Friday, November 30, 2012

Atoms Part 1: How Atoms Are Made

The very first atom, a simple union of a proton with an electron, appeared when the universe was just about 380,000 years old. It was hydrogen.

Big Bang Synthesis

Just after the Big Bang, our universe started out as a tiny seething environment filled with unimaginable energy. In just a millisecond, tiny particles called quarks formed. They come in two stable kinds - called up and down. The temperature was several trillion degrees Celsius. These quarks, attracted to each other by a fundamental force in the universe, called the strong force, quickly formed little groups of three quarks each, called neutrons and protons. About seven times more protons were created than neutrons. Neutrons contain two down quarks and one up quark. Protons contain one down quark and two up quarks.

Free protons are very stable. Each one, made right after the Big Bang, will outlast our universe. But free neutrons are unstable. They have a half-life of just over 10 minutes. This means that of all free neutrons today, only half will remain 10 minutes from now. What happens to them? They decay into stable protons. A down quark decays into an up quark.

Few free neutrons had a chance to decay, however. A few seconds after the Big Bang, almost every free neutron was paired up with another neutron and two protons to make a helium-4 nucleus. Helium-4 nuclei are very strongly bound. The strong force again attracted them to each other just as it attracted quarks together. By the time the universe was about three minutes old, the process of making atomic nuclei had stopped altogether, and once bound inside nuclei, neutrons became stable.

The universe had enough time to make a handful of simple atomic nuclei: a lot of helium nuclei, along with trace amounts of deuterium and lithium nuclei. The majority of protons didn't bind into nuclei at all. They remained free instead. Around these protons and nuclei, electrons and photons zipped around in all directions. A photon is a particle of light and radiation. The universe had to cool down much more before electrons could settle into orbits around nuclei to create atoms. It took about 380,000 years. When the electrons settled into atoms, photons were freed up to fly away in all directions. This is where something you may have heard of, the cosmic background radiation, came from. These photons started off as highly energetic radiation called gamma rays. While they traveled, space itself expanded as the universe expanded. This expansion stretched their wavelengths longer. These photon travelers from that distant time reach us today as faint (long wavelength) microwave background noise. You can see it as snow on an old fashioned TV when it isn't tuned to any station.

Hydrogen is the lightest and simplest elemental atom in the universe. It is assigned the atomic number 1 because it has just one proton in its nucleus. Every single kind of atom, or element, has its own atomic number. This hints at something very important about atoms. The number of protons in an atom's nucleus determines what kind of atom it is - it is what makes iron, with 26 protons, different from copper, with 29 protons, for example:
Of all the mass of all the atoms in the universe, 75% comes from hydrogen. Before stars formed, the only atoms in the universe were hydrogen, helium and lithium:
The early atom picture is just a bit more complex than this, however. Each of these three kinds of atom comes in different types, called isotopes. To understand what an isotope is, let's take hydrogen as an example. The simplest hydrogen atom contains a nucleus made of just one proton. However, all atomic nuclei larger than this hydrogen atom also contain neutrons. The number of neutrons in an atomic nucleus determines which isotope it is.

The simplest isotope of hydrogen is hydrogen-1 or 1H. Other isotopes of hydrogen were also made when hydrogen-1 was made: hydrogen-2, called deuterium, and hydrogen-3, called tritium, for example:
Protons and neutrons (shown in orange and purple, respectively, in this diagrams) in a nucleus attract each other through a glue-like force called the nuclear strong force, but protons naturally repel each other because they are of the same positive charge. These two forces compete with each other. Neutrons tend to stabilize a nucleus because they attract each other and protons equally. Too many neutrons, however, can destabilize the arrangement. When an atomic nucleus takes on more and more neutrons, it tends to get more and more unstable. The nucleus becomes unwieldy. Deuterium, like hydrogen-1, is stable. These atoms, once they were formed billions of years ago never decay. Tritium, on the other hand, isn't stable. It is radioactive, and it has a half-life of about 12 years. This means that in 12 years, half of all tritium atoms today will have decayed into another, stable, atom called helium-3. An unstable tritium nucleus, with one proton and two neutrons, decays into a stable helium-3 nucleus, with one neutron and two protons. A neutron decays into a proton, releasing an electron in the process.

