Elements

The Periodic Table

Just about everyone is familiar with the image of the periodic table.

Even the name is familiar, you say "periodic table" to some one and chances are, providing they have had some education, they are likely to picture something that looks a little like the image to the left.

I have always loved the periodic table, when I was a kid and I first saw one in a class room I was instantly struck by just how beautiful it was. In those days there were 103 elements, the last being Lawrencium named after Ernest Lawrence the Nobel prize winning physicist who had quiet a lot to do with radioactivity. These days there are more than 112, the last being Copernicium.

Livermorium at 116 has been added as of May 2012, so far 35 atoms of Livermorium have been produced in the 12 years since it was first discovered!

The periodic table as it is now was first dreamt up by Dmitri Ivanovich Mendeleev and presented by him in 1871, it looks a little different to the modern day, but that is partly because there were only 56 known elements at the time.


What fascinated me about the periodic table was that it listed each and every element that makes up the known universe. All in one little table. Each element having one more proton than its predecessor. The elements are stacked into rows consisting of 2 elements, H and He, then 8, that's Li through to Ne, then 8, then 18, then another 18, then 32 and finally the remaining 25 to make a total of 112. If we get as far as element 118 then chances are we will have to start a new row.

Carbon, which gives us diamonds the hardest known material, sits right next to nitrogen which is a gas! Yet the differ by only a single proton. Potassium, the highly reactive metal that burns with a purple flame has only one more proton than Argon, a gas. Some of the elements become superconducting when cooled, others do not. Some are great at carrying electricity, others are semi conducting or have insulator properties.

The chemistry of the elements seems to be governed by the number of electrons each atom has around it, this is governed by the number of protons, the number of electrons being equal to the number of protons in electrically neutral atoms. While this makes sense in a way, it is still quiet staggering when you consider the differences between the different elements.

Some elements have radioactive isotopes which means that can undergo some form of particle emission from the nucleus and turn into another element. Some of the large atoms, Uranium for example can decay into two elements!

What I also find completely mind blowing is that apart from Hydrogen and Helium, which we currently believe have been here since the beginning of the universe, and excluding those that have been made in the lab, the rest were actually made inside stars.

Every atom that makes up our bodies is listed in the periodic table. We are mainly made from Hydrogen that has been here since the beginning of time. There may be atoms in your body that are 13 Billion years old and were there to witness the birth of the universe (that is if the Big Bang theory is correct!).

How cool is that, to think that we contains atoms of Hydrogen from the very beginning of the universe. Know wonder there are some days I feel old, it's because my constituent parts are really, really, really old.

Beta emissions and quarks, really?

Up becomes down and out comes a positron

All atomic nuclei are made up of different numbers of protons and neutrons, the different combinations of protons and neutrons give us the elements and their isotopes.

Radioactivity is the emission of particles from the nuclei of atoms.


Of all the different types of radiation, I think my favourite is probably beta emission and it is this I am going to talk about in this post.


Beta emission is where an electron, or a positron, is ejected from the nucleus of an atom. Now for those of you paying attention, I am sure you have just raised an eye brow and thought, "hold on a minute, you said that the nucleus is made up of protons and neutrons, no mention of electrons there!" and this is absolutely true. There are NO electrons in the nucleus of an atom. None whatsoever and yet, they can still be ejected from the nucleus!

I think this is absolutely fantastic.

Many believe that we can view a neutron as a proton and an electron bound together. By that logic, positron emission, the emission of a positively charge electron, implies that a proton is a positron and a neutron bound together. This is not right.

The current thinking is that protons and neutrons are actually made up of quarks. This is part of the Standard Model, which appears to have had a rather spectacular result recently with the discovery of the Higg's boson.

Quarks can change and as a result of this change electrons or positrons can be produced that come whizzing out of the atomic nucleus.

Take Fluorine 18, this has 9 protons and 9 neutrons. The stable form of fluorine is fluorine 19, 9 protons and 10 neutrons. The extra 1 neutron in fluorine 19 makes it a happy bunny. But fluorine 18, completely unhappy and it shows this by kicking out a positron, like so

¹⁸F -> ¹⁸O + e⁺

one of the protons has changed to a neutron, so only 8 protons remain, and the 9 neutrons become 10, making it an Oxygen atom. Oxygen 18 is also a happy bunny and does not decay any further.

