The Alchemist's Dream

In the previous post we saw that chemistry is simply the movement of electrons between atoms. The sheer number of different types of atoms and ways in which they can exchange electrons is the reason chemistry is such a complex and diverse field of science.

Electrons, as you may recall, surround the nucleus (without touching it) in what is called an electron cloud. Although the exchange of electrons during chemical reactions will affect the electron clouds around the nucleus, the nucleus itself remains unaffected. In other words even the most violent of chemical reactions will have no effect on the protons and neutrons in an atom.

Let's forget everything we know about atoms for a moment and step back in time. If you've ever heard of the term "alchemy" before, you probably associate it with ancient mad scientist types trying ruthlessly to figure out a way to convert regular metals into gold. This was supposed to be possible by mixing them with a substance known as the "philosopher's stone" but was never successfully done. Towards the 18th century, the field of alchemy was surpassed and replaced by its offshoot: chemistry. Alchemy took a more philosophical/mystical approach, while chemistry was based on testing hypotheses against the results of repeatable experiments. It was this approach that led to the discovery of the elements as well as the atom and its constituents (which we discussed in The Indivisible(?) Atom).

Image courtesy of Wikimedia Commons

Recall for a second the goal of alchemy: the conversion of regular metals into gold. After centuries of trial and error, why didn't anyone succeed in making the transformation? It's true that alchemy was less robust than chemistry but alchemists still managed to figure out how to extract metals from rock and mix them into alloys like bronze. Even cavemen and cavewomen were able to produce the chemical reaction known as fire without any chemistry knowledge. You would think that someone over the centuries would have accidentally had the luck of creating gold from regular metals. That is of course, assuming it's actually possible. This is where chemistry enters into the picture, let's have a look at the periodic table.

Image courtesy of Wikimedia Commons

We can see that Gold (symbol Au) is an element with atomic number 79 (located near the centre of the table). The fact that it's an element is important because that means it cannot be made by mixing other substances. Gold is one of the fundamental types of atoms that make up our universe. It happens to have 79 protons and, when neutrally charged, 79 electrons as well. We saw at the beginning of the article that chemical reactions affect only the electrons around atoms. The nucleus, including all the protons and neutrons within it, remains unaffected. It is therefore impossible to create gold from any chemical reaction. No wonder nobody was ever able to do it.

The fact that the atoms themselves are not affected by chemical reactions was a powerful discovery. Early chemists like Antoine Lavoisier were able to figure this out by carefully capturing all byproducts of reactions and weighing them. They found that they always weighed the same as the initial reactants. Lavoisier through his own experiments and the careful review of others was able to show that water, long though to be an element, was in fact made up of hydrogen and oxygen. He deduced that water could be separated into hydrogen and oxygen and that they could be recombined again into water. "Nothing is created, nothing is destroyed, everything is transformed" as he put it.

Unfortunately he also collected taxes for the king and was executed during the French Revolution

So there you have it, atoms themselves do not change during a chemical reaction, they simply re-arrange themselves to form new compounds. This idea remained a cornerstone of chemistry for about 200 years. However this long-held belief was shaken in 1902 by Ernest Rutherford and Frederick Soddy while they were studying the radioactive element thorium (symbol Th, number 90 on the periodic table). The two noted that their thorium sample was spontaneously producing helium, among other substances. Conventional wisdom suggested this should not be possible. How could one element (thorium), be creating another element (helium)? They concluded that radioactivity is the result of atoms spontaneously decaying into other atoms. So much for atoms never changing!

Although the idea was ground-breaking, it is still true that in a chemical reaction the atoms themselves do not change, they simply exchange electrons and recombine. Reactions which involve atoms decaying into other atoms (also known as transmutation) are called nuclear reactions because they are the result of changes in the nucleus of an atom. These changes happen when the nucleus is unstable and can be induced artificially in the lab. Rutherford first demonstrated this in 1919 by transmutating nitrogen into oxygen.

So if nitrogen can be transmutated into oxygen, is it really possible after all to realize the alchemist's dream of creating gold out of regular metals? The answer is yes, in fact this was demonstrated by Glenn Seaborg in 1980 starting with the metal bismuth (atomic number 83 on the periodic table). However it required the use of a sophisticated particle accelerator, something no alchemist could have gotten their hands on. Alchemists were limited to using chemical reactions only and therefore had no hope of ever producing gold. Trasmutating one atom into another requires a nuclear reaction, something we will discuss further in the next article.

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What is Chemistry?

In a previous article, I discussed the structure of the atom and mentioned that it contains positively charged protons and neutrally charged neutrons in the nucleus, and negatively charged electrons forming a "cloud" around the outside. Below is a helium atom as an example.

Despite the fact that electrons are many times smaller than protons, their charge is of the same magnitude. That is to say that the amount of negative charge on one electron is exactly equal to the amount of positive charge on one proton. An atom therefore needs an equal number of protons and electrons to have a neutral charge (in the case of Helium, this works out to 2 protons and 2 electrons). Whenever the number of protons and electrons are not balanced, the atom is called an ion.

