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Atoms, molecules, and Ions
Chemistry is the science that studies the composition, structure and properties, and
interactions of matter
Matter is anything that occupies space, displays a property known as mass, and possesses inertia.
We classify matter (1) by its physical state as a solid, liquid, or gas, and (2) by its chemical
constitution as an element, compound, or mixture.
Before we classify matter by its chemical constitution, we must first distinguish between
physical and chemical change and between physical and chemical properties
What is property?
Any characteristic that can be used to describe or identify matter is called a property and
properties can be classified as
1) Intensive or extensive, depending on whether their value changes with the size of the sample.
2) Physical or chemical.
~ Physical properties and physical change
Commonly, a given kind of matter exists in different physical forms under different conditions.
For example: water exists as ice (solid), as liquid water and as steam (gaseous water).
Solid:
Liquid
Gas:
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The three form of matter-solid, liquid and gar-are referred to as the state of matter.
The temperature at which a matter change from the solid state to the liquid state is known as its
melting point, and when the reverse change occurs, this same temperature is referred to as the
freezing point. The temperature at which bubbles of the vapor begin to form in the liquid and
escapes to the surrounding is known as the boiling point. The boiling point of a liquid is affected
by the atmosphere pressure.
~ Chemical properties and chemical change
E.g. the rust that occurs when a bicycle is left out in the rain is due to the chemical combination
of oxygen with iron to give the new substance iron oxide. Rust is therefore a chemical property
of iron.
Table 1 Some Examples of Physical and Chemical Properties
Physical Properties
Temperature
Color
Melting point
Electrical conductivity
Chemical Properties
Amount
Odor
Solubility
Hardness
Rusting (of iron)
Combustion (of coal)
Tarnishing (of silver)
Hardening (of cement)
Chemistry in Action: cooking
Cooking is the oldest and most widespread application of chemistry. Every cook is a
chemist. The first chemical laboratories were kitchens in the Middle Ages. Recipes are the oldest
practical result of chemical research. Many chemical processes have been derived from cooking
techniques. These techniques include making sauces and meringues, baking, marinating and
tenderizing meat, and caramelizing sugar. Cooking is chemistry in action, with the added benefit
that you can eat the results. As you follow the recipe below for peanut brittle, make observations
about the changes that occur.
Recipe for Peanut Brittle
2 cups granulated sugar (sucrose)
1 cup corn syrup (glucose)
1 teaspoon water
3 cups peanuts, raw or roasted
3 tablespoons margarine
1 teaspoon vanilla
1 teaspoon baking soda
wooden spoon
hot mitts
candy thermometer
heavy, 2-quart saucepan
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CAUTION: Mixtures will be very hot.
1. To check the accuracy of your thermometer, bring a small pan of water to the boiling point.
Take the temperature of the boiling water. If the thermometer reads above or below 212°F, add
or subtract the same number of degrees from the temperatures in the following steps of this
recipe.
2. Use 1 tablespoon of margarine to lightly grease the surface of two baking sheets. Keep the
baking sheets slightly warm.
3. Combine the water, corn syrup, and sugar in the saucepan. Stir until the sugar is dissolved.
Place the thermometer in position in the pan. Be sure the thermometer does not touch the bottom
of the pan.
4. Heat the sugar solution rapidly to 280° F to ensure the desired concentration of sugar in
solution. As the mixture heats, the water evaporates and the concentration of the sugar solution
increases and a syrup forms. Only stir enough to keep the mixture from scorching.
5. When the syrup reaches 280° F, add the peanuts and margarine. Stir the mixture continuously
and heat to 300° F. The syrup will turn from clear to yellowish brown due to a process known as
caramelization.
6. At 300° F, called the hard crack stage, remove pan from heat immediately.
7. Immediately add the vanilla and baking soda and stir. The mixture will foam slightly due to
the release of carbon dioxide.
8. Pour approximately half of the mixture on each baking sheet and spread into as thin of a layer
as possible.
Problem 1: What is a physical change? List the physical changes that occur when one makes
peanut brittle.
Problem 2: What is a chemical change? List the chemical changes that occur when one makes
peanut brittle Chemical
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Classification of Matter by its chemical constitution
All the many kinds of matter can be classified as either pure substances or mixtures.
Matter
separate by
physical
process
no
yes
Substance
Mixture
chemically
decomposed
yes
uniform
throughout
no
yes
Compound
Ionic
compound
Element
Homogeneous
may be
Molecular
compound
no
Heterogeneous
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Chemistry and the Elements
The observation of changes such as wood burning to give off heat and turned to a small pile of
ash caused Greek philosophers to think about what different materials might be composed of and
lead to the idea of fundamental substances that we call element. An element is the most basic
form of matter that exists under ordinary conditions.