So, this is the thing that I am going to be pondering for the next week or two, Fluorine 19 - happy, Fluorine 18 - unhappy, Oxygen 18 happy. One neutron difference between F19 and F18 is enough to make the second, F18 unstable. It seems that the neutron has a calming effect. Will ponder this in a later post.

Now, to finish, fluorine had 9 protons and 9 electrons to balance it out. Oxygen, the final product, only has 8 protons so only needs 8 electrons to balance it out and make it electrically neutral. So what happened to that extra electron?

Well, maybe, the positron, which is the anti particle of the electron, interacts with the extra electron to form something that we could call positronium. The two particles could then annihilate each other and decay into gamma rays.

Now that would be really cool.

Size of an atom

Helium nucleus surrounded its electron cloud

The size of the atomic nucleus is an absolute wonder to me. Atoms are incredibly small and yet they are massive compared with the size of the nucleus. When I was a kid I once read that if an atom of Helium was the size of the Albert Hall in London, then the nucleus would be about the size of an orange.

The size does vary according to the atom in question, Uranium atom nuclei are about 8 times larger than a hydrogen nuclei, which only contains a single proton. The largest come in at about 15 fm (Femto metres).

15 fm is tiny, it is about 10,000 times smaller than an atom, and this is why people often say that atoms are mostly made up of nothing. I think this is wrong. Atoms cannot be any smaller than they are, you cannot make them any smaller. You increase the pressure and eventually atoms will collapse into neutrons, this is what we believe happens in neutron stars. They do not collapse into smaller atoms.

So I think that they are actually as full as they possibly can be. They are full to capacity, which got me pondering to what they are actually full off.

Take the hydrogen atom, a single proton and a single electron. The proton is tiny and the electron is also very tiny. They carry equal but opposite charges. The proton is about 1830 times the mass of the electron. The interact to create the hydrogen atom. They both spin. This leads us to imaging the proton like a large planet with a small satellite, the electron in orbit around it, which is wrong^*, but looks nice.

The size of the hydrogen atom is about 0.03 nanometers, which is huge compared with either the electron or the proton, yet this is the smallest it can possibly be.

The best explanation we currently have for the behaviour of an atom comes from quantum mechanics (QM). I have been told that the Heisenberg Uncertainty Principle (HUP) explains why the electron, which is negatively charged, does not drop straight into the positively charged proton to form a neutron, after all opposites should attract.  I've never really understood or been convinced by the argument, seems a little odd to me. That said

Atoms do change size as we move through the period table, Hydrogen is the smallest and the largest has a radius almost 10 times the size of Hydrogen, which is comparable to the change in size of the atomic nucleus for the two extremes. Is this coincidental or is there something in this?

The reason we think the atoms change size is due to the electrons having to pack in around the atomic nucleus. Hydrogen has a single electron so it is small, Uranium has a whopping 92 electrons surrounding each atomic nucleus, so it is larger!

All atoms are small compared to the wavelength of visible light, which is about 1000 times bigger, which really baffles me, because visible light should not be able to see something as small as an atom, and yet interactions occur between visible light and atoms. Just take a look in the mirror! This is just visible light striking atoms and being reflected back.

The interaction of light and atoms is currently explained by Quantum electrodynamics or QED for short. A brilliant theory that has been tested over and over and has come through time and again. Something I will cover in another post.

So what is the point of this post?

Atomic nuclei are typically 10,000 time smaller than atoms, which are in turn 1,000 times smaller that visible light. So from visible light to atomic nuclei that is a change is scale of 10 million. Let's see if we can get that in perspective. Imagine a ball with a diameter of about 1 metre then the wavelength of light would be comparable to the width of the planet earth. Which is once again, too big for us to image.

I suppose the point is, than even on tiny scales, those of visible light, atoms and nuclei, the distance from the largest to the smallest is absolutely massive and is so big to be beyond our imagination.

What I still can't figure out is why atoms are the size they are and why nuclei are the size they are, why approximately 10,000 times, why not 50, or 5,000,000? will have to ponder, when I come up with something I'll let you know.