ELEMENTS, COMPOUNDS, CHANGES, HISTORY, DENSITY
CHEMISTRY - the study of the structure and composition of matter, the physical and
chemical changes it undergoes and the energy that is exchanged.
structure is the way in which the atoms are actually hooked together
composition is what different elements are actually present
Often the terms theory, law and hypothesis are confused. Most students believe that a
hypothesis turns into a theory first and then finally turns into a law. THIS IS NOT THE CASE!
A theory attempts to explain WHY something happens but makes no attempt at all to tell WHAT
happens.
A law is a mathematical equation that tells exactly WHAT is going to happen; it makes no attempt
at all to explain WHY.
A THEORY NEVER BECOMES A LAW--THEY DESCRIBE TOTALLY DIFFERENT THINGS!
A hypothesis is an educated guess which will eventually be accepted as a theory or a law,
depending on whether the hypothesis attempts to explain WHY or WHAT.
Two LAWS you should be familiar with now. These can both be mathematically proven.
(1) Law of Conservation of Matter
(2) Law of Conservation of Energy
Einstein used his famous equation of E = mc2 and proved that these two laws could be
combined only through a NUCLEAR reaction, not just a simple chemical reaction. Chemical
reactions deal with altering only the ELECTRONS in an atom; a nuclear reaction alters the
NUCLEUS of an atom.
E = energy
m = mass
c = speed of light
You can see by the equation that the amount of energy obtained by converting even a small
amount of mass is tremendous.
Chemistry is all around you and affects every aspect of your lives. Name some ways
chemistry impacts your life.
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A knowledge of chemistry is required in making important decisions which affect our
society. What decisions?
Matter is the physical "stuff" of the universe. It is defined as anything which has mass and
occupies space. More recently it has been defined as anything having the property of
inertia.
Energy is a very intangible thing to define. Probably the best definition is the capacity to do work
or transfer heat. There are two types of energy
Potential
Kinetic
BE SURE YOU UNDERSTAND THE DIFFERENCE IN WEIGHT AND MASS!
mass
(1) is the amount of matter present
(2) does not change with position
(3) is measured on an instrument called a BALANCE (uses masses)
weight
(1) is a measure of gravitational force on an object
(2) varies slightly with position due to gravitational force varying from place to
place
(3) is measured on a SCALE (has a spring in it)
All matter can be classified into two categories--homogeneous and heterogeneous
homogeneous means that it is one phase throughout;; completely uniform.
An example of something homogeneous is water--every portion of the water looks (and
is) EXACTLY like every other portion of water; completely uniform throughout
heterogeneous means that the matter is made up of more than one phase
An example of something heterogeneous is a chocolate chip cookie—you can see
chocolate chips, pecans, cookie dough. etc,
HOMOGENEOUS SUBSTANCES CAN ONLY BE ONE OF THREE THINGS--elements,
compounds or solutions
1. ELEMENT - matter composed of only one type of atom; the simplest form of matter;
cannot be changed to another element except by a NUCLEAR reaction (not
chemical).
.
(1) 114 have been discovered but only 109 have been named and their
names accepted;
(2) only 88 elements are naturally occurring; all others are man made
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(3) the symbol for an element is 1 capital letter OR 1 capital letter and 1
lower-case letter.
Fill in the symbol for each of the following elements:
____
carbon
____
sulfur
____
hydrogen
____
magnesium
____
oxygen
____
calcium
____
chlorine
_____ nitrogen
(4) The oldest-known elements have their symbols taken from their Latin
names
Fill in the symbol for each of the following elements whose symbol is taken from Latin
____
iron
____
sodium
____
lead
____
copper
____
silver
____
potassium
____
gold
____
mercury
2. COMPOUNDS are substances composed of more than one type of atom chemically
joined
(1) compounds can be changed to other compounds and separated by chemical
reactions (electrons only) and do not require nuclear reactions
(2) compounds do NOT have the same properties as the elements which make up the
compound.
