Atoms

Then Why Is the Floor Solid

Nothing in an atom is a hard ball: it is a speck inside a soft cloud. So what holds you up? The quick answer is that electrons repel electrons. That is part of the story, but atoms are electrically neutral. As two atoms come close, each nucleus also pulls on the other atom’s electrons, and the pulls nearly cancel the pushes. That near cancellation is why atoms can stick together at all. Electric forces alone do not make a floor.

What makes the floor is the rule that no two electrons can share the same state. Press two atoms together and their filled clouds cannot simply overlap, because their electrons would have to share states. Instead the electrons are forced into shapes that cost far more energy, and the cost climbs steeply the harder the atoms are pressed. The push is electric in the end, but the rule is what makes it so stiff. It has been proved that without the rule, ordinary matter would collapse, packing itself tighter the more of it there was.

So when you touch something, your atoms never get inside its atoms. Their outer clouds press on each other, and the nuclei stay about an atom’s width apart. That press is all that touching is. It pushes the skin of your fingertip in a little, and nerves in the skin feel it. A knife cuts with the same push. Its edge is so thin that a gentle push on the handle is gathered onto a narrow line of skin: the edge of a razor blade is only a few hundred atoms across. There the edge wedges the skin’s molecules apart, harder than the bonds between them can hold, and the bonds let go. Bonds are electric pulls between clouds too, so the skin parts without a single nucleus meeting the steel.

Water touches you the same way, and its wetness is something the brain works out. Skin has no sensor for wetness. What it feels is the water’s soft touch and its cold: water molecules jostle against the skin’s, each bump the same press of clouds, and carry heat away far faster than air does. In tests, a cold, dry probe pressed on the skin is often felt as wet. Water stays on your hand because a little farther out than the push, the pulls between clouds win. Water molecules pull on the skin’s, so drops and a thin film cling. On a freshly waxed car that pull is weaker than water’s pull on itself, and the water gathers into beads.

Where gravity crushes matter far harder than any hand, the same rule still holds. In a white dwarf star the atoms are pressed until their clouds merge into one sea of electrons, and the tighter the sea is packed, the faster the rule forces its electrons to move. Their push alone holds up the star. In a neutron star most of the electrons have been pressed into protons, turning them into neutrons, and no two neutrons can share a state either, which helps hold that star up too. The rule that keeps your hand out of the table keeps those stars from collapsing.

Only Certain Shapes

The cloud is not a blur of something moving fast. It is a wave held in place around the nucleus, like the vibration of a drum skin, and like a drum it can ring only in certain patterns. The simplest is round. Higher patterns have nodes, places where the wave stands still and the electron is never found, and where the nodes fall gives each its shape: a round cloud with a hollow shell inside it, two lobes on either side of the nucleus, four lobes, and more. Each pattern has an exact energy. Measure an electron’s energy in an atom and you always find one of those values, never one in between.

So an atom takes in light only of exact energies. Light that brings exactly the difference between two patterns can change the cloud into the higher one. Light of any other energy is not taken in, unless it brings enough to free the electron. Left alone, the cloud soon falls back to the lower pattern and gives out the same energy as light. Neither change is an instant leap: in a tiny circuit that behaves like an atom, such a change has been watched under way and even turned back midway. Light that brings more than enough tears the electron out of the atom altogether, and leaves an ion: an atom with an electric charge.

Every element has its own set of patterns, and so its own set of colors that it gives out and takes in, as sharp as a barcode. Heat a gas of one element until it glows and its light splits into a few bright lines with darkness between them. Starlight that crosses a cooler gas loses the same colors as dark lines. Reading these lines is how we know what the Sun and galaxies billions of light-years away are made of. One element, helium, was found in sunlight before anyone found it on Earth. Hydrogen’s visible colors come from falls that end at its second-lowest energy. A fall to the lowest energy, as in the animation above, gives ultraviolet light, which no eye sees; the animation draws it violet.

i Hydrogen’s four brightest visible colors, nothing between ×

The patterns themselves have been seen. Physicists used lasers to lift the electrons of hydrogen atoms into a high, wide pattern, inside a steady electric field that shaped the pattern and let each electron go free and drift to a detector. On the way out, an electric lens spread the pattern’s image to millimeters. The electrons arrived one by one, and together they built up rings. The dark rings between them fell where theory puts the pattern’s nodes, the places where the electron is never found.

