People asked the question – for thousands of years: What is matter made of? The ancient
philosopher Democritus believed that the universe consisted of an infinite number of
indestructible atoms moving in empty space. He thought that atoms differed in size, weight
and shape, but that all atoms were made from the same material. Most ancient thinkers
disagreed with him and sided with Aristotle, who believed that matter was continuous,
infinitely divisible, and made up of four elements: earth, air, fire, and water.
Things changed in the 18th and 19th century when experimentation allowed scientists to
accumulate data that could explain atomic theory.
1803
Dalton’s Atomic Theory:
1. All matter consists of atoms: which are tiny indivisible particles of an element that cannot
be created or destroyed. Democritus, 2000 years previous to Dalton came up with the
idea of atoms)
2. Atoms of one element cannot be converted into atoms of another element by chemical
means. In chemical reactions, atoms from elements recombine to form different
substances.
C5H12
(l) +
8O2 (g)
5 CO2 (g) +
6 H2O (l)
Carbon atoms and hydrogen atoms from the molecule pentane combine with
oxygen atoms in the molecule oxygen to form the molecules carbon dioxide and
water.
3. Atoms of an element are identical in mass and other properties and are different from
atoms of another element.
4. Compounds result from the chemical combination of specific ratios of atoms of different
elements.
SO3 / SO2 ; CO / CO2 ; FeCl2 / FeCl3 ; UF3 / UF4 / UF6
Dalton’s Atomic Theory parallels the mass laws: all matter consists of indivisible atoms
of a fixed and unique mass, mass remains constant during a chemical reaction because
the atoms form new combinations, each compound has a fixed mass due to the fact that a
compound is composed of a fixed number of each type of atom, different compounds
made up of the same elements exhibit multiple proportions because they exist in whole
number ratios.
1897
J. J. Thomson: the atom as plum pudding
In the 19th century, various investigators were using discharge tubes to study the conduction of
electricity in gasses.
1
The discharge tube is a sealed glass tube that contains two metal plates called electrodes. One
electrode, the anode, is given a positive charge. The other electrode is the cathode, and is given
a negative charge. When the difference in charge between the electrodes was sufficiently large
enough, a beam of radiation, called a cathode ray, is emitted by the cathode. Under vacuum,
the ray can be detected when it impacts a fluorescent screen. It was noticed, that if a positively
charged object was placed near the beam that the beam bended towards the positive plate.
Thus, since opposites attract, the beam must be negatively charged!
J. J. Thomson had discovered electrons (although he called them “corpuscles”). Thomson found
that when different cathodes were used, the electron beam generated was the same, thus he
concluded that electrons are a fundamental and universal constituent of matter. He was unable
to determine the mass or charge of an electron separately, but he found the ratio of the
electron’s charge to its mass:
e
= 1.759 x 108 C/g
m
e = magnitude of the electron charge in Coulombs (C)
m = mass of the electron in grams
Thomson did not know how positive charge was dispersed in an atom, but since atoms were
found to be neutral species, the negatively charged electrons were counteracted by positively
charged species. Thomson believed the atom to appear something like:
+9 uniformly distributed positive
charge
-9 “corpuscles”
of negative
charge
_
_
_
_
_
_
_
_
_
Controlled electron beams are now used in TV picture tubes (see additional material at
the end of these notes), x-ray tubes, oscilloscopes, computer monitors, and electron
microscopes.
2
Cathode ray tube as beam bends towards a positive plate
Cathode ray tube as with beam uninterrupted
1909:
Robert Millikan: the oil-drop experiment
Millikan determined the charge of an electron. He used an apparatus, as shown below, to
produce tiny oil droplets. Very fine oil droplets were sprayed into a chamber and then were
allowed to fall between two charged plates where they were then observed, visually. The air
inside the chamber was exposed to x-rays, which displace electrons from air molecules resulting
in both negatively charged electrons and positively charged air molecules. These species come
in contact with the oil droplets. Some of the oil droplets pick up the electrons, thus acquiring a
net negative charge; other droplets pick up the positively charged fragments. Each charged
droplet is under the influence of two forces: gravity, and the force exerted by the charged
plates. By turning on the electric field, Millikan could control the rate of the droplet falling. He
could make the drop fall more slowly, rise, or even suspend it in midair! (Notice the lower plate
is negatively charged, and if the droplet is negatively charged, the plate will repel that oil
droplet). Millikan chose a particular oil droplet and measured its rate of falling with the electric
field turned on. Then, with the electric field turned off, the rat of falling of an oil droplet was
measured again. A comparison of the two rates allowed Millikan to calculate the droplet’s
charge.
