Atoms, Elements, and the
Periodic Table
Chapter Four
Structure of Matter
Section One
If you crushed a sugar cube, you
would find a number of small
fragments that were still sugar. If
you crushed a fragment, you would
find a number of small particles
that were still sugar. How long
could you divide the sugar until the
particles were no longer sugar?
This was the approach that
Democritus took in explaining the
atom. He said that you could take
a pair of shears and cut a piece of
copper in two and sometime, you
would reach a piece that couldn’t
be cut anymore. He named this
particle an atom meaning
indivisible.
Over two thousand years later,
Lavoisier used this concept to
develop the Law of Conservation of
Matter.
The Law of Conservation of Matter
states that matter is neither
created nor destroyed in ordinary
chemical or physical reactions.
Dalton’s Atomic Theory
In 1808, an English chemistry teacher
named John Dalton proposed the
atomic theory. From data gathered in
his student’s experiments, he
explained the theories mentioned
above and laid the foundation for
understanding the atom.
Dalton’s Atomic Theory can be
summed up in the following
statements:
All matter is composed of small
particles called atoms.
Atoms of the same element are
identical in size, mass, and other
properties; atoms of different
elements differ in size, mass, and
properties.
From the beginning, we all know
that atoms are very small.
However it has taken some good
science to develop a model of the
atom.
Gases at atmospheric pressure do not
conduct electricity well, however, gases at
very low pressures do conduct. Scientists
at the beginning of the twentieth century
found that a glowing current will pass
from a negatively charged cathode to a
positively charged anode in a glass tube
called a cathode ray. This stream was
called a cathode ray and the device was
called a cathode ray tube.
John Thompson found that the
cathode ray could cause a paddle
wheel to roll along rails through the
tube. This indicated that the ray was
made of a particles. Cathode rays are
deflected by a magnetic field and the
rays were deflected away from a
negatively charged object. This would
indicated that they carry a negative
charge.
Later, scientists established that
the particle was the smallest
known to man with a mass of
1/2000 of a hydrogen atom. It was
named the electron.
Thompson developed the
“chocolate chip cookie” model in
which he envisioned atoms as
being a ball of positive dough with
embedded electron “chips”.
Overall, the atom would have a
neutral charge.
In 1911, New Zealander Earnest Rutherford and his
associates Hans Geiger and Ernest Marsden
bombarded a thin gold foil with alpha particles
emitted from a radioactive source. They expected
an evenly charged force field in the foil so they
planned that the particles would pass straight
through. When the detector was studied, they
were greatly surprised to find about 1 in 8000
particles bounced straight back! Rutherford
exclaimed that this would be like firing a 15 inch
artillery shell into a piece of tissue paper and have
it bounce back.
Rutherford concluded that atoms must
contain a very small, dense, positively
charged nucleus. The nucleus of the
atom is very small. If it were the size of
a marble, the atom would be larger
than a football field. He named the
positively charged particles that made
up the nucleus protons.
Eventually, a student of
Rutherford’s, James Chadwick,
discovered a nucleur particle that
lacked a charge called a neutron.
Later in the twentieth century,
Niels Bohr established a model of
the atom in which the electrons
orbited the nucleus in fixed orbits
like the planets around the sun.
More recent studies have shown
that electrons don’t occupy orbits
like planets. Instead, they move in
spaces forming an electron cloud.
The higher the energy of the
electron, the farther from the
nucleus it is.
Studies have also found smaller
subatomic particles in the nucleus
called quarks.
The Simplest Matter
Section Two
The basic makeup of the earth is
simple substances made of only
one kind of atom called elements.
Of the total 115 elements, about
90 elements occur naturally on
earth and the remainder are manmade.
The elements are charted on the
periodic table. Atoms with similar
energies make up rows or periods.
Atoms with similar properties
make up columns or groups.
When the atoms are arranged this
way, certain properties reoccur
periodically giving the chart its
name.
The atomic number of an element
is the number of protons in the
nucleus. This number identifies
the element. It will be found near
the top of each grid on the periodic
table. For instance, hydrogen is 1,
lithium is 3, carbon is 6, and silver
is 47.
The relative atomic mass is the
number that is usually shown with
several decimals. When you round
it off to a whole number you get
the mass number which is the
number of protons and neutrons in
the nucleus.
Some atoms of the same element
have different numbers of
neutrons. They are called isotopes
and usually have special
properties. The relative atomic
mass is an average of the mass of
an elements isotopes so it usually
includes decimals.
Metals make up most of the chart
on the right side.
Metals:
•
•
•
•
•
•
Have a shiny surface luster
Conduct heat and electricity
Are malleable
Are ductile
Most are solid at room temperature
Most are denser that other
substances
Nonmetals make up the far left
side of the chart and lack the
properties of metals.
Metalloids are found between
metals and nonmetals and have
special properties. They are
sometimes known as
semiconductors.
Elements are divided into
three groups:
•Metals
•Nonmetals
•Metalloids
Compounds and
Mixtures
Section Three
A pure substance is only made of
one kind of particle. This would
mean that it is an element or a
compound.
A chemical compound is a pure
substance that is made of two or
more elements that are chemically
combined. Because of this
definition, we know that the
substances that make up a
compound change into a new
identity and cannot be separated
physically.
Compounds have a definite
chemical formula that does not
change.
H2O = Water
CO2 = Carbon Dioxide
Of the total 115 elements, about
90 elements occur naturally on
earth and the remainder are manmade.
The elements are charted on the
periodic table. Atoms with similar
energies make up rows or periods.
Atoms with similar properties
make up columns or groups.
When the atoms are arranged this
way, certain properties reoccur
periodically giving the chart its
name.
The atomic number of an element
is the number of protons in the
nucleus. This number identifies
the element. It will be found near
the top of each grid on the periodic
table. For instance, hydrogen is 1,
lithium is 3, carbon is 6, and silver
is 47.
The relative atomic mass is the
number that is usually shown with
several decimals. When you round
it off to a whole number you get
the mass number which is the
number of protons and neutrons in
the nucleus.
Some atoms of the same element
have different numbers of
neutrons. They are called isotopes
and usually have special
properties. The relative atomic
mass is an average of the mass of
an elements isotopes so it usually
includes decimals.
Metals make up most of the chart
on the right side.
Metals:
•
•
•
•
•
•
Have a shiny surface luster
Conduct heat and electricity
Are malleable
Are ductile
Most are solid at room temperature
Most are denser that other
substances
Nonmetals make up the far left
side of the chart and lack the
properties of metals.
Metalloids are found between
metals and nonmetals and have
special properties. They are
sometimes known as
semiconductors.