For example: water is a compound which is made of 2 atoms of hydrogen and one atom
of oxygen. Water is a clear liquid which is colorless, odorless, tasteless and does not
burn. The two elements which make up water have very different properties--oxygen is a
gas which is very combustible and hydrogen is also a gas which will burn explosively
(3) over 10 000 000 different compounds are known today, most of them organic
(4) the chemical formula for a compound will have more than 1 capital letter in it
because it contains more than one element
What is the chemical formula for each of the following compounds?
_____ carbon dioxide
_____ water
_____ table salt
_____ glucose
_____ carbon monoxide
(5) the PROPORTIONS of elements in a compound do not change or you have changed
the compound.
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For example H2O (2 hydrogens to 1 oxygen) is water, but H 2O2 (two hydrogens to
two oxygens) is hydrogen peroxide which is what you put on a cut on your finger. The
proportions of hydrogen and oxygen are very different in these two compounds, and the two
compounds are also VERY different.
3. SOLUTIONS are a uniform mix of two or more substances which are NOT chemically bonded and
therefore, unlike compounds, each component retains it original identity and properties
A solution has TWO component parts-- a solute and a solvent.
SOLUTE is the component which "disappears"; whose phase is changed. ONLY if both
components are the same phase, the solute is the one in the LESSER quantity
For example: If sugar is put into water and it dissolves, the sugar is the solute because
it has disappeared or changed its phase. BUT if water is mixed with alcohol (both are the liquid
phase) and neither are going to change phases, you must know which is present in the greater
quantity in order to be able to tell which is the solute and which is the solvent.
SOLVENT is the component which remains unchanged. ONLY if both components are the same
phase, the solvent is the one in the GREATER quantity.
For example: If 400 grams of sugar are dissolved in 350 grams of water, the water is
the solvent because it is the one whose phase does not change. However, if 400 grams of water
are mixed with 500 grams of alcohol, the alcohol is the solvent because you consider quantity
when both components are the same.
Example 3-1: Aluminum chloride is a white crystal. Alcohol is a clear liquid. When these are mixed, a
clear liquid solution results. If 100 grams of aluminum chloride are mixed with 95 grams
of alcohol, what is the solvent?
When two liquids are mixed, there are TWO words which can describe their interaction together.
If they dissolve in each other completely and mix uniformly in any proportions, they are said to be
MISCIBLE. An example of miscible liquids is vinegar and water.
If the two liquids do not dissolve in each other and form two distinct layers, they are said to be
IMMISCIBLE. An example of immiscible liquids is oil and water.
II. HETEROGENEOUS SUBSTANCES ARE ALWAYS MIXTURES! They are not uniform in
composition. There is more than one phase present, and 95% of all things in the universe are
mixtures--some homogeneous solutions, some heterogeneous mixtures.
TO CLASSIFY MATTER, YOU MUST ALWAYS USE TWO WORDS--HOMOGENEOUS
ELEMENT, HOMOGENEOUS COMPOUND, HOMOGENEOUS SOLUTION OR
HETEROGENEOUS MIXTURE.
How would you classify the following substances?
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__________________ Vinegar (look at the label; if there is more than one substance listed in
the ingredients and the vinegar LOOKS homogeneous, it is a solution; if
there is only one thing listed in the ingredients, it is a compound)
__________________ birdseed
__________________ cooking oil
__________________ mouthwash
__________________ catsup (look at the label; if there is more than one substance listed in the
ingredients and the catsup LOOKS homogeneous, it is a solution; if
there is more than one ingredient and it does not look homogeneous, it is
a mixture.
__________________ water
__________________ silver
__________________ milk
Physical Properties of matter describe the substance itself (alone).
There are two types of physical properties
(1) Intensive properties do not change with amount; are used for
identification
Examples: density, melting point, boiling point, color,
(2) Extensive properties depend on the amount of matter present; these change
constantly and therefore cannot be used for
identification
Examples: mass, volume, length, heat
Chemical Properties of matter describe how the substance reacts with something else.