3
After studying many droplets, Millikan confirmed that the charges on the droplets were always
some whole number multiple of a minimum charge. Different oil droplets are able to pick up
different numbers of electrons, so the minimum charge must be due to a single electron. He
determined that no charge existed that was less than 1.602 x 10-19 C. Thus, that must be the
charge of a single electron.
Using J.J. Thomson’s equation :
e
= 1.759 x 108 C/g
m
where e now equals: 1.602 x 10-19 C, Millikan could solve for the mass of a single electron:
9.109x 10-28 g.
Since electrons are negatively charged species we can place negative charges in front of
(-)1.759 x 108 C/g and (-) 1.602 x 10-19 C.
1910
Ernest Rutherford: the gold foil experiment
Thomson’s work with cathode rays and the discovery of positive ion beams led scientists to
believe that every atom contains electrons and that the removal of those electrons leaves behind
a positive ion that contains most of the atom’s mass. The inner structure of that atom was still a
mystery. J.J. Thomson has his theory (the plum pudding model), and soon, Rutherford, with
the help of his co-worker Hans Geiger, came up with their own. Early in the 20th century
scientists had discovered radioactivity. Rutherford used one of these radioactive particles, the
alpha particle, in his experiments.
4
They performed the experiment where a very thin piece of gold foil was bombarded with alpha
particles, and then they observed the path that the alpha particles took when they emerged from
the foil. Rutherford found that most of the particles went straight through the foil undeflected,
or deflected only slightly. These results were consistent with Thomson’s model of the atom. If
the atoms in the foil consisted of electrons embedded in a homogenous sphere of positive
electricity, then the electric field acting on the positively charged alpha particles would be
evenly balanced, and the particles would pass through the foil with very little deflection. The
embedded electrons were too small to cause deflections, it would be like a ping pong ball trying
to deflect a baseball thrown by Randy Johnson!
If Rutherford and Geiger had stopped there, they would have drawn the same conclusion about
the structure of the atom that J.J Thomson had – but they persevered, and several experiments
later they began to see alpha particles deflected backwards!
It seemed that in these experiments, the alpha particles were being repelled by something small,
dense, and positive within the gold atoms. Rutherford therefore concluded that there was a
region of positive charge within the atom (the nucleus), and as such, only when alpha particles
encounter this mass will they be deflected. And since most alpha particles passed through the
atom without being deflected, the size of this region must be small with respect to the overall
size of the atom. And since J.J. Thomson and Millikan had already shown that the electrons
could easily be removed from the atom, those electrons must have been separate from the
positive mass in the atom. Thus, Thomson’s model had to be abandoned and a new model, the
nuclear atom model was adopted.
In the nuclear atom model, the atom consisted of a small, but massive, positively charged
nucleus surrounded by negative electrons. Why massive? Almost all of the mass of the atom is
retained in the nucleus. He called the positive particles protons, and predicted nuclear charge
with remarkable accuracy. Rutherford’s model explained the charged nature of matter, but it
did not account for the atom’s mass. It would be another 22 years before James Chadwick
discovered another piece of the atomic puzzle, the neutron – an uncharged species that also
resides in the nucleus.
5
_
_
_
_
_
++
+ +
+
_
The atom is electrically neutral, spherical species that contains a positively charged nucleus
surrounded by one or more negatively charged electrons. The electrons move rapidly around
the nucleus and are held there in space by attraction with the positively charged nucleus (much
like the planets orbiting the sun). The nucleus, which takes up 1 ten-trillionth of the volume but
makes up 99.97% of the atom’s mass is incredibly dense. The atom’s total diameter is about
10,000 times the diameter of the nucleus!
The nucleus is composed of neutrons and protons. The protons are positively charged and the
neutrons have no charge at all, they just contribute to the overall mass of the atom. The
magnitude of the charge of a proton is equal to that of an electron, but the electron is negatively
charged. An atom is neutral because the number of protons ALWAYS equals the number of
electrons.
Atomic Notation: 3 Types
A = Atomic Mass = protons + neutrons
Z = Atomic Number = protons
X = Atomic Symbol = the elements letter designation
Type 1
Type 2
Type 3
A
Z
A
X
12
6
X
12
C
This type came first and gives the most
information
C
This type came second as the first type is
redundant. You do not need to indicate that
carbon has an atomic number of 6, all
carbon atomic numbers are 6.
X – A C -12
This type came last and is the easiest to type,
and still relays all the info you need. This
symbol is spoken, “carbon twelve.”
6
All atoms of a particular element have the same atomic number, and different elements have
different atomic numbers. On most periodic tables, the atomic number is indicated over the top
of the element.