One More Proton

What makes oxygen oxygen and gold gold is a single count: the protons in the nucleus. Hydrogen has one, carbon six, oxygen eight, gold seventy-nine. A neutral atom has as many electrons as protons, and they fill the patterns from the lowest energy up, two to each pattern, their spins pointing opposite ways. The patterns come in layers. The first layer has one pattern, the round one, and holds two electrons. The next has four, a round one and three with two lobes, one along each direction of space, and holds eight.

Across a row of the periodic table, each atom has one more proton pulling on the same outer layer. It has one more electron too, but that electron joins the same layer, beside the others rather than between them and the nucleus, so it hides the nucleus from them only a little. The inner pair does lie between, and hides two protons almost wholly. So the pull on the outer layer grows from atom to atom, and the layer is drawn in. From lithium to neon the outer cloud does not grow as it gains electrons. It shrinks, to less than half its width. Once a layer is full, the next electron has to start a new layer far out, so each row begins with a big atom: sodium, with eleven electrons, is wider than lithium with three.

The periodic table is this pattern laid out. Its first row fills the first layer’s two places, and the next two rows fill outer layers of eight. From the fourth row on, five more patterns, most of them four-lobed, fill in just inside the outer layer, adding ten places for metals such as iron, copper and gold, and the rows grow to eighteen. From the sixth row, seven more patterns add fourteen places, and the rows grow to thirty-two. The columns gather atoms whose outer layers look alike, so they behave alike. Lithium, sodium and potassium each have one electron alone far out, easily taken away, and all three react with water, each more violently than the last. Neon and argon have full outer layers and join with almost nothing. Fluorine and chlorine, near the ends of their rows, have room for one more electron in their outer layers, and pull so hard on one that they take electrons from almost anything. What an atom does in chemistry is decided by its outermost electrons.

Near the bottom of the table the heaviest atoms bend these rules. In gold, the pull of 79 protons is so strong that close to the nucleus, where every round cloud reaches in, electrons move at more than half the speed of light. At such speeds relativity draws gold’s round clouds in and shifts the energies between its patterns. Because of it gold takes in blue light and shines yellow, while silver, just above it in its column, sends back every color and looks white.

How Atoms Join

Atoms join when they have less energy together than apart, and something else must carry off the difference: a third atom that bumps into the pair as they meet and leaves faster, or a surface they meet on. Either way, what is around them ends up a little warmer. Two lone atoms that meet almost always bounce apart again; shedding the energy as a flash of light is possible, but rare. Two hydrogen atoms make the simplest case. As they come close, their two clouds merge into one, spread over both nuclei, with more of it between them than two lone atoms would have, holding both electrons, their spins pointing opposite ways. There each electron is pulled by both protons at once, and that shared pull holds the pair together. This is a covalent bond. Shared pairs of electrons hold together water, sugar, plastics and the DNA in every cell.

Table salt is held together differently, and the usual story about it is too simple. It says sodium wants to give away its lone outer electron and chlorine wants to take one, so that both end up with full layers. But taking sodium’s electron costs energy, and chlorine gives back only about seven tenths of it. What pays the rest, several times over, comes next: the sodium, now positive, and the chlorine, now negative, pull hard on each other, and in a grain of salt each is held by six neighbors of the other kind. Full layers come out of the bargain. They do not drive it.

In a metal, the outer electrons belong to no atom in particular. They roam through the whole piece, a shared sea that holds the atoms together and carries electric current. The sea is also why metals shine: light falling on the surface shakes the free electrons, and they send almost all of it straight back.

Even atoms that make no bonds pull on each other a little. A cloud has no rigid shell, and two atoms side by side fall into step. When the electrons of one are found a little to the left of its nucleus, those of the other tend to be found a little to the left of theirs, and two atoms lopsided the same way attract. This faint pull turns argon into a liquid when it is cold enough, and lets a gecko hang from glass by its toes.

Bonds can now be seen. The same kind of atom-sharp needle, tipped with a single small molecule, is lowered over another molecule lying flat on a surface. Where the electron clouds of tip and molecule begin to overlap, the push rises steeply, the same push that holds up the floor, and a map of that push draws the molecule’s rings of atoms and the bonds between them.

i One molecule’s rings of carbon, as such a needle maps them ×

Such microscopes have pictured single molecules bond by bond before and after a reaction, and found products the chemists had not expected.

