philosopher named
“Demokritus of Abdera”.
Although you can not “see” atoms in the same sense that
you can see the person next to you, we have many
methods that allow us to “see” atoms and much
evidence verifying their existence.
Hence, even though it is called “Atomic Theory”
because, of course, it could be wrong, no scientist thinks
in any other terms than atoms nor that there is another
explanation.
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All atoms occur as one of the 114 known elements.
Realistically, there are only 90 or so naturally occurring
elements and some of these are exceedingly rare.
The natural abundance of elements :
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Note that atoms, molecules, and elements are not
interchangeable terms.
An element is composed of atoms of the same type. For
example, elemental helium consists of atoms of helium
and these are all single atoms.
Elemental oxygen, on the other hand, consists of oxygen
molecules each of which has two oxygen atoms – it is O2.
Note that we refer to both the element “oxygen” and
“molecular oxygen” as “oxygen”. That is, air is
predominantly nitrogen (N2) and oxygen (O2) but we
would also say that carbon dioxide is a “molecule
composed of carbon and oxygen atoms” (CO2).
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Some elements occurring in their elemental form as
atoms:
Helium (He)
Neon (Ne)
Argon (Ar)
Krypton (Kr)
Xenon (Xe)
Radon (Rn)
Mercury (Hg)
Some elements occurring in their elemental form as
molecules:
Hydrogen (H2)
Oxygen (O2)
Nitrogen (N2)
Fluorine (F2)
Chlorine (Cl2)
Bromine (Br2)
Phosphorus (P4)
Iodine (I2)
Sulphur (S8)
NOTE: When we refer to these elements in the context
of chemistry, we are referring to the molecular form.
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All chemical compounds are composed of the 90 or so
naturally occurring elements. The difference between
one compound and another is:
a) which atoms – which elements – are present.
b) the relative number of atoms of each element.
c) how the atoms are connected to one another.
Compounds do not necessarily have the same chemical
properties as their elements. NaCl is the example usually
given:
Sodium is a shiny malleable metal which reduces
water to give an explosive mixture of hydrogen.
Chlorine is a diatomic poisonous gas.
Sodium chloride is a crystalline solid made of
sodium ions and chloride ions and found as table salt.
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+
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+
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Many compounds are “stoichiometric”
(“stoichiometry” is the measure of a compound – it tells
us the relationship between atoms/elements in a
compound or compounds in a chemical reaction).
This leads to the concept of a chemical formula which
shows the ratio of elements in a compound.
For example, carbon monoxide has a ratio of one
carbon to one oxygen (CO), whereas carbon dioxide
has one carbon to two oxygen atoms (CO2) . Carbonate
has three oxygens for each carbon (CO32-) while
percarbonate has four oxygens per carbon (CO42-). Each
is a separate chemical species with its own observable
properties and chemical characteristics.
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Chemical compounds can undergo transformation from
one compound to another. (Elements don’t change!)
Consider carbon monoxide. It can pick up an oxygen
atom to become carbon dioxide:
CO
+
½ O2
CO2
The chemical reaction involves a change in the
chemical compounds from reactants (those on the left
hand side) to products (those on the right hand side).
During a chemical reaction, there is a redistribution of the
atoms so that new substances are formed. The products
have different properties from the reactants – they are
different chemical compounds.
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Q2.3 For the molecule heme b (a common component
of hemoglobins) is C34H32O4N4Fe. This means that the
number of hydrogen atoms is:
a)
b)
c)
d)
e)
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Not determinable from the formula
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Q2.4 For the following reaction,
H2
+
O2
H2O
what are the stoichiometric factors?
a) 1, 1, 1
b) 2, 1, 1
c) 2, 1, 2
d) 2, 2, 2
e) 1, 0, 1
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Equipped with his five senses, man explores the
universe around him and calls the adventure
“Science”.
-Edwin Hubble, The Nature of Science, 1954.
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TUTORING Schedule for FALL 2012
Time
Monday
830
Tuesday
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Thursday
Kenton
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Friday
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1200
1230
Shirley
Shirley (12:30 to 2:30)
(12:30 to 4:30)
1300
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Travis
(2:30 to 4:30)
1500
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Atoms are not actually the smallest component of
matter. In the 1890s, it was realized that there are
actually smaller “particles” which build atoms.