The total number of protons and neutrons together is the atomic mass of an element. Again, on
the periodic table, the atomic mass is found beneath the element. Thus, given a particular
atom’s mass and knowing that the number of protons remains constant, the number of neutrons
can then be calculated.
All atoms of a particular element have the same number of protons (atomic number) but do not
have the same mass. All carbon atoms have 6 protons, but only 98.89% of carbon atoms have 6
neutrons. A very small percentage of carbon atoms have 7 neutrons.
Isotopes of an element are atoms that have the same number of protons and electrons but differ
in the number of neutrons.
Carbon isotopes include 12C, 13C, 14C. Each of these species have 6 protons and 6 electrons, they
are neutral species, but the number of neutrons changes.
Chemical properties, if you remember, depend on the electrons. Since each isotope has the
same number of electrons, their chemical properties are nearly identical, even though they differ
in mass.
Particle
p
n
e
Charge
+
n/a
-
Location
nucleus
nucleus
orbit-
AMU
1
1
0
Even though there are only 109 elements on the periodic table, the existence of isotopes has
given rise to more that 1450 different atomic species!
Most elements have one isotope that more predominant that the others, (e.g. carbon). But
others have significant percentages of their isotopes existing in nature: Mg consists of 78.7%
24Mg, 10.13% 25Mg, and 11.17% 26Mg. Other elements, such as Xe, and Sn, have 9 and 10
naturally occurring isotopes, respectively.
7
Hydrogen is the only element whose isotopes have been given special names. Most abundant is
hydrogen, whose nucleus consists of a single proton. The other isotopes are deuterium, with 1
neutron and 1 proton, and tritium, which has 2 neutrons and 1 proton.
Using a mass spectrometer, the isotopic make-up of an element can be determined, as well as
the relative abundance of each isotope. Each isotope contributes a certain percentage of its
mass to generate the overall atomic weight of a particular element. It is recognized that each
isotope contributes to the average mass in proportion to its abundance. Atomic masses
obtained in this way are said to be weighted averages.
For example: Gallium has two naturally occurring isotopes, 69Ga which has a mass of
68.9257 Da, and 71Ga which has a mass of 70.9249. The percent abundance of 69Ga is
60.27% while 71Ga makes up 39.73% of the element. Calculate the chemical atomic
weight of Ga.
Isotope
69Ga
71Ga
Isotopic Mass (Da)
68.9257
70.9249
*
*
Fractional Abundance
0.6027
0.3973
=
=
Product (Da)
41.54
28.18
________________
Sum =
69.72 Da
Examining the periodic table we see that this is the atomic mass of Ga!
Reassessing Modern Atomic Theory:
1. all matter is composed of atoms: but they are divisible and are composed of even smaller
units; protons, electrons, and neutrons, but the atom is the smallest body that retains the
unique identity of the element.
2. atoms of one element cannot be converted into atoms of another element by chemical
means: this can occur in nuclear reactions but not in a chemical reaction!
3. all atoms of an element have the same number of protons and electrons, they only vary in
their number of neutrons. It is the electrons that determine the chemical properties of the
element. We treat a sample of the element as having an average mass but still being
composed of the same species.
4. compounds are formed by the chemical combination of two or more elements in a
specific ratio.
: the central concept of counting in chemistry.
Typically, we perform a chemical reaction, and we take grams of substance A and react it with
grams of substance B. But if we examine what is really going on, we are talking about this
chemical reaction as each atom or molecule reacts with other atoms or molecules. HOW then
can we talk about quantities of things that are just soooooo small. We are not asked to go into
the balance room and weigh out 5 molecules of A . . . but we can, using this concept of the mole,
8
determine how many molecules/atoms are involved in chemical reactions. Please remember
this – it is not conceivable to give instructions to someone with final answers in molecules,
moles, ions, or atoms in experimental situations – as we do not have a way to measure these! If
you are asked to report amounts of reactants or products in a calculation, your final answer
(unless specifically indicated otherwise) will be in experimentally MEASUREABLE values
(grams, kg, mL, L and so on).
The mole (n): a unit derived by chemists to count up the chemical entities involved in chemical
reactions which can be determined by weighing the substance.
-defined as the amount of a substance that contains the same number of entities as there
are atoms in exactly 12 grams of 12C.
- the mole specifies the number of objects in a fixed mass, therefore 1 mole of a substance
represents a fixed number of chemical entities
1 mole of substance = 6.022 x 1023 entities
Think about the size of this number. 109 is a billion. 1018 is a billion billion. 1023 is one hundred
thousand billion billion. If you had 6.022 x 1023 dollars you could spend a billion dollars a
second for your entire lifetime and still have used less than 0.001% of your money. If C atoms
were the size of peas, 6.022 x 1023 of them would cover the entire surface of our planet to a depth
of over 100 m!!
those entities could be atoms, molecules, formula units, ions NEVER grams – EVER.
