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Atomic Basics

It would require a very much larger book than this one to cover the basics of our subject, electricity, in any real depth. The aim here is that you develop a feel for it. To help you do this, we are going to quickly look at some principles that will enable you to grasp the topic - bearing in mind that many aspects are still a mystery. We can, however, quite easily and quickly learn enough to enable us to work effectively (and safely) with electricity.

Before we get to practical calculations and ideas, we will very quickly check out some of the theory of the atom...

 

Chapter Outline

 


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Meet Electricity

The first observation of electricity was made (to our knowledge) by the ancient Greeks before 600 BC. They discovered that if they rubbed a rod of amber (which is fossilized tree resin) with fur, the rod would then attract small light dry articles and pick them up. The Greek for amber is elektron, and that is the origin of our words electron and hence electricity. At about the same time a substance with strange properties was discovered in Magnesia, also in Greece. This attracted certain metals to it. This is the origin of our word magnet. The word atom is derived from the Greek a-tomos meaning indivisible. Various Greek philosophers held that such an indivisible lowest-common-denominator of matter should exist, so when in the 1800's experimental evidence pointed to the existence of such a thing, the old name was used.

Since then we have discovered that the atom certainly is not indivisible - it appears to be made of a construction kit of smaller units, each of which is made of a construction kit of smaller units... . The atom does retain the distinction of being the smallest stable unit that retains a specific identity with individual properties - it is the smallest unit that can take part in a chemical reaction. Photograph 1   Open in popup window shows a message written in individual atoms. This was done by removing sulphur atoms from a molybdenum disulphide surface with a scanning tunneling microscope; you can probably guess which company financed the work. Magnetism and electricity are properties of the atom, so let's have a closer 'look' at an atom.

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The atom

Theories of the atom continue to diversify - at present there is no total description and all descriptions are highly mathematical. I am not highly mathematical, so we will use a set of 'models' or approximate descriptions that are good enough for particular applications. We will have to shift from one model to another as we look at different aspects.

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the theoretical atom

The very first thing to say is that an 'atom' is a theoretical construct. I mean by theoretical not that it is unreal, but that 'atom' is a name we use to label a set of phenomena - we find it useful to have such a name in our observations, and to make descriptions. But to label something does not actually describe it fully. We might be able to take 'pictures' of "atoms" but that doesn't make them real in the sense that stubbing your toe is real  [NOTE 1] .

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the simplest atomic model



Nevertheless, in Figure 1   Open in popup window there is a representation of an atom of hydrogen - this is the simplest and lightest atom we know of. It consists of two elementary particles (items of the 'construction kit' I mentioned) - one proton and one electron. Now remember this is a simple model - the particles are not really particles, and they certainly don't 'look' anything like this, but for our purposes they behave sufficiently like particles for this model to be useful.

In the drawings I have shown the electron on the circumference of a circle, with the proton at the centre. This harks back to an old model of the atom, the 'solar system' model, which is inaccurate (all models are!) but still useful. It assumes that the atom is a bit like our solar system, with the protons forming what is called the nucleus in the role (and position) of the sun, and the electrons orbiting around it at very high velocities whilst spinning on their axes, rather like planets.

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grasping the scale

This model is not too bad but not too good either. In terms of scale, the nucleus has a diameter of between 1/10,000th and 1/100,000th of the diameter of the atom. If you imagine yourself in a large sports arena, then the nucleus would be represented by a grain of sand in the centre of the arena and the electron would be about the size of a dust particle glittering in the sunbeams in the top of the back of the stands, this being the sort of relative sizes and distances involved. This means that most of the volume taken up by the atom is empty space  [NOTE 2] . You may find it useful to consider, when bashing into the teeth of a force 7 'yachtsman's gale', that both you and the wave that is wetting you are (being made up of atoms) essentially 99.99999999999...% empty space - though I must say it doesn't always seem to be the first thing that comes to mind.

