Yes, centre. I will fight you. I am from the UK, so it is centre. Anyone who says otherwise will get astatined. Over the past few weeks we’ve been looking in a bit of detail about the history of chemical elements – back in the day when they were just lumps of rock. Now we’re going to go deeper into a bit of atomic and subatomic theory. And get small. That’s the first thing. Atoms are really, really, really, really small. It’s hard to get your head around how really small they are. But I’ll try to give an example.
A standard tennis ball is 6.5405 cm across, or 0.065405m (that’s actually at the lower end of tennis ball sizes, but we might as well use that as a standard). An atom of helium is 31 picometres across. Or 0.00000000031 m. It’s hard to get your head around these numbers as well, so here’s it another way. If a helium atom was the size of a tennis ball, a tennis ball would be 210,000 km across. Jupiter, for comparison, is 139,822 km across. Atoms are really f**king small. But we need to go even smaller. Most people (I hope) will have seen this graphic of an atom.
This is a nice image of an atom, but it’s not massively accurate. The core features are right, sure. There’s a nucleus in the middle, made of neutrons and protons (red and blue) with electrons (grey) going round the outside. But the scale is very, very wrong. Here’s a more accurate drawing of an atom. I couldn’t get any exact numbers on the size of helium nuclei, so I’ll use the guide above as an estimate. If we have our tennis ball-sized atom, the nucleus will be about 0.00000065 m across. Which is small. But we want our nucleus to be the size of a tennis ball. The atom is now 654 metres across, the tennis ball the atom was part of is now 21,000,000,000 km across. The distance from the Sun to Pluto is 5,906,376,272 km – half the size of the radius of our tennis ball.
At this level we can see some interesting things. The nucleus itself is made of two different particles – protons and neutrons. No-one knows what they look like, because they are too small. But we do know that the nucleus itself has a positive charge. The neutrons have no charge, and the protons have positive charge. This nucleus is held together by the strong nuclear force, one of the four fundamental forces of the universe (if it’s not electromagnetic, strong nuclear, weak nuclear or gravity then it is illusion or lies or Illuminati).
The electrons (whizzing around the outside) are negatively charged – so attract themselves to the nucleus with standard magnet-type attraction. And we at last get to the main point. The electrons (which are still invisible to the naked eye) are going around the outside in shells. It’s these shells that give the element all of its properties – and as such, will be covered next week in more detail. However, the all-important electrons are still too small to see – only 0.006 mm across. Let’s take a closer look.
If we make the electrons the size of tennis balls, things become even more interesting. At this level, we aren’t even sure what sizes the fundamental particles are – but we do have their masses. Which are really, really small. I’ll make the (probably wrong) assumption that each of the particles has identical density, so their sizes can be extracted from their masses. The protons and neutrons are each about 10000 times more massive than the electrons, so let’s use that as a size reference. The electron is the size of a tennis ball, 0.0654 m across. The protons and electrons are each around 65.4 metres across, making the nucleus 654 m across. The whole atom is 6540km across. And the tennis ball is 210,000,000,000,000 km across, 22 light years.
And at last we have set the scene for the main point of this post – atomic identity. Only at this sort of zoom level can we see what makes an atom tick. And it all comes back to those three particles – electrons, protons and neutrons. I’ll deal with them one at a time.
Protons are what make elements what they are. You can mess around with the numbers of neutrons and electrons no problem, but change the number of protons and the atom has an identity crisis. The number of protons defines what element that atom is. That little number at the top – that’s the number of protons in the atom. You throw in another protons, and fluorine becomes neon. That fact is important later on, so keep it in mind.
Electrons determine the charge of the atom. Electrons have a negative charge (remember?) and atoms are by default neutrally charged – meaning the number of positive protons balances the number of negative electrons. As explained above, you can’t change the number of protons – but you can change the number of electrons – which will give you…
An ion is an atom with a non-zero charge – meaning it has a different number of protons and electrons. There are a few ways to make ions, which will also be detailed later, but they give us another important thing you can do with atoms. Opposite ions attract each other. Because salts are formed of an alkali metal (which forms a positive ion) and a halogen (which forms a negative ion) the two combine together to give us…
Salt. Ionic bonding will be discussed in more detail at a later date, check back regularly to view any updates.