Everything you see around you is formed from one or more of 116 presently known elements.
Only 92 of the 116 presently known elements occur naturally.
Chemical Symbol
For simplicity, chemists referred to specific elements using one or two-letter abbreviations of the
name of the element known as Chemical symbols. The first letter is capitalized; E.g. carbon, C;
neon Ne and silicon, Si.
Some elements have symbols based on their Latin name, E.g. Fe iron (ferrum), Pb lead
(plumbum), Na sodium (natrium for sodium carbonate), K potassium (kalium for potassium
carbonate). The symbol for tungsten, W, is based on the German wolfram.
Name and Symbols of Some Common Elements
Aluminum
Argon
Barium
Boron
Bromine
Calcium
Carbon
Al
Ar
Ba
B
Br
Ca
C
Chlorine
Fluorine
Helium
Hydrogen
Iodine
Lithium
Magnesium
Cl
F
He
H
I
Li
Mg
Manganese
Nitrogen
Oxygen
Phosphorus
Silicon
Sulfur
Zinc
Mn
N
O
P
Si
S
Zn
Copper
Iron
Lead
Mercury
Potassium
Silver
Sodium
Cu
Fe
Pb
Hg
K
Ag
Na
Atoms and the Atomic Theory
All the matters can be broken down into elements. Is matter continuously divisible into ever
smaller and smaller pieces, or is there an ultimate limit? What is an element made of? Although
most of thinkers including Plato and Aristotle, believed that matter is continuous, the Greek
philosopher Democritus disagreed. Democritus proposed the elements are composed of tiny
particles that we now call atoms, from the Greek word atomos, meaning indivisible.
Law of conservation of mass
1774 Joseph Priestley & Antoine Lavoisier –showed heating the red power of mercury (II) oxide,
HgO, causes it to decompose into the silvery liquid mercury and the colorless gas oxygen.
2HgO  2Hg +O2. Lavoisier then show that oxygen is the key substance involved in
combustion.
Furthermore, he demonstrated by careful measurements that when combustion is carried out in a
closed container, the mass of the combustion products is exactly equal to the mass of the starting
reactants.
(tin + air+ sealed glassed vessel)  (tin oxide + remaining air + glass vessel)
Law of conservation of mass
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E.g. An 8.4g sample of sodium hydrogen carbonate is added to a solution of acetic acid weight
20.0g. The two substances react, releasing carbon dioxide gas to the atmosphere. After reaction,
the contents of the reaction vessel weight 24.0g. What is the mass of carbon dioxide given off
during the reaction?
Law of constant composition
1799 Joseph Proust – reported one hundred pounds of copper, dissolved in sulfuric or nitric acids
and precipitated by the carbonates of soda or potash, invariably gives 180 pounds of green
carbonate.
Law of constant composition
E. g. Water is made up of two elements H and O. The two sample of water below have the same
proportions of the two elements, expressed as percentages by mass.
mass of sample
mass of oxygen
mass of hydrogen
sample A
10.00g
8.881g O
1.119g H
sample B
27.00g
23.979g O
3.031g H
Dolton’s Atomic Theory and Law of multiple proportions
How can the Law of conservation of mass and Law of constant composition be explained? Why
do element behave as they do? To answer these questions:
1803-1808 John Dalton: proposed a new theory of matter.
1 Each chemical element is composed of minute, indestructible particles called atoms. An
element is a substance made up of only a single type of atom.
2 All atoms of an element are alike in mass and other properties, but the atoms of one
element are different from those of all other elements.
3 In each of their compounds, different elements combine in a simple numerical ratio: e.g.
one atom of A to one of B (AB) or one atom of A to two of B (AB2). If all atoms of an element
are alike in mass (assumption 2) and if atoms unite in fixed numerical ratio (assumption 3), the
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percent composition of a compound must have a unique value, regardless of the origin of the
sample analyzed. This explains the law of constant composition.
4. A chemical reaction only rearrange the way that atoms are combined; the atom
themselves are unchanged. Atoms can be neither created nor destroyed during a chemical
change. Atoms can be neither created nor destroyed during a chemical change. (If atoms of an
element are indestructible, then the same remains unchanged. This explains the law of
conservation of mass).
