History: From ancient Greeks to early 1900s

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Chapter 2
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History From ancient Greeks to early
1900s Democritus vs. Aristotle
Joseph Proust (1799) Law of Definite
Proportions (see 2 below)
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John Dalton (1808) Modern Atomic Theory
1. Elements composed of small, spherical,
indestructible particles, called atoms. Atoms of
the same element are identical. Atoms of
different elements are different.
2. Compounds are composed of 2 or more atoms from
different elements combined together in a fixed
ratio of small whole numbers.
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3. A chemical reaction involves only the
rearrangement of atoms it does not result in the
creation or destruction of atoms.
Law of Multiple Proportions If 2 elements form
more than one compound, the masses of one element
that combine with a fixed mass of the other
element are in ratios of small whole s.
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Law of Conservation of Mass Matter cannot be
created nor destroyed, thus in a chemical
reaction, no atoms are created or destroyed only
rearranged (3 above)
Crookes Tube (1877) Cathode Rays and the
Cathode Ray tube Remove gas - not quite
completely - light disappears - but if a
fluorescent screen placed in tube it glows. This
glow was called cathode rays, because they seemed
to flow from the cathode electrode to the anode
electrode. What caused this glow ?
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J. J. Thomson (1899) Set up the following
experiment with a Cathode Ray Tube
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What this showed was that this glow was a stream
of negative charge, because it bent toward the
positive plate. The cathode ray was also bent by
a magnetic field. This showed that the stream of
negative charge was composed of material
particles. He could actually measure the ratio
of mass to charge m / e. He performed this
experiment, using different gases and different
metals for the anode and cathode. In every
experiment, the cathode rays were always the same
with the same m / e ratio.
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He concluded that these negative particles must
be coming from either the cathode metal or the
gas in the tube, but since they were always the
same, they must be a common part of all
substances and since they were negatively
charged, he said they must be the basic particle
of negative charge, which Ben Franklin had
described over a century earlier and named the
electron.
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Thomson went one step further and concluded that
since all substances, according to Dalton,
contained only neutral atoms, these electrons
must be coming from inside the atoms. This was a
contradiction to Dalton, who said the atom was
indivisible, and the smallest possible particle.
Daltons Theory was modified as a result.
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Thomson proposed his own model for the atom,
which he named the Plum-Pudding Model
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Subsequently, An American Physicist was able to
determine the exact charge on this electron
-1.6022 x 10-19 coulombs (C). he was also able
to determine the mass of the electron 9.10 x
10-28 g. Obviously the electron is a very small
particle.
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In 1893, Henri Becquerel accidentally discovered
a new phenomenon. Some substances spontaneously
gave off certain particles and radiation. This
process was named radioactivity. Three types of
radioactive emissions had been discovered by
1900
1. Alpha particles ? charged particles of high
energy (a)
2. beta particles ? - charged particles of very
high energy (b)
3. gamma rays ? High energy electromagnetic
radiation (g)
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Ernest Rutherford, a colleague of Thomsons at
Cambridge performed the following experiment,
expecting to show experimental support for his
colleagues model
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Rutherford expected some very slight scattering
of a particles due to their attraction to the
electrons and due to the pudding like inside of
the atom. Instead over 99 of a particles were
unscattered, while a few were scattered at high
angles. Some even bounced straight back. His
only conclusion was that Thomsons Model was
incorrect.
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Rutherford proposed what is called the Nuclear
Model of the atom. He said that almost all the
mass of the atom was concentrated in a tiny space
in the center of the atom, which also contained
all of the positive charge. Electrons were
scattered around, as in the Thomson Model. But,
quite remarkably, Rutherford concluded that 99
of all the space in atoms was a perfect vacuum,
totally empty.
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If you took all atoms of earth and removed all
the empty space you would have a 0.8 mile
diameter sphere. Proton discovered at about
same time Found to be equal in charge to the
electron but about 1800 times more
mass. Prediction of neutron around 1910.
Discovered in 1932 Found to have no charge but
a mass about equal to the proton.
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Two new terms based on these discoveries Atomic
(Z) - of protons inside a nucleus. Also
equal to electrons outside the nucleus of a
neutral atom. Atomic is unique for each
element.
Mass of protons of neutrons in any
atom.
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Also, with these new discoveries, another flaw in
Daltons Atomic Theory is brought out. He said
that all atoms of the same element are identical.
It turns out that every element, except F, has
more than one type of atom occurring in nature.
They have the same of protons (they have to, in
order to be the same element), but they have
different s of neutrons.
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Isotopes 2 or more atoms of the same element
with different Mass s (different of neutrons)
The common way to designate different isotopes
where Z represents the Atomic , A the Mass and
X the symbol of the element.
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The Periodic Table There were many attempts to
organize elements into some kind of logical
pattern, but none made sense until Dimitri
Mendeleev, a Russian chemist presented his model
in 1869.
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Mendeleev organized the elements by atomic
weight, left to right, but also vertically by
similar properties. There were a few exceptions,
which he attributed to inaccurate atomic weights.
In actuality, the mistake he made, was that the
arrangement should not be by atomic weight, but
rather by Atomic Number.
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The Periodic Table gets its name from the fact
that properties of elements repeat themselves
(vertical columns), just like a pendulum swings
back and forth and each swing is called a period,
which is each row in the Table. Each column is
called a group or family.