Neutrons are the last of the three main subatomic particles. They have no charge – so you might think they’re boring, but think again. It’s neutrons that give us the last defining feature of atoms – the isotopes. Isotopes are just atoms of the same element with different numbers of neutrons. Take this example to the left. The number at the top is the atomic weight, which is essentially the number of neutrons plus the atomic number. Atomic weight for an individual atom is the number of protons plus neutrons, as electrons have insignificant mass. Carbon has an atomic number of 6 (6 electrons, 6 protons) and usually has 6 neutrons. But you can throw neutrons at it (or take them away) and get four flavours of carbon. Carbon-11(synthetic), carbon-12(makes up 98.9% of all naturally occurring carbon), carbon-13 (1.1%) and carbon-14(an insignificant percentage of all carbon). Generally, the isotopes aren’t massively important. They can slightly change the properties of an atom – the way it forms crystals, the way it bonds – and can critically change the properties of the atom as it undergoes nuclear fission. This is the reason that uranium-235 can be used in a nuclear weapon but uranium-238 can’t be. The difference seems inconsequential but can be a matter of life and death in certain situations. So there you have the three numbers that determine the properties of an atom.
From those three you can get lots of other properties of the atom – atomic weight (as mentioned above) which is neutron number plus protons number being the main one. That is all the main things you need to determine the properties of an atom. So we should have finished our crazy zoom, right.
Wrong. The three subatomic particles aren’t the end of the line. Even protons and neutrons and electrons are made of something – quarks. These are the smallest particles of matter that we currently know of – and as far we can tell, it is the smallest particle that can exist. There are 6 ‘flavours’ of quark – up, down, top, bottom, strange and charm. These vary in size, so I’ll look at the smallest one – the up quark. It has a mass of 2.01 megaelectron-volts. About 0.214% of the mass of the proton.
Hang on! That would put the mass of the quark as twice that of the electron. That’s the strange thing. Quarks are one of the fundamental particles (they are used to build things that are similar sizes to electrons), but electrons manage to be smaller. There are hypothetical smaller particles – like photons or gluons – but they have irrelevant mass. The very smallest, the graviton, actually has zero mass.
But the photon comes a close runner up, with a mass around 1×10^-18 MeV/c²
Side note: The megaelectron-volt (MeV) isn’t exactly a measure of mass. Technically, it’s a measure of charge. But you can use e=mc² to extrapolate the mass from this number – so I’ll use it anyway. (Almost nothing in that last sentence was rigorous because I’m not a quantum physicist, so sorry). Also, Wikipedia lists mass of photons in megaelectron-volts, so I have to use it. For fear of catching the unwanted attention of our great Wikipedian overlords, praised be them.
In other words, 1,000,000,000,000,000,000 times smaller than a quark. So let’s use that as the final baseline.
If we make the photon the size of a tennis ball, things get very large indeed. The atom that we looked at is now 6,540,000,000,000,000,000,000 km from edge to edge. That equates to a staggering 691 293 310 light years across. That means that the tennis ball (remember, right back at the start?) is approximately 22,000,000,000,000,000,000 light years across, compared to our galaxy which is only 100,000 light years across.
This means our tennis ball is bigger than the observable universe – we can only see things 13,700,000,000 light years away, because light from the dawn of the universe is only just reaching us from that distance. And my quick back-of-the-envelope calculation says the air pressure at the centre of the ball (if it was at standard air pressure around the edge) would be around 100,000,000,000 yottapascals – 1×10^35 pascals. I don’t know how air would compress at that kind of pressure, but I reckon that 44602238100565491204200302326880195614575279686061502375654 cubic light years (or 4.5×10^103 tonnes) of air would almost definetly collapse into a black hole and maybe destroy the universe. I mean, 4.5×10^103 is more than 1×10^50 times denser than the whole universe. So it isn’t a good idea. Don’t try this at home.
Next time: Remember those electron shells? I hope you like those. If not, the next post will be about atomic nuclei with hula hoops. If you do like them, it is about electron shells. It is actually about both of them simultaneously. That idea will make more sense next week.