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Chapter One
Atoms and Elements
The fascinating science of chemistry is made even more intriguing by the fact that it is based on something no one has ever seen—an atom. Each atom is built from three kinds of pieces—electrons, protons, and neutrons—with the number of protons strictly prescribed and the number of neutrons varying only within a limited range. The electrons are systematically situated in mathematically defined regions of space, and they make transitions between positions by acquiring precisely predictable amounts of quantum energy. All atoms in all materials are in constant motion, but while an atom in a solid may require millennia to move a millimeter, atoms in air race around at incredible speeds. Some atoms have estimated lifetimes as long as the current age of the universe, while others exist for only billionths of a second.
All the complex materials of life are built of atoms and the compounds that they form. The journey from the discovery of the atom to the development of atomic theory and beyond is one of the great stories of science.
A Theory of the Atom
Of the ancient Greek, Asian, African, and Arabic philosophers whose ideas on atoms have survived, one of the best known is Democritus, a Greek philosopher of c. 460-370 BCE. As did many of his contemporaries, Democritus believed the world could be understood by the application of logic. As such, these thinkers inspired the rational approach to science, but they eschewed experimentation, believing rigorous logic to be the purer form of investigation. Democritus thought of the atom as being the smallest particle of all matter, but he also thought that all atoms were the same. Aristotle (384-322 BCE) believed that four elements—air, earth, water, and fire—were present in all materials. The modern concept of a large variety of elements that can exist as separate, pure materials was the inspiration of Antoine Lavoisier (1743-94), though the modern concept of an atom, the smallest unit of an element, had to await the work of John Dalton (1766-1844). Dalton is given credit for our understanding of the atom because he not only offered the idea that atoms of different elements were distinguishable by characteristic mass, but grounded his theory in experimental results. With compounds such as carbon monoxide and carbon dioxide, Dalton was able to show that the masses of the same element in different compounds stood in whole-number ratios to each other, indicating the element came in discrete mass packets. Dalton's "ultimate particles"—atoms—explained this observed law of multiple proportions and set the stage for a closer look at the atom.
Of the ancient Greek, Asian, African, and Arabic philosophers whose ideas on atoms have survived, one of the best known is Democritus, a Greek philosopher of c. 460-370 BCE. As did many of his contemporaries, Democritus believed the world could be understood by the application of logic. As such, these thinkers inspired the rational approach to science, but they eschewed experimentation, believing rigorous logic to be the purer form of investigation. Democritus thought of the atom as being the smallest particle of all matter, but he also thought that all atoms were the same. Aristotle (384-322 BCE) believed that four elements—air, earth, water, and fire—were present in all materials. The modern concept of a large variety of elements that can exist as separate, pure materials was the inspiration of Antoine Lavoisier (1743-94), though the modern concept of an atom, the smallest unit of an element, had to await the work of John Dalton (1766-1844). Dalton is given credit for our understanding of the atom because he not only offered the idea that atoms of different elements were distinguishable by characteristic mass, but grounded his theory in experimental results. With compounds such as carbon monoxide and carbon dioxide, Dalton was able to show that the masses of the same element in different compounds stood in whole-number ratios to each other, indicating the element came in discrete mass packets. Dalton's "ultimate particles"—atoms—explained this observed law of multiple proportions and set the stage for a closer look at the atom.
Classical Atomic Theories
The atomic theory of matter advanced by Dalton served to answer many questions concerning the behavior of chemical reactions, but these solutions always gave rise to new questions—and so it was with atoms. It became apparent by the early 1800s that the atoms themselves had structure, and by the early 1900s, positively charged nuclei and negatively charged electrons had been identified in the atom; but how these pieces fit together remained to be determined. British investigator J. J. Thomson (1856-1940), who may be credited with finding the persuasive evidence for the electron, thought that the negatively charged electrons might be embedded in a sea of positive charge, a concept that became known as the plum-pudding model because the electrons resembled the negative-charged raisins in a sea of positive-charge plum pudding, a British dessert favorite. Ernest Rutherford (1871-1937), however, in a ground-breaking experiment, fired small, positively charged particles at the atoms in a thin gold foil and saw some particles rebound straight back toward the source. Astonished by the result, Rutherford said it was as though he had "fired a cannonball at a tissue and found it bouncing back," but he correctly interpreted his results to mean that the atom has a very tiny, dense, positively charged nucleus, and the electrons are located outside the nucleus. The areas of space occupied by the electrons have come to be called orbitals.
The Structure of the Atom
Two students of Rutherford, Henry Moseley (1887-1915) and James Chadwick (1891-1974), went on to show that nuclei of atoms consist of two types of particles—protons and neutrons. Individual protons are assigned one unit of positive charge, electrons have one unit of negative charge, and neutrons have no charge. Chadwick provided evidence for the uncharged neutrons by listening to the advice of his mentor, Rutherford, who said, "Look for the invisible man in Piccadilly Square by those that he pushes aside."
Science 101: Chemistry. Copyright © by Denise Kiernan. Reprinted by permission of HarperCollins Publishers, Inc. All rights reserved. Available now wherever books are sold.
Excerpted from Chemistry by Denise Kiernan
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