A good theory should not only explain known observation but also predict the outcome of events
yet unknown. Dalton’s atomic theory does exactly that: it predicts what has come to call the law
of multiple proportions.
Law of multiple proportions
E.g. Oxygen and carbon can combine either in a 1: 1.333 mass ratio to make a substance or in a
1: 2.667 mass ratio to make a substance.
Atomic Mass Scale
An important feature of Dalton’s atomic theory is the idea that an atom of an element has a
characteristic mass. Because he cannot measure the exact mass of individual atoms, Dalton
measured the relative mass of elements required to form compound, and from this, deduced
relative atomic masses.
E.g. Consider calcium sulfide, which consists of 55.6% calcium by mass and 44.4% sulfur by
mass. Suppose there is one calcium atom for each sulfur atom in calcium sulfide. Because we
know that the mass of a calcium atom relative to that of a sulfur atom must be the same as the
mass % in calcium, we know that the ratio of the mass of a calcium atom to that of a sulfur atom
is
mass of Ca atom = 55.6 = 1.25
mass of a S atom 44.4
or mass of a Ca atom = 1.25 x mass of a sulfur atom
By continuing in this manner with other compounds, it is possible to build up a table of relative
atomic masses. We define a quantity called atomic mass ratio (usually referred to as atomic
mass), which is the ratio of the mass of a given atom to the mass of some particular reference
atom. The unit of atomic mass is assigned atomic mass unit (amu).
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The Structure of Atoms: Discovery of subatomic particles
So, what is an atom like? What is the atom made of?
After many ingenious experiments of scientists, the mysteries of atom has been slowly
discovered and understood. The first subatomic particle to be discovered is now we called
electron.
*1897 J.J. Thomson- cathode ray experiment
Thomson’s experiment involved the use of cathode-ray tube. When a sufficiently high voltage is
applied across the electrode, an electric current flows through the tube from negatively charged
electrode (the cathode) to the positively charged electrode (the anode).
Because the beam is produced at a negative electrode and is deflected toward a positive plate,
Thomson proposed that cathode rays are negatively charged fundamental particles found in all
atoms which, we now called electrons. Furthermore, because electrons are emitted from
electrodes made of many different metals, all these substances must contain electrons
Later experiments had shown that cathode-ray can be deflected by electric or magnetic field. By
careful measuring the amount of deflection caused by electric and magnetic fields of known
strength, He established the ratio of mass to electric charge for cathode ray that is,
m/e = -5.6857x10-9 g/coulomb.
1909 Robert Millikan determined the electronic charge through a series of oil-drop experiments.
The currently accepted value of the charge of the e is
–1.6022x10-19C.
Substituting into Thomson’s mass to charge ratio then gives the mass of electron as 1/1836(=
9.1094x10-28g).
Since atoms were know to be neutral, it was understood that the atom must also contain a
positive charge to counterbalance the negative charge. From this information, Thomson
suggested the “plum pudding” model of atom, which explained the fact known at the time. In
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this model, the positive charge is thought to be diffused and evenly distributed throughout the
volume of the atom.
The search for the positively charge particle in an atom and for an overall picture of atomic
structure led to a landmark experiment published in 1911 by Ernest Rutherford.
He identified two type of radiation from radioactive materials, alpha () and beta (). -particles
(He2+)carry two fundamental units of positive charge and have essentially the same mass as He
atoms. -particles are negatively charged particles produced by changes occurring within the
nuclei of radioactive atoms and have the same properties as electrons.
A third form of radiation, that is not affected by an electric field was discovered in 1900 by Paul
Villard. This radiation, called -ray, is not made up of particles; it is electromagnetic radiation
of extremely high penetrating power.
Properties of the three radioactive emissions discovered
Original name
Modern name
Mass (amu) Charge
-ray
-particle
4.00
+2
-ray
-particle (electron) 5.49x10-4
-1
-ray
-ray
0
0_______
1909 Ernest Rutherford
~ use  particle to study the inner structure of atoms. When he directed a beam of -particles at a
thin gold foil, he found that
 The majority of -particles penetrated the foil undeflected.
 Some  particles experienced slightly deflections.
 A few (about one in every 20,000) suffered rather serious deflections as they penetrated
the foil.
 A similar number did not pass through the foil at all, but bounced back in the direction
from which they had come.
1911 Rutherford explained his results by proposing a model of the atom known as the nuclear
atom and having these features.
1 Most of the mass and all of the positive charge of an atom are centered in a very small region
called the nucleus. The atom is mostly empty space.
