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.
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.
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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