Remember this – 6.022 x 1023 never appears with the unit grams
1 mole 12C = 6.022 x 1023 carbon atoms
1 mole H2O = 6.022 x 1023 H2O molecules
1 mole NaCl = 6.022 x 1023 formula units
1 mole of electrons = 6.022 x 1023 electrons
1 mole of Na+1 ions = 6.022 x 1023 Na+1 ions
1 mole of electrons = 6.022 x 1023 electrons
There are several relationships to be familiar with: the first is:
1Da = 1u = 1 amu
The central relationship between the mass of 1 atom and the mass of 1 mole (which is a
collection of a LOT of atoms!) of atoms comes from the periodic table. The atomic mass is
expressed in amu (u, or Da)– which is numerically equivalent to the mass of one mole of atoms
of the element expressed in g/mole.
- Thus, 1 Na atom has a mass of 22.99 amu = 1 mole of Na atoms with a mass of 22.99 g
- 1 Si atom has a mass of 28.09 amu = 1 mole of Si atoms with a mass of 28.09 g
- Therefore, our units can be g/mole. This means that we can say that 1.0078 amu for
Hydrogen = 1.0078 grams/mole for Hydrogen.
Molar Mass: the mass of a substance per 1 mole of its entities (atoms, molecules, ions, or
formula units). Units for molar mass are grams/mole. Once again, the periodic table is used to
calculate the molar mass (previously we called this molecular mass – it’s the same thing!!)
9
1.) to find the molar mass of an element, simply look at the atomic mass of an element on
the periodic table.
a.) monatomic elements: Cu, Fe, Mg etc . . . Their molar mass value is the same as
their amu value – thus the molar mass of Cu is 63.55 grams/mole ; for Fe it is 55.85
grams/mole ; and for Mg it is 24.31 grams/mole.
b.) diatomic elements: X2: H2, N2, O2, F2, Cl2, Br2, I2: have to take into account that
there are 2 things present! So the molar mass of O2 is 2xmolar mass of 1
oxygen:
O2 molar mass
2 x 15.9994 = 31.9988 grams/mole
c.) the same applies to sulfur, whose major form exists as S8
S8 molar mass
8 x 32.066 = 256.528 grams/mole
2.) to find the molar mass/molecular mass of a compound, we perform the same
calculation . . .
a.) for ionic compounds we call this the formula weight: Cr2(SO4)3
this species results from Cr+3 + SO4-2
it would be named: chromium (III) sulfate
The formula weight would be determined by taking the sum of
all the atoms present in the compound:
Cr2(SO4)3
Cr = 2 x 51.9961 = 103.9922
S = 3 x 32.066 = 96.198
O = 12 x 15.9994 = 191.9928
Adding the masses of the atoms together we get:
103.9922 + 96.198 + 191.9928 = 392.183 grams/mole
The KEY point is that the subscript in the formula refers to the number of
atoms/ions present in the compound – and the parentheses indicate
multiplication – remember (SO4)3 indicates 3 SO4-2 groups: so we have SO4-2 +
SO4-2 + SO4-2 adding up the atoms present we see the 3 sulfurs and the 4+4+4 = 12
oxygens. The subscripts also indicate the number of moles of atoms or ions
present.
The mole is useful as it allows one to calculate the mass (g, kg, µg) of a substance in a
particular sample if you know the moles of the substance (we’ll use this when we talk
about solutions!!!). If you know the mass of a substance you can inversely calculate the
number of moles:
moles
grams
OR grams
10
moles USE molecular weight!
Remember the units for molecular weight are grams/mole
So it can be used as the conversion factor between the two
species!!
