Download Grades 6-8 Text Exemplars Informational Texts: Science

Grades 6-8 Text Exemplars
Informational Texts: Science, Mathematics and Technology
“Elementary Particles.” New Book of Popular Science. New York: Scholastic, 2010. (2010)
Since ancient times, people have tried to discover the basic units of matter. What, they have asked,
are the smallest particles from which all the objects in the universe are made?
Many people in ancient Greece thought that all matter was made of various combinations of four
basic “elements”— earth, fire, air, and water. But one Greek philosopher, Democritus (c.460–c.370
B.C.), had a different theory. He suggested that matter was composed of tiny particles called atoms.
The word “atom” comes from a Greek word meaning “unable to be cut” or “indivisible.”
The theory of Democritus was largely ignored for 2,000 years. Then, in 1802, an English chemist and
physicist named John Dalton (1766–1844) revived the atomic theory. He was the first scientist to
define the atom as it is understood today—the smallest particle of an element that behaves
chemically like that element.
Atomic physics is the study of atoms and their behavior. Atoms are incredibly small. A tiny speck of
dust contains many millions of atoms. Some molecules, such as certain of the protein molecules,
contain hundreds of thousands of atoms. Yet a protein molecule is so small, compared with things we
can see with the unaided eye, that a powerful electron microscope is needed to view it. Even then,
the individual atoms cannot usually be seen.
Small as the atom is, however, it is not the smallest component of matter. Particle physics is the study
of the smallest, most elemental building blocks and the basic forces of nature.