At this point I would like to elaborate a bit on exactly what I mean by electron "cloud". In the early days it was thought that electrons circle around the nucleus in orbits called "orbitals". It seemed to make sense at the time that each atom must look like a little solar system with the nucleus playing the role of the sun and electrons the role of planets. As elegant as this sounds, the real answer turned out to be much more strange. 

Whenever charged particles change direction or speed, they give off a little energy in the form of electromagnetic radiation. This law of nature is the reason radio towers can send music to your car radio receiver and why power stations can produce electricity from spinning turbines. If electrons were truly orbiting around the nucleus, as they turn they should constantly emit some electromagnetic radiation and in doing so lose some energy. This constant loss of energy should cause the electrons to spiral ever closer and eventually crash into the nucleus meaning that atoms would only be stable for a fraction of a second. Scientists couldn't help but notice that all the atoms in the universe don't seem to be spontaneously destroying themselves so it was back to the drawing board on the structure of the atom.

The solution to this problem required an entirely new branch of physics known as quantum mechanics, which is a study of motion on a very small scale. We will discuss quantum mechanics in a different article but it suffices to say that the solution to the problem is that electrons do not orbit at all, instead they exist around the nucleus in a sort of probability cloud. There are several different types of electron "clouds" (also referred to as "shells"). Each has a different shape and is capable of holding a different number of electrons.

Let's return to the periodic table of the elements for a moment (see the previous article The Indivisible(?) Atom for further details on the periodic table). We can use the table to instantly find the number of electrons needed for any given atom to have a neutral charge. Recall that the numbers given below are the number of positively charged protons in the nucleus of that atom. For example carbon (symbol "C") has 6 and argon (symbol "Ar") has 18. boron would therefore need 6 electrons to be neutral and argon would need 18. 

Image courtesy of Wikimedia Commons

We can also use the periodic table to determine how the electrons will organize themselves into various types of electron clouds. These clouds will fill with electrons starting from the inside (close to the nucleus) and moving outwards. Interestingly enough, most of the chemical behaviour of an element depends only on the number of electrons in its outermost cloud. These electrons are referred to as valence electrons. You can get a sense of how many valence electrons an element has by its position on the table. The elements in the far left row have one valence electron and the elements on the far right have a full outer cloud. So sodium (Na) has one valence electron and helium (He) has a full outer cloud. The elements in the second row from the right are one electron away from having a full outer cloud. chlorine (Cl) is an example of this.

So what does it matter whether the outer cloud is full or not? As it turns out, atoms will exchange electrons with other atoms in order to achieve an outer cloud that is either full or empty. For example sodium (Na) will readily give up its only valence electron to a chlorine (Cl) atom. Sodium empties its outermost cloud and chlorine fills its outermost cloud so its a good deal for both atoms. The result is a positively charged sodium atom (since it lost a negative electron) and a negatively charged chlorine atom (since it gained a negative electron). The two opposing charges attract and the two atoms stick together forming sodium chloride (which you may recognize as the chemical formula for good ol' table salt). I should note that this reaction is incredibly violent and yet the result is a chemical that is safe enough to sprinkle on your fries.

Image by Petr Kratochvil

Great, now what's the chemical formula for some ketchup?

So why exactly do atoms care whether their outermost clouds are filled? They don't. It is often taught in introductory chemistry classes that atoms are "striving" to fill their outer clouds as if atoms are some sort of sentient beings trying to keep up with their neighbouring atomic Joneses. The reason an electron will move from the sodium atom to the chlorine atom when the two pass close enough is because the result is a lower energy state. Admittedly this isn't a very satisfying answer at first glance so let's look at an example.

Think of a ball sitting on a table. If undisturbed the ball will stay put. If you give it enough of a bump however, it will roll off the table onto the floor and stay there. If you bump it again, it will not hop back up onto the table because in the gravitational field of the Earth, the floor is a lower energy state than the higher table. Once it moves to a lower energy state, it takes a significant amount of energy to reverse the change (in this case you would have to pick the ball up to put it back). The outermost electron in a sodium atom is in the same situation. It's sitting there just fine, however if that sodium atom bumps into a chlorine atom, the electron will "fall" away from the sodium electron cloud towards the lower energy state in the chlorine electron cloud. One the electron has been transferred, it would take a lot of energy to move it back.

Image courtesy of photos-public-domain.com

Uhh... a little help?