All the tritium in the universe would have disappeared billions of years ago if it were not made naturally when cosmic rays interact with gases. Neutron-heavy hydrogen-4, 5, 6 and even hydrogen-7 have been made in the lab, but they are very unstable isotopes and they last just tiny fractions of a second before decaying into more stable atoms.

Besides stable hydrogen-1 and deuterium, helium-3, helium-4 and lithium-7 atoms, all of which are also stable, were made when atoms began to form in the universe. Almost a quarter of the atomic mass of the universe is made up of helium-4, with trace amounts of deuterium, helium-3 and lithium-7. No other elemental atoms were made at this time. They didn't exist in the universe until the first stars formed about 200 million years later, and began to fuse lighter elements into heavier ones.

Star Core Synthesis

During most of a star's life, hydrogen fuses into helium under the enormous pressure and heat deep inside its core. Small stars, like our Sun, will eventually run out of hydrogen and nuclear fusion will stop. Only in very massive stars, much larger than our Sun, do carbon, oxygen and even iron atoms form in significant quantity, all from successive fusion reactions.

The production of carbon atoms requires even more energy than the fusion of hydrogen into helium. When a large star begins to run out of hydrogen, its core begins to collapse. This pressure heats it up even further. Helium nuclei begin to fuse fast enough to compete with the decay of their unstable product, beryllium-8. If they don't fuse fast enough, beryllium-8 simply decays back into two helium nuclei before any new fusion reaction can happen. Now, however, some beryllium-8 sticks around long enough to fuse with other helium nuclei. When they do, stable carbon-12 nuclei are created. Three helium nuclei fuse into a carbon nucleus. The two fusion reactions involved in making carbon occur almost simultaneously:

Carbon nuclei can fuse with other helium nuclei to create oxygen nuclei, and so on. Carbon atoms didn't form right after the Big Bang because the universe cooled down before carbon nuclei could form.

When we think about atomic nuclei fusing with each other to make bigger nuclei, we are describing a special high-energy environment inside a star. Here, the pressure and temperature are so high that atoms don't exist as atoms, with electrons orbiting nuclei. Here, the electrons are just too excited. They fly around freely. When this happens you get a special state of matter called plasma. Free nuclei float around in a "soup" of highly energized electrons flying around in all directions. This plasma within a star is similar to the plasma that filled the universe just before atoms formed.

In the plasma inside very large stars, all the available carbon nuclei eventually fuse into even larger nuclei such as oxygen, sodium and magnesium. Once the carbon is gone, the star contracts once again and gets even hotter. Oxygen fuses into silicon and sulfur nuclei as well as other elements. Eventually the core of the star contains nothing left but sulfur and silicon. It contracts further and it gets even hotter. It soon has enough energy to make even heavier elements. Each new nucleus is made by fusing a helium nucleus to an existing nucleus, in a step-by-step fashion. For example, a silicon nucleus fuses with a helium nucleus to make sulfur. The sulfur nucleus fuses with another helium nucleus to make argon. The argon nucleus fuses with another helium nucleus to make calcium, and so on. This process continues along to create larger and larger nuclei. It all happens very fast. The entire step-by-step process lasts about five days in total, until a nickel-56 nucleus is eventually made. Then it suddenly stops. This is when things get very interesting for the large star.

Stars burn brightly for many millions of years because massive amounts of energy are released every time light elements such as hydrogen fuse into helium. This energy is called nuclear binding energy. It's the energy needed to remove a proton or a neutron from a nucleus and it's the same energy that's released when a proton or a neutron is added to a nucleus.

