| The
word atom is derived from the Greek word atom which means indivisible.
The Greeks concluded that matter could be broken down into particles to small
to be seen. These particles were called atoms Matter
has mass and takes up space. Atoms are basic building blocks of matter, and cannot
be chemically subdivided by ordinary means. Atoms are composed of three
type of particles: protons, neutrons, and electron. Protons and neutrons are responsible
for most of the atomic mass, e.g in a 150lb. person 149 lbs, 15 oz are protons
and neutrons while only 1 oz. is electrons. The mass of an electron is very small
(9.108 X 10-28 grams). Both the protons and neutrons reside
in the nucleus. Protons have a postive (+) charge, neutrons have no charge --they
are neutral. Electrons reside in orbitals around the nucleus. They have a negative
charge (-). It is the number of protons that determines the atomic number,
e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92)
but neutron number may vary, so mass number (protons + neutrons) may vary. The
same element may contain varying numbers of neutrons; these forms of an element
are called isotopes. The chemical properties of isotopes are the same, although
the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning
they "radiate" energy as they decay to a more stable form, perhaps another element
half-life: time required for half of the atoms of an element to decay into stable
form. Another example is oxygen, with atomic number of 8 can have 8, 9, or 10
neutrons. Historical Models of the atom JOHN DALTON -- John
Dalton in 1803, proposed a modern theory of the atom based on the following assumptions:
matter is made up of atoms that are indivisible and indestructible; all atoms
of an element are identical; atoms of different elements have different weights
and different chemical properties; atoms of different elements combine in simple
whole numbers to form compounds; and, atoms cannot be created or destroyed. BOHR
MODEL
Bohr model shows electrons circling the nucleus at different levels
or orbitals much like planets circle the sun. Electrons move from one energy state
to another but can only exist aft defineite energy levels. The energy absorbed
or released when electrons change states is in the form of electromagnetic radiation.
THE WAVE MODEL AND QUANTUM THEORY
The Bohr model was only able to explain
the very simplest atoms, like hydrogen. Today's modern day theory is based on
mathematics and the properties of waves. The wave model forms the basis for the
Quantum Theory. This theory gives the probability of locating electrons in
a particular location, unlike assuming electrons orbit the nucleus as in the Bohr
model. How are electrons organized around the nucleus?
All atoms would like to attain electron configurations like noble gases.
That is, have completed outer shells. Atoms can form stable electron configurations
like noble gases by: - losing electrons
- sharing electrons
- gaining
electrons.
For a stable configuration each atom must fill its outer
energy level. In the case of noble gases that means eight electrons in the last
shell (with the exception of He which has two electrons). Atoms that have 1,
2 or 3 electrons in their outer levels will tend to lose them in interactions
with atoms that have 5, 6 or 7 electrons in their outer levels. Atoms that have
5, 6 or 7 electrons in their outer levels will tend to gain electrons from atoms
with 1, 2 or 3 electrons in their outer levels. Atoms that have 4 electrons in
the outer most energy level will tend neither to totally lose nor totally gain
electrons during interactions. The Periodic Table of Elements will show you
the electron configuration for any element you click on.
Making
a model of an atom for your school Do you need to make a model or a
drawing of an atom for science class? Jefferson
Labs has an excellent page that takes you through this project.
Visualizing
Atomic Orbitals The atomic orbitals of the hydrogen
atom can be visualized as a cloud around the nucleus. The orbital represents a
probability of finding the electron at a particular location. Darker regions signify
a greater probability. Shown below are the 1s (lowest orbital and the 2s orbital.
 1s 2s Atomic orbitals
do not always have the shape of a sphere. Higher orbitals have very unusual shapes.
 2px 3px These orbitals
were prepared by Dr. Yue-Ling Wong from the University of Florida for more images
click here.
How
do atoms arrange themselves in molecules?
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Sodium Chloride (NaCl) is composed
of sodium (purple) and chlorine (green atoms). To view the 3-D structure
of a salt crystal click here.
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Carbon atoms
in graphite- These substance appear as a single giant molecule made up of
an almost endless number of covalent bonds. To view the 3-D structure of
graphite click
here.
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| Ice
structure showing atoms of oxygen in red and hydrogen atoms in white. To view
the 3-D structure of ice click here.
|  | Metallic
solids are repeating units made up of metal atoms.
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