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The elements are arranged in other ways, also, in
the Periodic Table. They are divided into
metals, non-metals, metalloids and Noble
gases. Metals For now we will define metals in
terms of physical properties 1. Conduct heat
and electricity 2. Are malleable Solid state
can be banged into thin sheets without shattering
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3. Are ductile Solid state can be formed into
thin wires 4. Solid state can be
polished 5.Have a wide range of melting and
boiling points
Metals are the largest group on the Periodic
Table. Every element left of the bold stepped
line beginning between B and C are elements.
This includes the 2 rows at the bottom of the
Table.
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Non-metals Basically the opposite of metals.
They dont conduct heat or electricity, are not
ductile or malleable, can not be polished and
generally have low to medium melting and boiling
points. Non-metals are found between the bold
stepped line and the last column.
Metalloids They are the bridge between metals
and non-metals. They have some properties of
both. They are the elements that touch the bold
stepped line.
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Noble Gases The elements in the last column
(beginning with He). All are gases at room
temperature. They have the lowest melting and
boiling points of all the elements. They either
do not chemically react at all or react very
little. We will discuss this later.
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Some elements only exist naturally as 2 atoms
bonded together in a molecule, such as H2. These
are called diatomic elements. There are 7 common
ones H2, N2, O2, F2, Cl2, Br2 and I2.
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Atoms form ions by gaining (anions) or losing
(cations) electrons. Nothing happens to the
nucleus in these type of reactions.
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Molecular Formula is the actual formula of a
compound.
Empirical Formula is the formula with the
smallest possible ratio of whole numbers. Many
times the molecular and empirical formulas for a
substance are the same. Some examples are H2O,
NaCl, H2SO4 however there are many cases in which
the molecular formula is not the same as the
empirical formula. Some examples are (the
empirical formula will be in parentheses) H2O2
(HO), Na2S2O4 (NaSO2) and C6H12O6 (CH2O).
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We can usually predict the molecular formula of
binary (containing 2 elements) ionic compounds or
compounds containing any 2 ions. The subscript
of the cation is the same number as the charge on
the anion and the subscript of the anion is the
same as the charge on the cation. Examples
potassium bromide, zinc iodide, ammonium nitrate
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Chemical Nomenclature The naming of
compounds. To become a chemist, it is necessary
to be able to speak and understand the language
of chemistry. The Naming of compounds, at one
time, was somewhat haphazard. We call those
names today, common names. Some are still used,
but by and large, we now have systematic methods
for naming compounds. We will begin to learn how
to do this with the simplest cases, those of
binary salts. A binary salt is a compound
between a metal and a non-metal. In other words,
binary means there are 2 and only 2 elements in
the compound.
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To name a binary compound involving only
Representative elements, the metal is named first
(with no changes) followed by the non-metal (with
any ending, such as gen, ine, ur, orous,
ic or ium, replaced by ide. For
example NaCl sodium chloride Al2O3
aluminum oxide Note that there is no need to tell
the of each type of atom with these names.
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When the 2 elements are both non-metals, it
frequently becomes necessary to use prefixes in
front of both names to indicate the number of
each of these elements in the compound. The
prefixes are listed in Table 2-4 on page 56. If
there is only one atom of the first element, the
prefix mono is usually omitted. ide is still
used for the ending. If the compound is an
oxide, frequently the last a of the prefix in
front of oxide is frequently omitted. For
example N2O4 is named dinitrogen tetroxide
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When a transition metal is involved, the naming
becomes a little more complicated. Usually,
transition metals have 2 common valences (Cu can
be 1 or 2, Fe can be 2 or 3). It thus
becomes necessary to distinguish between the 2
possibilities. The modern method is to use Roman
Numerals in parentheses after the metal name to
indicate its charge. For example FeO is
iron(II)oxide. (We know that Fe is 2 here
because O is almost always 2) Fe2O3 is
iron(III)oxide
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There is an older method that is still used by
some chemists and books, which attaches the
suffix ic or ous to the metal name, depending
on its charge. For the higher of the 2 possible
charges, ic is used and for the lower of the 2
charges ous is used. Also, the Latin name for
the metal is frequently used because it sounds
better, thus FeO is ferrous oxide and Fe2O3 is
ferric oxide.
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For now, we can define an acid as a substance
that yields H ions when dissolved in water. All
acids contain at least one H atom. Naming
acids Binary Acids HX Name as "hydro"
followed by X's name with "ine" or "ium" ending
replaced with ic acid. Example HCl is
hydrochloric acid.
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Oxygen containing acids (oxoacids) 1. If there
are 2 possibilities, with only O atoms
different, such as HNO3 and HNO2 or H2SO4 and
H2SO3 Use only the anion name Replace "ate"
ending with "ic acid" or sometimes "uric acid"
and "ite" ending with "ous acid" or sometimes
"urous acid". The above acids are nitric acid
and nitrous acid and sulfuric acid and sulfurous
acid.
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2. If there are 4 possibilities, add the prefix
"hypo" to the one with less O atoms than the "ous
acid" and add the prefix "per" to the one with
more O atoms than the "ic acid". For example
HClO4 perchloric acidHClO3 chloric
acidHClO2 chlorous acidHClO
hypochlorous acid
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A base can be defined as a substance that
produces OH-1 ions when dissolved in water. Most
are ionic compounds containing OH-1 and are named
as any ionic compound.
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Hydrate Sometimes, if a crystalline solid forms
in water, as the solid crystal forms, molecules
of water get trapped inside the crystal. This
process is not random but rather occurs the same
way every time the situation arises. The water
molecules are only weakly attracted to the rest
of the substance and basically retain their own
identity. To indicate this, a hydrate is written
with the of water molecules shown separately
from the rest of the formula, separated by a ?.
For example CuSO4? 5H2O is copper sulfate
pentahydrate.