Atom is the tiny basic building block of matter. All the
material on Earth is composed of various combinations of atoms. Atoms are the
smallest particles of a chemical element that still exhibit all the chemical
properties unique to that element. A row of 100 million atoms would be only
about a centimeter long. Understanding
atoms is key to understanding the physical world. More than 100 different
elements exist in nature, each with its own unique atomic makeup. The atoms of
these elements react with one another and combine in different ways to form a
virtually unlimited number of chemical compounds. When two or more atoms
combine, they form a molecule. For example, two atoms of the element hydrogen
(abbreviated H) combine with one atom of the element oxygen (O) to form a
molecule of water (H20).
(Encarta Encyclopedia)