Given: 2.50 moles of Fe want to know ? grams
2.50 moles Fe x
55.847 grams Fe
= 1.40 x 102 grams Fe
1 mole Fe
Given: 45.789 grams of Fe want to know ? moles
45.789 grams of Fe x
1 mole Fe
= 8.1990 x 10-1 moles
55.847 grams Fe
Given: 25.12 moles of Al2(SO4)3 want to know ? grams
Step 1: calculate molecular weight of Al2(SO4)3
Step 2: convert moles to grams
following steps above:
Al2(SO4)3
Al: 2 x 26.9815 = 53.9630
S: 3 x 32.066 = 96.198
O: 12 x 15.9994= 191.9928
Sum = 53.9630+96.198+191.9928 = 342.154 grams/mole
25.12 moles Al2(SO4)3 x
342.154 grams Al 2 (SO 4 ) 3
= 8.595 x 103 grams Al2(SO4)3
1 mole Al 2 (SO 4 ) 3
Given: 500.23 grams of Al2(SO4)3 want to know ? moles
500.23 grams Al2(SO4)3 x
1 mole Al 2 (SO 4 ) 3
= 1.4620 grams Al2(SO4)3
342.154 grams Al 2 (SO 4 ) 3
The periodic table is your answer key. Many of the things that we will talk about in class can be
found directly on the periodic table! The atomic mass of an element, the number of protons, the
number of electrons, and thus, through some simple math, the number of neutrons. Given the
talk about weighted averages, when you calculate the weighted average mass of an element,
you can check your answer by looking on the periodic table!
11
1. each element has a box that contains its atomic number, atomic symbol, and atomic mass.
The elements are arranged by increasing atomic number (increasing number of protons)
and with few exceptions this translates into being arranged by increasing atomic mass
(notice Co is heavier than Ni, Th is heavier than Pa, U is heavier than Np)
2. the elements are arranged in periods (by rows) and also by groups (columns). Each row,
or period, is numbered 1-7 and each group, or column, is numbered from 1-8 and
includes either the letter A or B.
3. the groups with the letter A are considered to be the main group, or representative
elements. The B group elements are known as the transition or the inner transition
elements. The inner transition elements run from Ce to Lu and Th to Lr.
There is a clear marking on the periodic table which indicates the division between the metals
and non-metals. This staircase line that runs from boron (B) down to polonium (Po) divides the
metals, which are on the left hand side of the line, from the non-metals, which are to the right of
the line. All elements which share a SIDE with the line (NOT a corner!!) are known as the
metalloids, which retain properties of both metals and non-metals.
Additionally, groups have been termed families, and just like real people families, elements in a
family share characteristics. These elements have similar chemical properties while the
elements in a period (row) have very different chemical properties.
Group IA – except for H: alkali metals
Group IIA – alkaline earth metals
Group VIIA – halogens
Group VIIIA – noble or inert gasses
_____________________________________________________________________________________
12
Supplemental Material:
from http://entertainment.howstuffworks.com/tv2.htm
The Cathode Ray Tube
Almost all TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their images. LCDs and plasma
displays are sometimes seen, but they are still rare when compared to CRTs. It is even possible to make a television screen out
of thousands of ordinary 60-watt light bulbs! You may have seen something like this at an outdoor event like a football game.
Let's start with the CRT, however, because CRTs are the most common way of displaying images today.
13
The terms anode and cathode are used in electronics as synonyms for positive and negative
terminals. For example, you could refer to the positive terminal of a battery as the anode and the Phosphor
A phosphor
negative terminal as the cathode.
In a cathode ray tube, the "cathode" is a heated filament (not unlike the filament in a normal light
bulb). The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of
electrons that naturally pour off a heated cathode into the vacuum.
Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode.
In a TV's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight
beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons
flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This
screen is coated with phosphor, which glows when struck by the beam.
is any
material
that,
when
exposed to radiation,
emits visible light. The
radiation
might
be
ultraviolet light or a beam
of
electrons.
Any
fluorescent color is really
a phosphor -- fluorescent
colors absorb invisible
ultraviolet light and emit
visible
light
at
a
characteristic color.
As you can see in the above drawing, there's not a whole lot to a basic cathode ray tube. There is
a cathode and a pair (or more) of anodes. There is the phosphor-coated screen. There is a
conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the
tube. However, in this diagram you can see no way to "steer" the beam -- the beam will always In a CRT, phosphor coats
the inside of the screen.
land in a tiny dot right in the center of the screen.
When the electron beam
That's why, if you look inside any TV set, you will find that the tube is wrapped in coils of wires. strikes the phosphor, it
makes the screen glow.
The following pictures give you three different views of a typical set of steering coils:
In
a
black-and-white
screen, there is one
phosphor
that
glows
white when struck. In a
color screen, there are
three
phosphors
arranged as dots or
stripes that emit red,
green and blue light.
There are also three
electron
beams
to
illuminate
the
three
different colors together.
There are thousands of
different phosphors that
have been formulated.
They are characterized
by their emission color
and the length of time
emission lasts after they
are excited.
(Note the large black electrode hooked to the
tube near the screen -- it is connected internally
to the conductive coating.)
14
The steering coils are simply copper windings (see How Electromagnets Work for details on coils). These coils are able to
create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field
that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils,
you can position the electron beam at any point on the screen.
15