WHAT IS MATTER?

WHEN SCIENTISTS TALK about "matter," they mean every substance in the universe, from the tiniest speck of dust to the largest star. Indeed, matter is everything that is not simply empty space, or, rather, matter is anything that takes up space. Matter is often thought of as something solid, but it can also be either a liquid or a gas.

Evolution of the Atomic Theory (Matter)

People have wondered about the true nature of matter for thousands of years. In the days of ancient Greece, over 2,000 years ago, many scientists, or "natural philosophers" as they were called, thought that all matter could be divided into four basic types or elements - earth, water, air, and
fire. This sounds odd, but it is not so different from our modem division of matter into solids, liquids, and gases.  Each element, they thought, had its own natural resting place, with earth at or below the ground, water and air above, and fire at the top surrounding the Earth's atmosphere. Every substance tried to return to its natural resting place, they argued, which was why stones fell when you dropped them and flames leaped upward.

Thinkers such as Aristotle (384-322 B.C.) also believed matter was really made of one continuous substance such as some fluid or gas--so it is always possible to shop a lump of matter into smaller pieces. However, there were other Greek thinkers, such as Democritus (c. 460-400 B.C.), who argued that matter was made of clumps of tiny particles with empty space in between, just as scientists believe today. These particles were the smallest possible pieces of matter and could never be chopped up, which is why they were called "atoms," the Greek word for "uncuttable." But it was Aristotle's view that seemed more convincing at the time, and for more than 2,000 years, the idea of atoms was forgotten.

The Alchemists
 In the meantime, many Greek and Arab philosophers studied the moon and the stars and argued that there must be a fifth natural element, called "ether," that filled the heavens. Others began to study a particular kind of chemistry called "alchemy," which flourished not only among the Arabs but also in medieval Europe. Alchemists experimented with matter in order to learn how to change it. Soon they began a long and fruitless quest for the "philosopher's stone," the substance that would change ordinary metal into gold.

Alchemists were as much interested in magic and superstition as in science, and they are sometimes thought of as wizened wizards stirring strange potions rather than serious scientists. But alchemy attracted attracted some powerful minds, and they made some important discoveries, such as nitric and sulfuric acids, and how to make drugs from herbs. Indeed, the word "chemistry" comes from the Arabic al quemia; or alchemy.

Elements and compounds
 It was not until the 17th century that the alchemists' view of the world and Aristotle's four elements (earth, water, air, and fire) were really challenged. Then the Irish scientist Robert Boyle (1627-91) suggested the idea of basic pure chemicals or elements which could be combined to . make particular "compounds." Each of these elements, Boyle argued, had its own characteristics and could exist as a solid, liquid, or gas. Boyle also urged scientists to accept the old idea of atoms again.

Soon many experiments showed that Boyle was right. Joseph Priestley (1733-1804) and the brilliant French scientist Antoine Lavoisier (1743-94), for example, proved that air, one of Aristotle's basic elements, was actually a mixture of different gases, including oxygen and nitrogen. Lavoisier also showed that another of Aristotle's basic elements, water was a mixture of hydrogen and oxygen.

Not long afterward, in 1808, English-Chemist John Dalton (1766-1844) put forward his atomic theory. He suggested that all the atoms of an element are identical -- but different from the atoms of every other element. He also argued that compounds were formed by the joining of an atom of one element with an atom of another. Water, he thought, was made when a hydrogen atom linked up with an oxygen atom.
 

Molecules
Three years later, the Italian physicist Amedio Avogadro (1776-1856) showed that in water, each oxygen atom joined up not with one hydrogen atom as Dalton had said but with two. Moreover, neither hydrogen nor oxygen atoms ever existed alone, but only in pairs. These pairs of atoms are now called "molecules".

As Dalton was working out his ideas, chemists were racing to discover new elements and compounds. Progress was astonishing. Dalton's notebooks from 1803 list just 20 different elements. By 1830, chemists knew of 55. By the end of the 19th century, they knew 80 elements and many more compounds.  For a long while after Dalton put forward his atomic theory, people thought that atoms could never be split up, destroyed, or created. They seemed to be like minute billiard balls, solid and indestructible. But chemists, gradually began to wonder whether this was true.
 

Electrons
It was the British physicist J. J. Thomson (1856-1940) who showed they were right to wonder. In 1897, Thomson's experiments with cathode ray guns (devices something like TV tubes) led him to discover tiny panicles, 1,800 times smaller than the smallest atom. These particles were called "electrons" because they have an electrical charge. Thomson believed electrons sit on the atom something like raisins in raisin bread. But further experiments suggested otherwise.

In 1911, New Zealand born Ernest Rutherford realized that at the heart of every atom is a tiny but very dense "nucleus." Around this, nucleus, he believed, electrons whirl in a relatively vast cloud, which is largely empty space. Two years later, the Danish physicist Neils Bohr suggested that electrons spin around the nucleus in orbits, like planets around the Sun. These orbits, Bohr proposed, are arranged in layers or "shells" like the layers of a onion.

Then, in 1919, Rutherford split the atom for the first time -- it was not indestructible at all. In fact, he smashed the nucleus of an atom of nitrogen gas with a stream of alpha particles (helium nuclei). What was left were nuclei of atoms of hydrogen gas, the smallest atoms of all. It soon became clear that the nucleus of atoms of every element contained hydrogen nuclei, or "protons" as Rutherford called them. Like electrons, protons are electrically charged, but their charge is opposite to that of electrons. In fact, ,every, atom contains equal numbers of protons and electrons, so their charges balance each other out.

Neutrons and more

Thirteen years after Rutherford split the atom, the nucleus was shown to contain another kind of particle called a "neutron," as well as protons. For everyday chemistry, you can still think of the atom as electrons orbiting in shells around a nucleus of protons and neutrons. But physicists have since shown that the story of matter is unimaginably more complicated. Atoms have been smashed many times to reveal the existence of scores of different particles, and these particles are not like billiard balls at all but ghostly packages of energy or "quanta." Scientists are still far from understanding the true nature of matter.