Examples of chemical properties are "How does it react with acids?" "Does it burn?" "Does it
REACT with water to form a new compound (chemical) or does it simply dissolve in water?
(physical)?
Physical changes in matter will change its physical state but not its composition; frequently physical
changes are reversible
Examples: grinding, bending, dissolving, and all physical state changes
Some physical state changes have specific names
solid changing to a liquid
_____________________
liquid changing to a vapor
_____________________
vapor changing to a liquid
_____________________
liquid changing to a solid
_____________________
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solid changing to a vapor
_____________________
vapor changing to a solid
_____________________
Chemical changes involve a NEW substances being formed. These types of changes are usually not
reversible
Examples: tarnishing, burning, digesting, rusting,
Clues to look for that indicate that chemical change has taken place are:
(1) a color change
(2) a texture change
(3) a gas produced
(4) a precipitate is formed (a precipitate is a solid product which forms in a
chemical reaction)
(5) a mass change
The main differences in solids, liquids and gases is the proximity of their particles.
In solids, the particles are very close together, touching each other and packed tightly enough
that the particles cannot move at all except to rotate on their axes.
In liquids, the particles touch each other lightly but can slide past each other as the liquid
FLOWS.
In gases, the particles are completely independent and do not touch each other at all.
SOLIDS - have definite shape and definite volume; usually HIGH density; expands only
slightly when heated; almost incompressible because particles are packed
very close together.
LIQUIDS - has definite volume but not definite shape; usually is less dense than solids;
expands more than solids when heated; almost incompressible; takes the
shape of its container; flows; particles are not quite as tightly packed
together as in solids
GASES - has neither definite shape nor definite volume; very LOW density; expands
greatly when heated; highly compressible; takes the shape of the
container; flows; particles are completely independent and do not touch
each other except when they bump into each other.
Density is the mass of a substance per unit volume or how much it weighs per given
volume; it is an intensive property and therefore can be used for identification since it does not
change with amount.
Putting the definition into a mathematical equation
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m
D = -----
V
In the beginning, you will probably choose to work most density problems using this equation
rather than dimensional analysis, and this means you must remember two things:
(1) if you are going to solve density problems with the density equation,
you must write the equation before you use it
(2) you must “play by the rules” and use only the proper units in the equation.
The “proper” units in this equation are:
(1) mass MUST be in grams; if it is given to you in other units, use DA to convert it to grams
BEFORE you put it in the equation
(2) volume MUST be in mL for liquids, cm3 for solids and L for gases.
If it is given to you in other units, you must use DA to convert it to the proper units before
putting it in the equation.
A MILLILITER and a CUBIC CENTIMETER ARE EXACTLY THE SAME AMOUNT of volume!
These are just two different terms which mean the same volume and are sometimes used to
designate whether a substance is a solid or a liquid—ml for liquids and cm 3 for solids. However,
particularly in medicine, liquids are almost always designated cm 3.
(3) if you use these labels, the density of a solid will therefore be g/cm 3; the
density of a liquid will be g/mL and the density of a gas will be g/L.
THERE ARE 3 VARIABLES IN THE EQUATION, AND YOU WILL BE ASKED TO SOLVE FOR ANY
ONE OF THESE WHEN THE OTHER TWO ARE GIVEN.
Example 3-2. A certain solid has a volume of 35.7 cm3 and weighs 85 grams. What is
its density?
Example 3-3 A liquid fills a graduated cylinder to the 45.8 mL mark. The empty graduated cylinder
weighs 26.5 grams and when it is filled with an unknown liquid to the 45.8 mL mark, the
cylinder and the liquid together weigh 70.0 grams. What is the density of the unknown
liquid?
Example 3-4
If the density of gold is 19.3 g/cm 3, what is the volume of 200 g of gold in cm 3?
Example 3-5 The density of liquid mercury is 13.6 g/mL. How much would 35.0 mL of mercury weigh in
pounds?
Example 3-6 Find the density of a rectangular solid whose dimensions are 3.4” by 1.2” by 1.7” if it is
known to weigh 1.500 kg.