We now know that atoms are made up of electrons,
protons, and neutrons.
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Atoms are composed of a nucleus containing protons
and neutrons, surrounded by electrons. Protons have a
positive charge and electrons have a negative charge –
leading to electrostatic attraction between the two
particles. Neutrons do not have a charge or are neutral.
Neutral atoms have equal numbers of protons and
electrons. If an atom loses electrons, it can become a
positively charged “cation”. If an atom gains electrons, it
can become a negatively charged “anion”. The number
of nuclear components (protons + neutrons) does not
change in chemical reactions – just electrons.
The number of protons in the nucleus determines the
value of the atomic number, Z, and is distinct for each
element.
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Note: the electrons around the nucleus do NOT occur in
circles – they do not “orbit” around the nucleus.
This model of the atoms dates
back to Bohr (circa 1910) and
is not accurate!
The structure of the atom is
much more complicated but
much more explicable and
the subject of quantum
mechanics which we will get
to in Section 8.4.
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The number of protons – the atomic number – defines
which element an atom is. All carbon atoms, for
example, have 6 protons. All oxygen atoms have 8
protons. All hydrogen atoms have only one proton.
But not all carbon, oxygen, or hydrogen atoms are the
same. The number of neutrons that are found in the
nucleus can vary. These atoms with the same atomic
number but different mass are called “isotopes”.
For example, carbon-12 has 6 protons and 6 neutrons
carbon-13 has 6 protons and 7 neutrons
carbon-14 has 6 protons and 8 neutrons
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The mass number of an isotope is the sum of the atomic
number and the number of neutrons:
Mass number, A = Z + number of neutrons
= protons + neutrons
The atomic mass or “amu” reflects the mass number of
the isotopes and the relative abundances of the
different isotopes. For example,
98.89% of carbon atoms are 12C,
1.11% are 13C,
0.01% are 14C
as a consequence, Carbon is assigned a mass of
12.0107(8) amu – based on 12C having an exact mass of
12.
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Masses and Abundances of Some Isotopes, Atomic
Weights of the Elements
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Isotope Abundance
Mass
58Ni
66.034%
57.9353
60Ni
27.359%
59.9307
61Ni
3.911%
60.9311
64Ni
2.696%
63.9280
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Atomic Weight (or Mass) is a weighted distribution based
on the relative abundances:
At. Wt. = (% abundance of isotope 1) x (mass of isotope 1)
+ (% abundance of isotope 2) x (mass of isotope 2)
+ (% abundance…….) etcetera
For Hydrogen, this gives:
At. Wt. = 0.99985 x 1.0078 + 0.00015 x 2.0141 + 0 x 3.0161
= 1.00794 amu
For Oxygen, this gives:
At. Wt. = 0.99757 x 15.99492 + 0.00038 x 16.99913
+ 0.00205 x 17.99916 = 15.9994 amu
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How do we know what the weight of the isotopes is? How
do we know their abundance?
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How do we know what the weight of the isotopes is? How
do we know their abundance?
Answer: Mass Spectroscopy
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Q2.5 Chlorine occurs with two natural isotopes, 35Cl and
37Cl. If the atomic weight for chlorine is 35.453 amu,
roughly what are the percentages for these two
isotopes?
a)
b)
c)
d)
e)
10% 35Cl and 90% 37Cl
25% 35Cl and 75% 37Cl
50% 35Cl and 50% 37Cl
75% 35Cl and 25% 37Cl
90% 35Cl and 10% 37Cl
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Q2.6 Chlorine occurs with two natural isotopes, 35Cl and
37Cl and an atomic number of 17. The number of
neutrons in each, respectively, is:
a)
b)
c)
d)
e)
18 and 18
18 and 20
19 and 21
20 and 20
20 and 22
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Although everything around us is made up atoms, they
are too small to be useful at the laboratory level. A mass
spectrometer can count individual atoms, but it is
impossible to do so on the bench top.
