The atom is the smallest particle of an element that
has all the properties of that element. Since
atoms are so small, scientists develop models to represent them.
In science, a model is a tool used to visualize something that cannot be
seen.
HISTORY OF THE ATOMIC MODEL
A)
DEMOCRITUS
The first person to develop a model of the atom was Democritus. His model stated that all matter was made up of small indivisible particles called atoms. He also stated that atoms of different elements had different sizes and shapes.
In the early 1800s, a British scientist, named
John Dalton, developed a model he called the atomic theory.
This theory has 4 main points.
1) All matter is
made up of tiny particles called atoms.
2) Atoms cannot be created
or destroyed.
3) Atoms of the same
element have the same properties, and atoms of different
elements have different properties.
4) Atoms of different
elements can combine to form new
substances.
In
the early 1900s, a British physicist, named J.J. Thomson, discovered that the
atom contained particles with a negative charge.
He called these particles electrons.
He also discovered that somewhere in the atom there must be an equal
number pf positively charged particles.
In
1911, a physicist from New Zealand, named Ernest Rutherford took Thomsons
model one step further. He stated
that all the positive charge of the atom was located at a very small point in
the center of the atom. He called
this point the nucleus. He called
the positively charged particles protons. He
also stated that the electrons were circling around outside the nucleus and that
most of the atom was empty space.
In
1913, a Danish physicist named Niels Bohr, took Rutherford's theory to the final
step. He stated that the electrons
move around the nucleus in definite paths.
Each path being a certain distance from the nucleus.
The distance between the paths were not the same.
He called these paths energy levels and stated that the electrons were
found in the energy levels and not between them.
The electrons in each energy level have a definite distance from the
nucleus. The farther the energy level is from the nucleus, the greater
the amount of energy they have. He
also found out that the electrons can move from one energy level to another.
If the atom absorbs energy, the electron moves away from the nucleus.
If the electron moves closer to the nucleus the atom gives off energy.
** All these points put together form our modern day atomic
theory.
ATOMIC
STRUCTURE
Matter exists in 3 possible forms, as elements, as
compounds, and as mixtures. Some
common elements are Hydrogen (H), Helium (He), Uranium (U), Sodium (Na),
Chlorine (Cl), Zinc (Zn), and Oxygen (O). Mercury
is the only element that exists as a liquid at room temperature.
The atom has 2 main areas, the nucleus which is the center and the
largest part, and the electrons, which circle around outside the nucleus.
The atom has 3 main subatomic particles.
I) PROTONS,
a.
Symbol is p
b.
Located in the nucleus
c. Largest
of the 3 main subatomic particles
d.
Has a positive electrical charge
e.
The number of protons is the atomic number
II) NEUTRONS,
a.
Symbol is a n
b.
Located in the nucleus
c.
Almost as large as a proton
d. They are
electrically neutral (no charge)
e.
The number of protons plus the number of neutrons equals the
atomic weight.
f.
Since the neutrons are have no charge, the nucleus of the
atom is positive.
g.
Hydrogen is the only atom without a neutron
III)
ELECTRONS
a. Symbol is an e
b.
Located outside the nucleus
c.
They have a negative electrical charge
d.
They are the smallest of the 3 main subatomic particles
e.
In all single, uncombined atoms, the number of electrons
equals the number of protons.
This makes them electrically
neutral.
f.
They circle the nucleus in areas called energy levels or shells
g.
The first shell is the K shell, it holds a maximum of 2
electrons
The second shell is the L shell,
it holds a maximum of 8
electrons.
The third shell is the M shell, it holds a maximum 18
electrons
The fourth shell is the N shell, it holds a maximum of 32
electrons.
All shells after that go alphabetically, and to find the number
of electrons there, look
them up on the periodic table of
elements.
The nucleus contains other subatomic particles such as
the positron, the meson, the neutrino, and the quark. The SI unit used to express the masses of particles in the
atom is the AMU (atomic mass unit). Scientist
have assigned each proton a mass of one AMU.
Remember that mass number and atomic weight mean the same thing.
Under certain conditions atoms of an element may not be the same. These atoms are called isotopes.
Isotopes are atoms that have the same number of protons, (which give the
atom its identity), but different numbers of neutrons.
Atoms that are isotopes of each other are always the same element because
they have the same number of protons.
FORCES IN THE ATOM
There are 4 main forces that act within the atom:
a. Gravity this force is
very small within the atom.
b.
Electromagnetic Force this force is responsible for keeping
the electrons in
orbit around the nucleus. This
occurs because
of the attraction between
the negative electrons and the
positive nucleus.
c.
Strong Force this force is responsible for keeping the
nucleus together.
d.
Weak Force this force is believed to play a key role in
allowing atoms to change
when they combine with other atoms.
MAKING MODELS OF THE ATOM
RULES FOR MAKING ATOMIC MODELS:
1.
Make a data table. Using the
periodic table of elements, find the information needed to fill out the data
table. This include the number of
protons, neutrons, and the electron configuration.
Remember, to find the number of protons, look up the atomic number.
To find the number of neutrons, round off the atomic weight and subtract
the atomic number. The electron
configuration, located in the upper left hand corner of each box on the periodic
table, tells you the number of shells, the order they are in, top number is the
first shell (closest to the nucleus), and the number of electrons located in
each shell.
2.
Draw in the nucleus
3. Draw in the proper number of electron shells.
4.