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THE ONLY DENSITY WHICH MUST BE MEMORIZED IS WATER WHICH IS 1.00 g/mL.
This means that 1.0 gram of water has a volume of 1.0 mL and that 1.0 mL of water will weigh
1.0 gram. This is actually only true at ONE temperature (3.98o C); at all other temperatures the
density of water is slightly less than 1.0 g/mL, but it will round to 1.0 g/mL.
Example 3-7 How much would 500 mL of water weigh in grams?
1.0 liter of water in grams?
SUBSTANCES WHICH ARE LESS DENSE THAN WATER WILL FLOAT assuming that they do not
dissolve in the water nor react with the water.
Example 3-8 A certain substance has a mass of 3.2 kg and a volume of 1.5 cubic feet. Would you
expect this substance to float in water, assuming that it did not dissolve in the water nor
react with it?
(b) Would you expect this substance to float in mercury (density of mercury = 13.6 g/ml),
assuming that it did not dissolve in the mercury nor react with it?
SPECIFIC GRAVITY IS THE DENSITY OF A SUBSTANCE COMPARED TO THE DENSITY OF A
DESIGNATED STANDARD (which on earth is water). Therefore, if the density of lead is 11.1
g/cm3, the specific gravity of lead is 11.1 g/cm 3 compared to (divided by) the density of water
which is 1.0 g/cm3 or simply 11.1. Notice that specific gravity is a PURE number—it does not
have a label. It is simply an indication of how many times more dense that substance is than
water. A quick, easy way to think of specific gravity is that it is the density of the substance
without the label (since it is being compared to 1.0). Conversely, when the specific gravity of a
substance is given, you know that the density of that substance is the same number with a proper
label (g/cm3 for solids, g/mL for liquids and g/L for gases).
Example 3-9 Milk has a specific gravity of 1.12. How much would 1.0 gallon of milk weigh in pounds?
Example 3-10. What is the specific gravity of an unknown metal sphere whose volume is known to be
340.2 mm3 if it is known to weigh 35.9 pounds. Would this sphere float in water
assuming that it did not dissolve in the water nor react with it?
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(b) Would this sphere float in mercury (density of mercury = 13.6 g/ml) if it did not
dissolve in it nor react with it?
Atom - the smallest particle of matter which will exhibit the properties of that element. When broken
down smaller than an atom, the parts (protons, electrons, and neutrons) of different elements look
exactly the same. You cannot tell a proton in a gold atom from a proton in oxygen gas.
Atoms are very small—typically about 1 X 10-8 cm in diameter). 1.0 gram of lead contains
2.9 X 1021 atoms of lead. By comparison, the earth’s entire population is only 5 X 10 9 people.
(1) DEMOCRITUS, an ancient Greek and student of Aristotle, first suggested the existence of the atom
about 400 BC. He hypothesized that matter could be subdivided again and again until it would
finally reach a particle which could not be divided. This smallest “indivisible” particle he called
“atomos”.
(2) ARISTOTLE rejected this theory and banished Democritus from the country for his radical beliefs.
Aristotle hypothesized that matter was “continuous” in nature. That is, matter could be subdivided
again and again an infinite number of times, and you would never reach a basic particle which
could not be divided. It was not until the late 1700’s that anyone dared to question Aristotle’s
wisdom. Aristotle may have been a great philosopher, but he was not a scientist. I personally
hold Aristotle responsible for setting chemistry back 2200 years!
(3) ROBERT BOYLE AND ISAAC NEWTON were the first men to question the validity of Aristotle’s
hypothesis.
(4) JOHN DALTON was an Englishman who was the first to develop and publish a theory about how
atoms looked and behaved. He conceived of the atom as a solid sphere, much like a billiard ball.
The following are statements of John Dalton’s ATOMIC THEORY with “•” beside the parts
which are known NOT to be true today.

(1) all elements are composed of very small particles called atoms which are indivisible.