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2 The magnitude of the positive charge is different for different atoms and is approximately onehalf the atomic weight of the element.
3 There are as many electrons outside the nucleus as there are units of positive charge on the
nucleus. The atom as a whole is electrically neutral.
Rutherford’s nuclear atom suggested the existence of positively charged fundamental particles of
matter in the nuclei of atoms- called protons. He predicted the existence in the nucleus of
electrically neutral particles.
* 1932 Jame Chadwick
~ verified that there is another type of particles in atom called neutron.
Therefore
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Conclusion: Modern physics has revealed successively deeper layers of structure in ordinary
matter. Matter is composed, on a tiny scale, of particles called atoms.
Atoms are in turn made up of minuscule nuclei surrounded by a cloud of particles called
electrons. Nuclei are composed of particles called protons and neutrons, which are themselves
made up of even smaller particles called quarks. Quarks are believed to be fundamental, meaning
that they cannot be broken up into smaller particles.
Atomic number
What is that makes one atom different from another?
Elements differ from one another according to the number of protons in their atoms, a value
called the element’s atomic number. All atoms of particular element have the same atomic
number, Z, on the other hand all atoms with the same number of protons are the same element.
Isotopes
Contrary to what Dalton thought, we know that atoms of an element do not necessarily all have
the same mass. Atoms that have the same atomic # (Z) but different mass numbers (A) are
called isotopes. Most elements occur naturally as a mixture of different isotopes.
An isotope is specified by its atomic # and its mass #. The notation used to designate isotopes is
the chemical symbol of the element written with its atomic # as a left subscript and its mass # as
a left superscript.
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Ions
~ is an electrically charged particle obtained from an atom or chemically bonded group of atoms
lose or gain electrons. The charge on an ion is equal to the # of protons minus the # of electrons.
An atom that gains extra electrons becomes a negatively charged ion, called an anion. An atom
that loses electrons becomes positively charged ion, called a cation.
E.g. Determine numbers of electrons in Mg2+ cation and the S2- anion?
Isotopic Mass
As mentioned previously, we cannot measure the exact mass of individual atoms but their
relative masses. We arbitrarily choose one atom as reference and assign it a certain mass. By
international agreement, this standard is an atom of the isotope carbon-12, which is assigned a
mass of exactly 12 atomic mass unit (amu). Therefore, one atomic mass unit is a mass of exactly
1/12 of the mass of 12C.
Mass of one 12C atom = 12 amu (exactly)
1 amu = Mass of one 12C atom = 1.660539 x 10-24 g
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Next, we determine the mass of other atoms relative to carbon-12 by mass spectrometer. All
atomic masses are given relative to the mass of carbon-12 isotope.
E.g From the mass spectral data, the ratio of the mass of 16O to 12C is found to be 1.33291. What
is the mass of an 16O atom?
E.g. From precise measurement show that the mass of 10B is 0.83442 times the mass of 12C.
What is the isotope mass of 10B?
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Atomic Mass
is the weighted average of the isotopic masses of the element’s naturally occurring isotopes. The
naturally occurring percentages of isotopes of a particular element are referred to as the percent
natural abundance of that element
E.g. Chlorine consists of the following isotopes:
Isotope
isotope mass (amu) natural abundance
Chlorine-35 34.96885
75.77%
Chlorine-37 36.96590
24.23%
What is the atomic mass (weight) of chlorine?
Introduction to Periodic Table
With discovery of many elements
1869 Mendeleev and Meyer
~ independently proposed periodic table organized the elements
In modern periodic table, The periodic table of the elements is organized into 18 groups and 7
periods. Elements are represented by one or two-letter symbols and are arranged according to
atomic number.
The names, symbols, and other information of all 116 elements are organized in a form called
periodic table.
It is customary also to divide the elements into two broad categories known as
Metals: Except mercury (liquid), metals are solid s at room temperature. They are generally
malleable, ductile, good conductors of heat and electricity, and have a lustrous or shiny
appearance.
Nonmetals: generally have opposite properties of metals; e.g. poor conductors of heat and
electricity.
Metalloid (semimetal): is an element having both metallic and nonmetallic properties.
Or into three groups
Main group elements are those in groups 1, 2 and 13-18. when form ions, group 1, 2 lose the
same number of electrons as their group number; group 13 lose group #-10; group 14-18 gain 18group #.
Transition elements: from group 3 to 12, and because all of them are metals, they are also
called the transition metals (TM). The # of electrons lost in TM is not related to their group #.
Inner transition metals which include Lanthanides and Actinindes.