# The Atomic Nature of Matter

## The Atomic Hypothesis

The idea that matter is composed of atoms goes back to the Greeks in the fifth century Be. Investigators of nature back then wondered whether matter was continuous or not. We can break a rock into pebbles, and the pebbles into fine gravel. The gravel can be broken into fine sand, which then can be pulverized into powder. Perhaps it seemed to the fifth-century Greeks that there was a smallest bit of rock, an “atom”, that could not be divided any further.

Aristotle, the most famous of the early Greek philosophers, didn’t agree with the idea of atoms. In the fourth century BC, he taught that all matter is composed of various combinations of four elements-earth, air, fire, and water. This view seemed reasonable because, in the world around us, matter is seen in only four forms; solids (earth), gases (air), liquids (water), and the state of flames (fire). The Greeks viewed fire as the element of change, since fire was observed to produce changes on substances that burned. Aristotle’s ideas about the nature of matter lasted for more than 2000 years.

The atomic idea was revived in the early 1800s by an English meteorologist and school teacher, John Dalton. He successfully explained the nature of chemical reactions by proposing that all matter is made of atoms. He and others of the time, however, had no direct evidence for their existence. Then, in 1827, a Scottish botanist named Robert Brown noticed something very unusual in his microscope. He was studying grains of pollen suspended in water, and he saw that the grains were continually moving and jumping about. At first he thought the grains were some kind of moving life forms, but later he found that dust particles and grains of soot suspended in water moved in the same way. This perpetual jiggling of particles-now called Brownian motion-results from
collisions between visible particles and invisible atoms. The atoms are invisible because they’re so small. Although he couldn’t see the atoms, Brown could see the effect they had on particles he could see. It’s like a super-giant beach ball being bounced around by a crowd of people at a football game. From a high- flying airplane, you wouldn’t see the people because they are small relative to the enormous ball, which you would be able to see. The pollen grains that Brown observed moved because they were constantly being jostled by the atoms (actually, by the atomic combinations referred to as molecules) that made up the water surrounding them.

All this was explained in 1905 by Albert Einstein, the same year that he announced the theory of special relativity, Until Einstein’s explanation- which made it possible to find the masses of atoms-many prominent physicists remained skeptical about the existence of atoms. So we see that the reality of the atom was not firmly established until the early twentieth century.

In 1963, the importance of atoms was emphasized by the American physicist Richard Feynman, who stated that, if some cataclysm were to destroy all scientific knowledge and only one sentence could be passed on to the next generation of creatures, the statement with the most information in the least words would be: “All things are made of atoms-little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.” All matter-shoes, ships, sealing wax, cabbages, and kings-any material we can think of-is made of atoms. This is the atomic hypothesis, which now serves as a central foundation
of all of science.

## Characteristics of Atoms

Atoms, the building blocks of matter, are incredibly tiny. An atom is as many times smaller than you as an average star is larger than you. A nice way to say this is that we stand between the atoms and the stars. Or another way of stating the smallness of atoms is that the diameter of an atom is to the diameter of an apple as the diameter of an apple is to the diameter of the Earth. So, to imagine an apple full of atoms, think of the Earth solid-packed with apples. Both have about the same number.

Atoms are numerous. There are about 100,000,000,000,000,000,000,000 atoms in a gram of water (a thimbleful). In scientific notation, that’s $10^{23}$ atoms. The number $10^{23}$ is an enormous number, more than the number of drops of
water in all the lakes and rivers of the world. So there are more atoms in a thimbleful of water than there are drops of water in the world’s lakes and rivers. In the atmosphere, there are about $10^{22}$ atoms in a liter of air. Interestingly, the volume of the atmosphere contains about $10^{22}$ liters of air. That’s an incredibly large number of atoms, and the same incredibly large number of liters of atmosphere. Atoms are so small and so numerous that there are about as many atoms in the air in your lungs at any moment as there are breathfuls of air in the Earth’s atmosphere.

Atoms get around. Atoms are perpetually moving. They migrate from one location to another. In solids, the rate of migration is low; in liquids, it is greater; and in gases, migration is greatest. Drops of food coloring in a glass of water, for example, soon spread to color the entire glass of water. The same would be true of a cupful of food coloring thrown into an ocean: It would spread around and later be found in every part of the world’s oceans.

Water dilution is a main reason for salmon being able to return to their birthplaces. Atoms and molecules from soil and vegetation in a lake or a stream make that water unique. Likewise with salmon spawning habitats. Once hatched, young salmon remain in local streams for two years before beginning their voyage to the ocean, where they remain for an average of four years. What also goes into the ocean, of course, is water from the region in which they grew up. The original water composition is diluted as it travels to the ocean. In the ocean, it is further diluted-but never to zero, When the time comes to return to their original habitat, salmon follow their noses. They swim in a direction where concentrations of familiar water become greater. In time, they’ll encounter the source of that water. Humans can discern different bottled waters, and salmon have enormously more ability to sense the difference in waters, as bloodhounds have a similar sensitivity to air composition.

