Cosmic Chemistry

Note that the element symbol is merely an abbreviation of the element (though sometimes it does not align with the English word for the element — take lead's abbreviation, Pb). These element symbols can be found on a periodic table.

Nuclear Equations and Reactions — Alpha, Beta, and Gamma Decay

We can describe reactions on the nuclear scale with nuclear equations. The logic behind a nuclear equation is straightforward — in a reaction where atoms are combined, ripped apart, or anything else, protons, neutrons and electrons cannot simply "disappear" or "appear". That is, their totals must be conserved before and after a reaction. This information makes solving nuclear equations straightforward. Let's start with a few examples:

The piece on the left of the arrow represents what existed before the reaction (in the first example, an atom of Uranium-235 is combined with a neutron), and the piece on the right of the arrow represents the configuration afterwards (in the last example, a Helium-3 atom and an electron are present).

If we recall that the bottom numbers indicate the number of protons, then one can indeed see that the sum of the bottom numbers on the left always equals the sum of the bottom numbers on the right (the number of protons stays the same). Likewise, the top number represents the number of protons plus neutrons — so if we know that the number of protons does not change, and the number of neutrons does not change, we would expect the top number to not change as well. This is indeed verifiable fairly straightforwardly.

As it happens, there are a few kinds of reactions which occur frequently and are thus classifiable. We call a Helium-4 nucleus an alpha particle, and a reaction which involves an "ejection" of an alpha particle an alpha decay. (Note that an "ejection" simply means "there originally was an atom, but it split up into an alpha particle and some atom by the end".) We can thus see that the second equation above is an example of alpha decay.

Similarly, an electron is called a beta particle, so the third equation is an example of beta decay.

There is a third kind of named particle which is somewhat more elusive — the gamma particle. The gamma particle is pure energy and as such has zero mass (top number) and zero protons (bottom number). It is often released in reactions involving atoms which have too much energy to be stable, whereupon the atom will release a gamma particle to lower its energy. Such a reaction is called (unsurprisingly) gamma decay.

Nuclear Fusion and Fission

There are two more kinds of common nuclear reactions: fusion and fission. Nuclear fusion refers to a reaction involving two nuclei being "smashed together" with extreme force to produce one larger nucleus. As such, fusion requires an environment with extreme pressure and temperature to occur, and large amounts of energy are released in the process. Fission, on the other hand, involves one nucleus being "ripped apart" to create two smaller nuclei and some leftover neutrons. Fission also involves a release of large amounts of energy in an extreme environment. Two examples of nuclear equations for fusion and fission are shown below.

Notice how the fusion reaction involves two smaller nuclei forming one larger one, while the fission reaction involves one large nucleus forming two smaller ones.

Radioactive Decay & Half Life

Sometimes, an element is inherently in an unstable state. This "instability" occurs from a nucleus having too much energy for its current configuration. In this unstable state, the forces that bind and repel the nucleus' protons and neutrons are unbalanced, meaning some of these nucleic particles may drift away. This loss of particles is precisely radioactive decay, the slow decay over time of an unstable element into an eventually stable form (when the nucleus loses particles, it eventually loses enough that the forces are balanced once more).

Because this process of particle ejection is stochastic, at any given moment it is up to chance whether a particle will happen to be ejected. Intuitively, that means that the more particles there are, the more chance there is that one will be ejected, so this decay should happen quickly at first (while there's lots of particles) but slow down over time (once most of the particles are lost to decay). To model such a decay, we use an exponential curve, which essentially means that the mass of the nucleus will "halve" after a certain amount of time. This amount of time is called the half life of the element.

To make this more clear, let's use a concrete example. The half life of strontium-90 is 29 years. That means that if you take 100 grams of strontium-90, in 29 years you'll only have 50 grams of it left (the other 50 grams will have been lost to decay). In 29 more years (that's 58 years total), you'll only have 25 grams of it left, since 25 is half of 50. In 29 more years (87 total), you'll only have 12.5 grams of it left, and so on.