Same Element, Different Weight

The protons decide the element. The neutrons beside them add weight but no charge, so they leave the electrons almost as they were, and the atom’s chemistry with them. Atoms of one element with different numbers of neutrons are called isotopes. Carbon always has six protons and, in nature, six, seven or eight neutrons. Carbon-12, with six, makes up about 99 percent of all carbon. Carbon-14, with eight, is made in the air by cosmic rays and slowly decays: half of any amount of it is gone in about 5,700 years. Living things keep taking it in and stop when they die, so the carbon-14 left in wood, bone or cloth tells how long ago they died, back some fifty thousand years. Hydrogen’s isotopes are the simplest: a lone proton, a proton with one neutron, called deuterium, and a proton with two, called tritium.

i All three are hydrogen: one element, three weights ×

Only some numbers of neutrons last. In a nucleus the strong nuclear force binds protons and neutrons together, while the protons push each other apart electrically. With too few neutrons or too many, a nucleus sooner or later changes, turning a neutron into a proton or the other way round, or throwing out a piece of itself, until its balance is better. Protons and neutrons, like electrons, cannot share a state with their own kind, so each kind fills its own set of states from the bottom up, and a light nucleus has the least energy with about as many of each. But every proton pushes on every other, so in a heavy nucleus that electric cost grows fast, and extra neutrons become cheaper than extra protons. Lead-208, with 82 protons and 126 neutrons, is the heaviest nucleus that has never been seen to decay, and every heavier one decays in time. Uranium, with 92 protons, is the heaviest found in nature in quantity. Heavier nuclei have been made one atom at a time, up to 118 protons, and the heaviest lasts less than a thousandth of a second.

Where Your Atoms Come From

The first nuclei formed in the hot, young universe. Single protons, the nuclei of hydrogen, appeared within its first twenty millionths of a second or so. Over the next few minutes some of the protons and neutrons fused into nuclei of helium, with a trace of lithium. Whole atoms, nuclei holding electrons, came far later. Helium, which holds its electrons tighter, kept them first, and by about 380,000 years universe had cooled enough for hydrogen’s protons to keep theirs too. Since then most ordinary matter has lost its electrons again: the gas in stars and the thin gas between galaxies is a hot mix of nuclei and free electrons. Whole atoms belong to the cooler places, such as planets and the dark clouds where stars are born.

Nearly everything heavier was made in stars. The exceptions, beryllium and boron, are mostly chipped off heavier nuclei by cosmic rays crossing space. Carbon, oxygen and the other elements up to iron are fused in stars, in their cores and in the explosions that end some of them, and are spread into space when stars shed their outer layers or blow apart. Beyond iron, fusion no longer releases energy, and heavier nuclei grow instead by catching neutrons. About half of them are built slowly, a neutron at a time, inside aging giant stars. Most of the rest, including most of the gold, platinum and uranium, are made in a flash in violent events. Collisions of neutron stars have been seen making heavy elements of this kind. Whether those collisions make most of them, or other rare blasts do, is not settled.

i An exploding star scatters what it made into space ×

So the nuclei of nearly all the atoms in your body are older than Earth. And there are a great many atoms: a body holds about 7 × 10²⁷. A single glass of water holds about 8 × 10²⁴ molecules, so many that counting one a second would take some 18 million times the present age of universe.

They do not stay. Water holds most of the hydrogen and oxygen in a body, and half of a body’s water is replaced in about a week to ten days. Most proteins are rebuilt within weeks or months, and bone over about a decade.

Carbon-14 shows which cells last, though not by its decay, which is far too slow to notice in a lifetime. The bomb tests in the open air in the late 1950s and early 1960s nearly doubled the carbon-14 in the air, and since then it has fallen year by year as the oceans and plants take it up. A cell builds its DNA from the carbon in food when it is born and keeps that DNA for life, so the carbon-14 in it gives the year of the cell’s birth. The nerve cells of the brain’s outer layer turn out to be as old as the person: their DNA keeps the carbon it was built with. About a tenth of the body’s fat cells are replaced every year, and the fat inside them within a year or two. Outside a few long-lived molecules like that DNA, what lasts is the arrangement, while the atoms pass through it.