We've spent a lot of time discussing the electron clouds and transfer of electrons between these clouds, but what does all this have to do with chemistry? The answer is: everything! The movement of electrons between atoms is chemistry. When you see a firework explode, there is a rapid reaction taking place between oxygen and other chemicals that generates a lot of gas and heat in a very short time. The heat creates bright light and the rapidly expanding gases make a loud BANG! What about the old vinegar and baking soda trick? The atoms in vinegar (acetic acid) and those in baking soda (sodium bicarbonate) bump into one another and the electrons flow towards lower energy states. This results in the atoms recombining to form salt, water, and carbon dioxide which quickly bubbles out of the mixture. Every reaction you see in daily life, whether it be cooking food to improve its taste or watching metal rust, is the result of electrons moving between atoms. The sheer number of different types of atoms and the limitless number of ways they can be combined are the reason that chemistry is such a massive and complex field of science. And yet behind all this complexity is the simple movement of electrons from higher to lower energy states.

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The Indivisible(?) Atom

What if you were to cut the computer or handheld device you are reading this on in half. Then cut those pieces in half, and those pieces in half, and so on. Would you ever get to a point where this was no longer possible? Is there a building block so fundamentally small that it cannot be divided into smaller pieces?

This question has been asked since ancient times, in fact the word atomos was first coined by Leucippus sometime between 400-500 BCE. Leucippus, like many in his time, was a philosopher and was trying to one-up another philosopher named Zeno. Zeno had come up with the rather mindblowing idea that all motion must be an illusion. He reasoned that in order to walk across a room you must first cross half that room. But in order to cross half the distance across the room, you must first cross half that distance too and so on. Because you can divide the distance between any two points into an unlimited number of halves, there must be an infinite number of points. Since crossing an infinite number of points is impossible, all motion must be an illusion.

My mind hasn't been this blown since I saw Inception

Leucippus clearly didn't buy into Zeno's idea and proposed the following clever counterargument. It is impossible to divide a given distance into an infinite number of points because eventually you would reach some sort of fundamental building block that cannot be divided further. He called these building blocks "atomoi" meaning "indivisible". Leucippus passed his ideas onto his students and one of them named Democritus took it even further. Democrites claimed that atomoi each have different shapes which allow them to hook together an various ways. He said that the atomoi themselves never change, they simply recombine in different ways to make different materials.

The ideas of Leucippus and Democrites were surprisingly ahead of their time considering it wouldn't be until the year 1800 before the first atomic theory would be proposed by John Dalton. They were correct that atoms were the building blocks of matter and that the atoms themselves never change, they simply combine in different ways. A substance which is only made of a single type of atom is referred to as an element, and a substance made of two or more different types of atoms combined is called a compound. There are about 98 naturally occuring elements but upwards of 118 have been either already been synthesized in labs or probably will be in the near future.

Image courtesy of Wikimedia Commons

You probably remember the periodic table from high school chemistry (or possibly from your nightmares depending on how much you like chemistry). This chart quite literally contains all the building blocks required to make everything around you including yourself. In fact there's a good chance that nearly all of the 98 naturally occuring elements are somewhere in your body right now, if only in trace amounts. Just like Democrites said, atoms can combine with other atoms to make compounds, however the atoms themselves never change.

Ironically, despite being correct about the existence of atoms and their ability to combine to form compounds, Leucippus and Democritus were actually wrong about the most important part of their arguement. Atoms, a.k.a "indivisibles", can actually be divided!

So was Zeno right? Are there really an infinite number of points between any distance? Should I post the moving green stick man picture again with a clever caption? As it turns out, the atom is made up of three smaller particles: positively charged protons, negatively charged electrons, and neutrally charged neutrons. The protons and neutrons bond together in the nucleus of the atom and the electrons stay around the outside forming a so-called "electron cloud". An atom can be identified by the number of protons in its nucleus (this is the number it is given on the periodic table). The number of protons will determine the number of electrons needed to make a neutrally charged atom. In other words you need 2 electrons to have a neutrally charged Helium atom since Helium has 2 protons. If the electrons and protons are not balanced, the atom has a charge and is called an ion.

Structure of a Helium Atom

The neutrons however are a different story, they play a role in allowing the nucleus to bond together. Changing the number protons would change the element, changing the number of neutrons however changes the isotope. Any given element can have several different isotopes, but not all isotopes are stable. Any time there isn't a proper balance between neutrons and protons, the nucleus will try to rearrange itself into a more stable configuration. This can result in neutrons becoming protons, protons becoming neutrons, or even in the atom splitting into two (known as nuclear fission).

So if the "indivisible" atom can actually be broken down into protons, neutrons, and electrons, can these particles be further broken down? As far as we can tell, electrons are truly fundamental particles. They cannot be further broken down and are part of a family of fundamental particles known as leptons. As for protons and neutrons, they are made up of even smaller particles called quarks. A proton is made of two Up quarks and one Down quark and a neutron is two Down quarks and one Up quark. Quarks, like electrons, are fundamental particles and cannot be further broken down.

And so we have finally arrived at the answer to our original question. If you were to cut the device you are holding now into progessively smaller and smaller pieces, you would eventually end up with quarks and leptons (in addition to a voided warrenty) which cannot be broken down further. Looks like Democrites and Leucippus have the last laugh on this one.

Democrites always has the last laugh


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