As atomic nuclei get larger and larger, the increase in binding energy as protons and neutrons are added to the nucleus begins to wane, and eventually it doesn't increase at all. It becomes negative. The graph below charts binding energy as a function of nucleus size. Initially, as protons and neutrons are added to the nucleus, the binding energy of the nucleus increases dramatically. See the sharp upward incline to the left. You can see the increase start to wane at around carbon and oxygen nucleus size. (Notice the little jagged up-tick in energy at helium - this shows how much binding energy this particular nuclear arrangement has. This nucleus is especially tightly bonded together.) Now look at iron-56. This nucleus has the highest binding energy in this graph:

Actually, nuclei with 58 and 62 nucleons have the highest possible binding energy of all nuclei. We don't get them here in this massive star, because each new element must be a multiple of 4 (a helium nucleus has 2 protons and 2 neutrons). We get nickel-56 instead (that's 14 helium nuclei that have bound up together). As nickel-56 tries to fuse with one more helium nucleus to make the next largest element, zinc-60, the process actually requires energy instead of producing it. Rather than releasing energy when neutrons and protons are added, heavier elements release energy when protons and neutrons are removed ? fission rather than fusion is favoured. Nickel-56, therefore, is the last element this star or any star can make in its core.

With all this nickel being made, you may wonder why the cores of rocky planets and meteorites contain lots of iron-56 in them, not nickel. That's because nickel-56 is unstable. It has a half-life of about 6 days as it decays into cobalt-56, which is also unstable with a half-life of about 77 days. Cobalt-56 decays into iron-56, and that is a stable nucleus. It is the heaviest stable element made inside a star.

So what happens next when nickel-56 is made inside the star? And where do even larger atoms come from?

Supernova Synthesis

When stars are busy fusing nuclei, they are in a state of equilibrium. The outward force of thermonuclear fusion balances the inward force of gravity. When fusion reaches nickel-56 size, it stops altogether. The core begins to collapse once again but this time there's no new kind of fusion to ignite and restart the process. No more outward force is created to balance the crushing inward force of gravity. The star completely collapses in on itself. This collapse is catastrophic - it can reach a velocity as high as 70,000 km/second! The temperature and density of the core skyrocket. The inner core of the star eventually reaches a density comparable to that of an atomic nucleus. Consider that atoms are 99.9% empty space! Here, all that empty space has been squeezed out and atomic nuclei and electrons are packed in on each other. They are so tightly squeezed that protons begin to capture electrons and turn into neutrons. This process creates a dense hot neutron soup that can't decay any further. The temperature is about 6000 times higher than it was before the collapse. It is an environment where large and highly unstable neutron-rich nuclei form through a process called neutron capture, rather than through nuclear fusion. Free neutrons are basically squeezed into large nuclei so fast the nuclei don't have a chance to decay. There is an upper limit to nuclei formed this way. When the number of nucleons approaches 270 (that's larger than any even unstable atom scientists have created), the nucleus rapidly and spontaneously decays through fission, releasing a lot of energy, until it reaches a stable isotope.

The core collapses, creating a slew of new heavy unstable atoms all within a few seconds. The rapid collapse of the large star releases a staggering amount of gravitational potential energy. It is this energy that drives a supernova explosion. Heavy nuclei blow far and wide into surrounding space.

As the energy of explosion begins to subside, smaller unstable (radioactive) nuclei formed in the super-heated core undergo a series of nuclear decays (rather than fission) until they reach stable nuclei. Lead-204, with 82 protons and 122 neutrons, is the heaviest stable atomic nucleus in the universe. All elements in the periodic table with atomic numbers higher than 82 are unstable, although some of these have very long half-lives. Bismuth-209 for example, with 83 protons, has a half-life longer than the age of the universe, and therefore can be thought of for all intents and purposes as stable, and non-radioactive.

Every subsequent supernova seeds the universe with more and more large atomic nuclei. While the number of very small atoms remains about the same today as when they were created shortly after the Big Bang, the number of larger atoms continues to increase as they are created in the cores of large stars and during supernovae. These three kinds of atom creationBig Bang synthesis, star core synthesis and supernova synthesis - explain why there is so much hydrogen and helium in the universe, why there are significant amounts of oxygen, carbon and silicon - major components of rocky planets like Earth, and why heavy metals such as gold-79 (Au) and platinum-78 (Pt) are precious because they are present in the universe only in tiny amounts.

Next, take a look at Atoms Part 2, which explore how atoms emit light.

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