Atoms and molecules in the atmosphere spread around much more than they do in the ocean. Atoms and molecules in air zip around at speeds up to ten times the speed of sound. They spread rapidly, so oxygen that surrounds you today may have been halfway across the country a few days ago. Your exhaled breaths of air quite soon mix with other
atoms in the atmosphere. After the few years it takes for your breath to mix uniformly in the atmosphere, anyone, anywhere on Earth, who inhales a breath of air will take in, on the average, one of the atoms in that exhaled breath of yours. But you exhale many, many breaths, so other people breathe in many, many atoms that were once in your lungs-that were once a part of you; and, of course, vice versa. Believe it or not, with each breath you take in, you breathe atoms that were once a part of everyone who ever lived! Considering that the atoms we exhale were part of our bodies (the nose of a dog has no trouble discerning this), it can be truly said that we are literally breathing one another.

Atoms are ageless. Many atoms in your body are nearly as old as the universe itself. When you breathe, for example, only some of the atoms that you inhale are exhaled in your next breath. The remaining atoms are taken into your body to become part of you, and they later leave your body by various means. You don’t “own” the atoms that make up your body; you borrow them. We all share from the same atom pool, as atoms forever migrate around, within, and among us. So some of the atoms in the nose you scratch today could have been part of your neighbor’s ear yesterday!

Atoms are ageless. Many atoms in your body are nearly as old as the universe itself. When you breathe, for example, only some of the atoms that you inhale are exhaled in your next breath. The remaining atoms are taken into your body to become part of you, and they later leave your body by various means. You don’t “own” the atoms that make up your body; you borrow them. We all share from the same atom pool, as atoms forever migrate around, within, and among us. So some of the atoms in the nose you scratch today could have been part of your neighbor’s ear yesterday!

So the origin of the lightest atoms goes back to the origin of the universe, and most heavier atoms are older than the Sun and the Earth. There are atoms in your body that have existed since the first moments of time, recycling throughout the universe among innumerable forms, both nonliving and living. You’re the present caretaker of the atoms in your body. There will be many who will follow you.

## Atomic Imagery

Atoms are too small to be seen with visible light. You could connect an array of optical microscopes atop one another and never “see” an atom because light is made up of waves, and atoms are smaller than the wavelengths of visible light, The size of a particle visible under the highest magnification must be larger than the wavelengths of visible light. This is better understood by an analogy with water waves. A ship is much larger than the water waves that roll on by it. As the figure below shows, water waves can reveal features of the ship.

The waves diffract as they pass the ship. But diffraction is nil for waves that pass by the
anchor chain, revealing little or nothing about it. Similarly, waves of visible light are too coarse compared with the size of an atom to show details of the size and shape of atoms. Atoms are incredibly small.

Yet in the figure below see a picture of atoms-the historic 1970 image of chains of individual thorium atoms, The picture is not a photograph but an electron micrograph-it was not made with light but with a thin electron beam in a scanning electron microscope (SEM) developed by Albert Crewe at the University of Chicago’s Enrico Fermi institute. An electron beam, such as the one that sprays a picture on an early television screen, is a stream of particles that have wave properties. The wavelength of an electron beam is smaller than the wavelengths of visible light, and atoms are larger than the tiny wavelengths of an electron beam. Crewe’s electron micrograph is the first high-resolution image of individual atoms.

In the mid-1980s, researchers developed a new kind of microscope-the scanning tunneling microscope (STM). It employs a sharp tip that is scanned over a surface at a distance of a few atomic diameters in a point-by-point and line-by-line fashion. At each point, a tiny electric current, called a tunneling current, is measured between the tip and the surface. Variations in the current reveal the surface topology. The image on the figure below beautifully shows the position of a ring of atoms. The ripples shown in the ring of atoms reveal the wave nature of matter. This image, among many others, underscores the delightful interplay of art and science.

Because we can’t see inside an atom, we construct models. A model is an abstraction that helps us to visualize what we can’t see, and, importantly, it enables us to make predictions about unseen portions of the natural world.

The model of the atom most familiar to the general public is akin to that of the solar system. As with the solar system, most of an atom’s volume is empty space, At the center is a tiny and very dense nucleus in which most of the mass is concentrated. Surrounding the nucleus are “shells” of orbiting electrons. These are the same electrically charged electrons that constitute the electric current in your calculator. Although electrons electrically repel other electrons, they are electrically attracted to the nucleus, which has
a net positive charge. As the size and charge of the nuclei increase, electrons are pulled closer, and the shells become smaller. Interestingly, the uranium atom, with its 92 electrons, is not appreciably larger in diameter than the lightest atom, hydrogen. This model was first proposed in the early twentieth century, and it reflects a rather simplified understanding of the atom. It was soon discovered, for example, that electrons don’t orbit the atom’s center like planets orbit the Sun. Like most early models, however, the planetary atomic mode) served as a useful stepping stone to further understanding and more accurate models. Any atomic model, no matter how refined, is nothing
more than a symbolic representation of the atom and not a physical picture of the actual atom.