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
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 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
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
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
- 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
Labs has an excellent page that takes you through this
Visualizing Atomic Orbitals
Orbitals for 3D Jmol images of 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.
Atomic orbitals do not always have the shape of a sphere. Higher
orbitals have very unusual shapes.
These orbitals were prepared by Dr. Yue-Ling Wong from the
University of Florida for more images
How do atoms arrange themselves in molecules?
Chloride (NaCl) is composed of sodium (purple) and chlorine
(green atoms). To view the 3-D structure of a salt
crystal click here.
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
structure showing atoms of oxygen in red and hydrogen atoms
in white. To view the 3-D structure of ice click
solids are repeating units made up of metal atoms.