Much of the difficulty scientists had in discovering, and then proving the existence of, atoms, is their size. Something so small had never before been defined.
It took well over two thousand years between the time Democritus first postulated that all matter might be made up of countless little particles called “atoms,” and the time these little things finally entered into respectable scientific parlance with the work of John Dalton, Robert Brown and Albert Einstein. Still, even in the 19th and in some respects all the way into the 20th centuries, there remained holdouts – scientists who simply couldn’t bring themselves to believe in what they could not see.
In many respects, this is understandable. After all, if all matter was made out of atoms, then atoms, in turn, had to be very, very small. Smaller than anything yet known.
The Size of Atoms
It is certainly not difficult to understand why it took so long for atomic theory to catch on. After all, there truly were no practical methods available to scientists prior to the nineteenth century that might have been able to discover these tiny little things. And even then, in the work of Dalton and others, such experiments had to be exceptionally clever. To this very day, atoms remain, by definition, invisible to science.
Viewing Atoms
For something to be seen by the human eye, it has simply to be larger than the wavelength of light (for things are seen by the light reflecting off of them). Any smaller than that, and the thing is invisible. The shortest wavelength of visible light is just under 400 nm (that is, nanometers, or billionths of a meter). In comparison, the widest of all atoms sits comfortably somewhere around 500 pm (picometers = trillionths of meters). In other words, it would take more than a thousand of the largest possible atoms laying side by side for them to be even possibly seen by the most powerful optical microscope in existence.
Atoms have been viewed, however, in a sense. Modern technology has brought about such advances as scanning tunneling microscopes, which uses fundamental principles of quantum mechanics (specifically, quantum tunneling) to view “images” on an atomic scale (though these images appear visually to be little more than “bumps” in a surface, representing atoms.
Taking Small to the Next Level
While there are countless analogies and mental pictures which may be utilized to give one a perception of just how small atoms are, in physics one must take the idea of “small” and tweak it even further, for atoms only seem small on a human scale.
In reality, atoms are some of the bigger things in the universe. Intimidating as this idea might seem, atoms are the giants of the “quantum world.” This didn’t become evident to physicists until the latter decades of the nineteenth century, when the first signs began to show that these tiny little things might be made up of even tinier things.
After a while, scientists began to realize that an atom, in actuality, is made up of mostly empty space. The parts that make them up (protons, neutrons and electrons) compose only a tiny fraction of the total atom. The nucleus, in fact (which is where 99.9% of all the mass is located) is roughly ten thousand times smaller than the total size of the atom.
Today, many physicists believe matter (including atoms themselves) to be made up of the tiniest things of all – little vibrating strings. If this were the case, the theory would dictate that these strings be somewhere around the Planck length, which is thought to be the smallest possible size in existence, somewhere around a millionth of a billionth of a billionth of a billionth of a centimeter. To these things, atoms might seem as large as the entire universe is to humanity.
In other words, one must truly change their entire perception of size in order to even begin to understand modern physics. But at least it should give everyone an appreciate of the difficulties involved with the formation of atomic theory.
Further Reading:
Democritus and Ancient Atomic Theories
John Dalton and the First Modern Atomic Theory
Brownian Motion and the Proof of Atoms
J.J. Thompson and the Discovery of Electrons
Ernest Rutherford and the Discovery of Protons and the Atomic Nucleus
James Chadwick and the Discovery of the Neutron
References:
The New York Public Library. (1995). Science Desk Reference. New York, NY: Macmillan.
Isaacs, A. (2003). Dictionary of Physics. London: Grange Books.
Barnes & Noble Books. (2006). Ultimate Visual Dictionary of Science. New York: Barnes & Noble.
