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 --

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.

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:

1. losing electrons
2. sharing electrons
3. 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.

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

Atomic orbitals do not always have the shape of a sphere. Higher orbitals have very unusual shapes.

2px

These orbitals were prepared by Dr. Yue-Ling Wong from the University of Florida for more images

How do atoms arrange themselves in molecules?

	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.