The Electromagnetic Spectrum

"Radiation" may conjure images of Chernobyl, but it's simply the word for the emission of energy. Sometimes, that energy might be dangerous, like in nuclear accidents, but that's not always the case. To classify the different kinds of radiation, we use the electromagnetic spectrum.

To understand the electromagnetic spectrum, we need to understand that all radiation is really just a form of light. It's not necessarily the light that we can see, but the light that we can see is a part of the electromagnetic spectrum. Take a look at it below:

We encounter forms of radiation in our everyday lives — when we listen to the radio, when we turn on the microwave, when we look around, or when we get an X-ray. All of this radiation is carried by photons — or more accurately, photon waves.

Because light and everything else on the electromagnetic spectrum is actually a wave, we can organize them by wavelength, and this is precisely why the electromagnetic spectrum is called a spectrum! In the spectrum above, larger wavelengths are shown on the left, while higher wavelengths are shown on the right. Notice that visible light is around the middle, things that we encounter on a day-to-day basis tend to be on the left, and really dangerous things tend to be on the right. In fact, this is no accident.

When wavelength increases, the "size" of the wave decreases. That means that if we imagine the wave of radiation passing through you, there's less wave "passing through you" at any given moment. Think about that for a moment; if the wave is really long, then it's going to take a long time for just that one crest of wave to pass through you, while if it's really short, then it won't take long for the whole crest to pass through you. This provides an intuition for why longer wavelengths mean lower energy levels (and vice versa). The reason that the radiation on the right is dangerous is because it carries high amounts of energy, while radiation on the left does not.

Quantitatively, we can describe the relationship between wavelength and energy as follows:

E = hc / λ

Where E is the energy of the wave, h is Planck's constant (6.62607015 × 10^-34 m² kg / s), c is the speed of light (2.998 x 10^8 m / s), and λ is the wavelength. Making sense of the equation, as λ increases, E decreases, so as the wavelength increases the energy decreases (and vice versa).

We can also work backwards using the equation. Given an energy, we can find the wavelength of the light that is emitted. This is important in the flame test, a procedure involving examining the color produced by a given metal when it is placed into a fire. When the metal is heated, its electrons become "excited", jumping outward to a higher energy level. They cannot remain excited forever, so they put their energy into emitting light in order to return to their ground state (moving back down towards the nucleus again). Given this change in energy, one can compute the wavelength of light emitted, or vice-versa, using the equation.

The Big Bang Theory

Billions of years ago, the universe began as a tiny point. The point was an infinitely small singularity, and the point was truly the extent of the entire universe. Then, a violent "explosion" caused the universe to start expanding outwards, cooling as it expanded. As the universe cooled, matter began to form — first quarks formed into subatomic particles, and then those particles formed into small atoms like hydrogen and helium. Over time, this "cosmic dust" was compacted together by gravity, forming ever larger structures like planets and stars. Stars, in turn, performed fusion in their extremely hot, pressurized insides, forming larger and larger elements. When the stars ran out of elements to fuse, they lacked the energy to continue burning, and for stars where the pressure inside won out over gravity a massive supernova occurred. These supernovae created such hot, pressurized environments that even larger elements could form, thus explaining the creation of the elements we know today. (In actuality, a few of the heaviest elements known were created by scientists in a lab.)

This idea is likely not surprising, but it was understandably novel when first proposed. When George Lemaitre, a priest and physics professor, first put forward the idea, it was laughed away and given the joking nickname "The Big Bang". (The name, as it happened, stuck.)

But scientists have found extensive evidence supporting the idea of the Big Bang. Perhaps most obviously, hydrogen and helium make up a vast majority of the universe, suggesting that they were created first (in the Big Bang) and then other elements formed from them. We can also see the "leftover" radiation from the violent explosion itself in the Cosmic Microwave Background, pictured below. (Essentially, it is a map of sky showing how much leftover radiation can be seen by a microwave telescope. Redder hues indicate more radiation.)

Finally, the universe is in fact still moving apart to this very day — when we look at galaxies, they are redder than we would expect them to be, and this redshift indicates that they are moving away. Redshift is best explained with a diagram; essentially, because the object is moving away, the light wave gets "stretched out", giving it a longer (redder) wavelength.