Today we know that atoms can be divided into protons, electrons, neutrons and almost 200
other subatomic particles

(2) all atoms of the same element are identical. We know that this is not true today due to
the presence of isotopes. Isotopes are atoms with the same number of protons (same
element) but different numbers of neutrons (different masses). For example, “regular”
carbon has 6 protons and 6 neutrons, but carbon-14 (radioactive carbon used in carbon
“dating”) has six protons and 8 neutrons.
(3) atoms of different elements are different
(4) atoms of different elements can combine with each other only in simple
ratios to form compounds
whole number
Chemical reactions occur when atoms are separated, joined or rearranged. However, atoms of one
element ARE NOT changed into atoms of another element by a chemical reaction. (only by
nuclear reactions)
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About 100 years ago, 2 of the 3 main subatomic particles were discovered.
(5) J. J. THOMSON discovered both electrons and protons by means of his CATHODE RAY
EXPERIMENT
Electrons are negatively charged particles, abbreviated e-; very lightweight—their mass is
considered negligible when describing the “mass” of an atom” because they actually weigh
about 1/ 1837 the mass of a proton or a neutron.
CATHODE TUBE RAY EXPERIMENT. (Draw it and describe it)
Know WHO, HOW and WHAT was discovered.
Electrodes are small pieces of metal through which electrical current is input.
In this case, the electrodes are embedded in the ends of a sealed glass tube and electricity is
input through the cathode. The (-) negative electrode is called the cathode; (+) positive electrode
is called the anode
Before their true nature was known or understood, electrons were called “cathode rays.” Now we know
that they are small negatively-charged particles which move at a very high speed. A stream of
electrons is called electricity.
Thomson conceived of the atom as a plum pudding model—the protons and electrons were mixed
together like a fruit salad (or plum pudding). The larger pieces of “plum” were the protons
swimming in a pudding of electrons.
(6) ERNEST RUTHERFORD worked for Thomson in his lab for a while. He performed the GOLD FOIL
EXPERIMENT which proved that the atom was mostly space and that all of the positive charge
was located in a very small central nucleus.
GOLD FOIL EXPERIMENT (draw it and describe it)
(7) ROBERT MILLIKAN – the only American who added to the concept of the atom by determining the
exact amount of electrical charge on an electron. He did this by means of the Oil Drop
Experiment.
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OIL DROP EXPERIMENT (draw it and know what happened and why)
(8) ANTOINNE LAVOISIER – a French chemist who is known as the “Father of Chemistry” because he
was the first to actually do experiments. In one experiment He actually weighed the reactants
(total) and then collected and weighed the products (total) and found that the mass did not
change as the reaction proceeded. Lavoisier is thus responsible for the Law of Conservation of
Matter.
(9) HENRY MOSELEY – a very young brilliant scientist who discovered the atomic number of all
elements known at that time by using X-rays. His name is not a household word because he
was killed at a very young age in World War I. Because of him, Great Britain does not require
military service for scientists.
(10) NIELS BOHR – a Danish scientist who originally worked for Thomson and Rutherford and was the
first to suggest the “planetary model” of the atom in which the electrons orbited the nucleus much
as the planets orbit the sun. Rutherford’s experiment (Gold Foil) indicated that all of the protons
were located in a nucleus but did not speculate as to where the electrons were located.
(11) JAMES CHADWICK – a British chemist who discovered the neutron in 1936. Scientists had looked
for another subatomic particle (other than the protons and the electron) for many years but
couldn’t find it. Chadwick also found out that the neutron is approximately the same mass as a
proton but had NO electrical charge whatsoever. Today we know that a neutron is a proton and
an electron which are joined in the nucleus.
(12) MAX PLANCK – a German physicist who is responsible for the quantum mechanical model of the
atom which we believe is true today. In this concept the electrons do not actually “orbit” the
nucleus but are found only in definite areas
based on the amount of energy they have.
