Transcript Chapter 2

Chapter 2
History: From ancient Greeks to early 1900’s
Democritus vs. Aristotle
Joseph Proust (1799) – Law of Definite
Proportions (see #2 below)
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.
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.
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 ?
J. J. Thomson: (1899) – Set up the following
experiment with a Cathode Ray Tube:
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.
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.
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. Dalton’s Theory was
modified as a result.
Thomson proposed his own model for the atom,
which he named the “Plum-Pudding” Model:
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.
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)
Ernest Rutherford, a colleague of Thomson’s at
Cambridge performed the following experiment,
expecting to show experimental support for his
colleague’s model:
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 Thomson’s Model was incorrect.
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.
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.
PARTICLE
SYMBOL
LOCATION
Relative Charge
Relative Mass
(Mass #)
Electron
e-
Outside Nucleus
-1
0.005 = 0
Proton
p+
Inside nucleus
+1
1.000
Neutron
n0
Inside nucleus
0
1.000
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.
Also, with these new discoveries, another flaw in
Dalton’s 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.
Isotopes – 2 or more atoms of the same element
with different Mass #’s (different # of neutrons)
The common way to designate different isotopes;
A
Z X
where Z represents the Atomic #, A the Mass #
and X the symbol of the element.
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.
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.
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.
The elements are arranged in other ways, also, in the
Periodic Table. They are divided into metals, nonmetals, 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
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.
Non-metals – Basically the opposite of metals. They
don’t conduct heat or electricity, are not ductile or
malleable, can not be polished and generally have
low to medium melting and boiling points. Nonmetals 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.
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.
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.
Atoms form ions by gaining (anions) or losing
(cations) electrons. Nothing happens to the
nucleus in these type of reactions.
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).
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
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.
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.
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
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
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.
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.
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.
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 acid
HClO3 = chloric acid
HClO2 = chlorous acid
HClO
= hypochlorous acid
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.
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.