Imagine you were stranded on a desert island with only a block of pure gold and a very, very sharp knife. Given that there’d be nothing much else to do, you might just take that knife and cut the block of gold in two. Having managed that feat, you might eventually get around to cutting one of the pieces in half again, then repeating, to get a couple blocks one eighth the size of the original. Before expiring in the sizzling sun, you might keep cutting and cutting, creating smaller and smaller pieces of gold—each identical to the first in every way except for size.
But could you continue forever, in theory, cutting the bits of gold into smaller and smaller pieces indefinitely? If you were to ask Leucippus of Miletus, a Greek philosopher who pondered such things 2,500 year ago, he most certainly would have said: “No.” Leucippus was of the opinion that everything about us was composed of huge numbers of indivisible particles that filled otherwise empty space.
He reasoned that any everyday object—say, a block of pure gold—was formed by an infinitely large number of identical particles. These fundamental building blocks could not be divided any further, no matter how sharp your knife. There were an infinite variety of these particles, and they could be joined in different combinations to form all types of matter in the world.
Democritus of Abdera, a student of Leucippus, carried this concept forward and even gave these little guys a name: atomos, Greek for “indivisible.” A few years later, Titus Lucretius Carus immortalized this concept in his poem De rerum natura (“On the Nature of Things”).
QUANTUM LEAP
Though Newton and Maxwell disagreed on the particle nature of light, they both believed in the Greek notion of the atom. In his Opticks, Newton wrote of “solid, massy, hard, impenetrable moveable particles … so very hard, as never to wear or break in pieces.” Maxwell wrote in 1872 that atoms were “the only material things which still remain in the precise condition in which they first began to exist.” It is a shame that the one thing they agreed on, however, would turn out to be wrong!
Time passed, and after the collapse of a good many empires, this clever notion was eventually lost. For nearly two millennia it lay neglected. Then it was rediscovered, one day in the 15th century, just as the Enlightenment began to stir. The Greeks’ atomos received a good deal of attention over the ensuing years, and ultimately proved quite useful to scientists of the day who were trying to explain some of their own observations.
By the late eighteenth century, chemists had inferred that all material objects were composed from a short list of unique ingredients. These ingredients were stable, and one could not be transformed into another. Given this elementary nature, these were called elements by the French chemist Antoine Laurent de Lavoisier. He identified 33 of them, including oxygen, hydrogen, and gold. Although elements could not be converted from one type to another, they could join together in distinct combinations to form compounds (for example, table salt, which is formed by equal parts of sodium and chlorine).
ATOM TRAP
We now know that only 20 of Lavoisier’s original 33 elements are truly elemental. For example, his list included many “earthy substances” like chalk and clay, which we now identify as compounds. He also had a category of “simple substances belonging to all the kingdoms of nature,” which included light and heat. Today, scientists count a total of 118 chemical elements.
The English scientist John Dalton would eventually draw inspiration from his Greek predecessors in order to explain the observations of Lavoisier and others. In 1803, he postulated that each individual element is composed from tiny, identical particles that are unique to that element. These particles seemed to be fundamental in that they could not be divided any further, nor could their properties be changed by any chemical process. As for compounds, he reasoned that these were composed of specific combinations of the elemental particles. Given the similarity between the properties of his particles and those of his Greek forebears, Dalton named them atoms.
DEFINITION
An element is a type of matter composed of only one type of atom, with an atom forming the smallest possible portion.
A compound is a type of matter composed of fixed proportions of two or more elements, and therefore fixed proportions of two or more types of atoms.
An atom is a tiny particle of matter, possessing unique properties that remain unchanged during chemical processes.
How tiny is tiny? Scientists by the late 1800s recognized that atoms were so small they could not be directly observed, even with the most powerful of lenses. However, they could infer the approximate size of an atom by applying the laws of thermodynamics. The answer was tiny indeed: atoms have an approximate diameter of about 1 × 10-10 m, which implies that about 100 trillion of them could fit within the period at the end of this sentence.
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