Consider that a single hydrogen atom would weigh
1.6735 x10-24 grams, it would be impossible to weigh out
individual atoms for a chemical reaction.
Instead, we use a convenient quantity – the mole. This is
the SI Unit for the amount of substance, which we will
frequently designate by N.
“A mole of atoms of an element is the amount of it whose
mass in grams is numerically equal to its atomic weight.”
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That is, “one mole of hydrogen atoms” is equal to 1.0107
grams. Note that “one mole of hydrogen” is equal to
2.0214 grams since we would normally find hydrogen as a
diatomic molecule (H2).
The most important thing to realize is that one mole of
hydrogen has the same number of particles as one mole
of oxygen which has the same number of particles as one
mole of water. That one mole is one mole.
That is, one mole is 6.0221415 x1023 somethings – be they
molecules of water, atoms of iron, or chickens.
This is Avogadro’s constant, NA = 6.0221415 x1023 mol-1
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The concept of a mole allows us to work out the mass of
molecules by simply adding up the atomic weights of the
elements according to their appropriate stoichiometric
factors.
That is, the mass of a molecule is simply the sum of the
molar masses of its atoms.
Consider the reaction:
2H2 (g) +
1O2 (g)
2H2O (g)
This means that “2 molecules of hydrogen react with 1
molecule of oxygen to give 2 molecules of water.”
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2H2 (g) +
1O2 (g)
2H2O (g)
But it could also tell us that 2 moles of hydrogen gas
react with 1 mole of oxygen gas to give us 2 moles of
water.
Maybe, more importantly, it tells us that 4.0428 grams of
hydrogen react with 31.9988 grams of oxygen to give
36.0416 grams of water.
All of these same ways of thinking say the same thing.
They tell us the stoichiometric relationships for the
reaction of hydrogen with oxygen to generate water.
We are operating at the symbolic level but it gives us
something that is observable.
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Q2.7 Given an amu for carbon of 12, for hydrogen
of 1, and oxygen 16, what is the molar mass of
glucose – C6H12O6?
a)150 amu
b) 162 amu
c) 176 amu
d) 180 amu
e) 186 amu
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In Figure 2.12, we have a comparison of the molar
amount of a variety of substances:
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Q2.8 One mole of different elements occupy different
volumes. This is a consequence of:
a) larger atoms requiring more mass.
b) some elements are gaseous, some are liquid.
c) the size of the nucleus which is much bigger for
heavier elements.
d) the volume that something occupies is a function of
its density.
e) no one is really sure – it is one of the remaining
mysteries of chemistry.
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Information about atoms, atomic weights or mass,
reactivity, and a whole bunch more is all contained in the
Periodic Table.
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Developing the Periodic Table
Many people contributed to the development of the
Periodic Table – scientists that discovered the elements,
scientists that tried to make sense of chemical properties,
scientists that tried to organize chemical properties into
an ordered picture.
However, in 1869, Dmitri Mendeleev published his
thoughts in the form a table that recognized the periodic
nature of the chemical properties known at the time.
He also recognized that there were elements that had
not been discovered and left blanks in his table where
these elements would occur while predicting their
properties. For this reason, the Periodic Table is his
invention.
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Mendeleev based his table on “atomic weights”. As the
values for the weights become more refined, it became
clear that there were some problems. Some elements
were out of order (i.e. Ar and K).
In 1913, H.G.J. Moseley corrected Mendeleev’s
assumption. His work on core x-rays led to precise
measurement of the charge on the nuclei of the
elements – and the supremacy of the atomic number (Z)
as the organizing principle behind the periodic table.
The Law of Chemical Periodicity became:
The properties of the elements vary periodically with their
atomic numbers.
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“If, in some cataclysm, all scientific knowledge were to
be destroyed, and only one sentence passed on to the
next generation of creatures, what statement would
contain the most information in the fewest words?
I believe it is the atomic hypothesis (or atomic fact, or
whatever you wish to call it) that all things are made of
atoms — little particles that move around in perpetual
motion, attracting each other when they are a little
distance apart, but repelling upon being squeezed into
one another.
In that one sentence you will see an enormous amount
of information about the world, if just a little imagination
and thinking are applied.”
- Richard Feynman
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