PROTON – a positively-charged particle which is found in the nucleus. It is considerably more
massive than the electron (1837 times heavier)
ELECTRON – a very lightweight negatively-charged particle which is found somewhere outside
the nucleus. Its mass is considered to be negligible when determining the mass of an
atom. It weighs only 1/1837 that of a proton, but its negative charge is the same
magnitude as the positive charge of a proton.
NEUTRON – a particle found in the nucleus which is approximately the same mass as a proton
but does not have an electrical charge associated with it. It is neutral.
ATOMS ARE ALWAYS NEUTRAL PARTICLES – atoms contain the same number of protons as
electrons, and it will have varying numbers of neutrons depending on the particular
isotope.
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IONS – particles which do not have the same number of protons and electrons and therefore they
DO have an electrical charge associated with them. If a particle has more protons than
electrons, the ion will have a positive charge. If a particle has more electrons than
protons, it will have a negative charge.
ATOMIC NUMBER – the number of protons found in the nucleus of an atom. On the Periodic
Chart, it is the large whole number in the upper corner. Notice that the only thing which
makes one element different from another is the number of protons it contains. An atom
with six protons is always carbon; an atom with 5 protons is always boron; an atom with 7
protons is always nitrogen.
MASS NUMBER – the sum of the protons and the neutrons in the nucleus of an atom. The
mass number DOES NOT show on the Periodic Chart. The atomic mass (which does
show on the chart) is an average of all of the different isotopes of that element weighted
by their abundance.
ATOMIC MASS – the atomic mass that shows on the Periodic Table for each element is really a
weighted AVERAGE of the masses of all of the isotopes of that element, weighted by
their percentage of abundance. Every element on the Periodic Table has at least 2
isotopes; some of them have 20 or 30 or more isotopes.
Here is the equation to use to find the average atomic mass of an element:
Atomic Mass = (% isotope #1)(mass of #1) + (%isotope #2)(mass of #2) +
(%isotope #3)(mass of #3) + . . . . .
Please notice that even though you are finding an average, you do NOT divide by the number of
isotopes. The percentages of abundance take care of this for you. You must either change the
percentages to decimals before putting them into the equation OR you must be sure to remember
to press the % key on your calculator when you enter each abundance number.
Example 3-11
Naturally occurring chlorine is 75.53% chlorine-35 and 24.47% chlorine-37. What is
the average atomic mass which should be placed on the Periodic Chart for the element
chlorine?
Example 3-12 The element neon consists of three isotopes with masses 19.99, 20.99 and 21.99amus.
These three isotopes are present in nature to the extent of 90.92%, 0.25% and 8.83%
respectively. Calculate the atomic mass of neon.
Example 3-13 The element silver consists in nature of two isotopes, Ag-107 and Ag-109. The accepted
atomic mass of silver is 107.87 amu. What are the percentages of abundance of Ag-107
and Ag-109?
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CHEMICAL CONFIGURATION is a way of displaying an atoms or ions showing lots of information.
Mass number
charge (if any)
SYMBOL
Atomic number
Example 3-14 Write the chemical configuration for a particle containing 5 protons, 6 neutrons and 5
electrons.
Example 3-15 Write the chemical configuration for a particle which contains 8 neutrons, has a mass
number of 17 and contains 10 electrons.
Example 3-16 Write the chemical configuration for a particle which contains 18 electrons, has a mass
number of 32 and has a –2 charge.
Example 3-17 Write the chemical configuration for a particle which has a charge of +3 and contains 10
electrons and 14 neutrons.
Example 3-18 Write the chemical configuration for a particle which has a charge of +2, an atomic
number of 24 an contains 28 neutrons
Example 3-19 Write the chemical configuration for a particle which has a mass number of 31, contains
18 electrons and has a –3 charge.
Sometimes you are not given enough information to write the entire chemical configuration for a particle,
but you can at least identify it as to what element it is and whether it is an atom or an ion.
Example 3-20 What particle has a +2 charge and 12 protons?
What particle has 50 protons and 48 electrons?
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What particle has 44 protons and 44 electrons?
What particle has a –3 charge and contains 36 electrons?
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