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properties of elementary particles

Each of these 'elementary' or fundamental atomic particles has its own properties. The proton has a mass and a charge, and the electron has charge but an incredibly tiny mass. What are mass and charge? Mass is experienced in a daily way as weight. We casually say that a problem is 'massive', meaning difficult to move. The mass is the quantity of matter that an object contains. Matter is that which has mass  [NOTE 3] . You will see the circularity of this definition - physicists had to start somewhere! Weight is the effect of gravity on a mass, and depends on where the mass is. If you go to the top of a high mountain your weight will reduce (very slightly) compared with that you have when in the cockpit of your yacht, but your mass will not change. Charge is a property of some particles - we call it electric charge since it is was first associated with the electron, and the charge on the electron is the smallest amount of electricity that we have found (another circular definition, I'm afraid.)

We experience the effects of electric charge whenever we try to take off a woolen jumper when wearing a polyester shirt, or when we receive a shock from a door knob after walking across a plastic carpet. The charge on the proton is of one polarity and that of the electron is of the opposite polarity. By definition the charge on the electron was called 'negative', (which really was a mistake, as we will see, but we're stuck with it) and therefore the charge on the proton is called 'positive'. The charges are identical in quantity, being of one unit. Charge in fact only comes in integral units, 1,2,3 and so on. The polarities given are the fault of the amber rods. The process of rubbing them with fur transferred one sort of particle from the fur to the rod, so that the rod had an excess of them - therefore later the particles that were transferred were called 'electrons' and the sort of effect they produced was called negative. Incidentally even now we don't really know why this charge transfer works! (or much else, when you get down to it).

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masses and measures

By experiment the masses of the electron and the proton have been measured. The electron has a mass 1/1836th of that of the proton, so the bulk of the mass of an atom is in its nucleus . The significance of this will show up later, but one fairly obvious prediction from this fact is that it will be much easier for the light (low mass) electrons to move around than the heavy (high mass) protons, or the nucleus which is made of protons.

Early on it was discovered that opposite electric charges attract each other and that like charges repel each other. It happens that the charge on an electron is exactly equal but opposite in polarity to that on the proton, so in the hydrogen atom shown the net charge is +1 plus -1 which of course is 0. The electron and proton can be considered to be held together by their opposite charges, which attract each other. Why don't they join together and cancel out? I don't know, but my approach to the problem is to imagine a pair of billiard or snooker balls Figure 2   Open in popup window. These are set spinning to the same degree in opposite directions. If you add up the two spins the sum is zero spin. But if you just push the two balls into contact, the spins do not cancel out- they encourage each other to continue spinning. This is very inexact, but it gives the idea (see  [NOTE 4] ).

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some more simple atoms

Next to the hydrogen atom I have shown a few more simple atoms. The next on the list is helium, Figure 3   Open in popup window. Helium is very like hydrogen - it contains one electron, one proton and two new particles - neutrons. The neutrons have a mass very similar to the proton (1837 times the electron mass), but they have no charge.

For this reason they do not affect the equation - +1 plus 0 plus 0 plus -1 still equals 0. The atom of helium is much heavier than hydrogen, however, as it has four massive particles not just one - its mass is approximately 4 times as much. Then we have an atom of lithium. Lithium (Figure 4   Open in popup window) has 3 electrons, three protons and four neutrons so its mass is approximately 7 times that of hydrogen. In the case of lithium you can see that the third electron is not on the same circle or 'orbit' as the first two. We have discovered that there are, in materials found on the Earth, up to seven possible 'orbits' that electrons can take around the atom at fixed distances from the centre, and each 'orbit' can only hold a certain maximum number of electrons - the first 'orbit' can hold two, the second eight, the third eighteen and so on. Because lithium has three electrons, and the first orbit can only hold two, the third has to go to a different orbit. The 'orbits' have been given letters to identify them - from K to Q, and in lithium the K and L orbits are in use  [NOTE 5] .

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Magnetism

Earlier I mentioned magnetism. Well, we all have some experience of magnets. The origin of magnetism appears to be in the movement of the electron. It is, it seems, spinning round very fast, and rotating around the nucleus very fast, and this motion of the negative charge produces the effect which we call magnetism - Figure 5   Open in popup window. Magnetism and electricity are different faces of the same coin, as it were. A moving electron causes a magnetic effect, and a magnetic effect influences the movement of electrons.

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magnetic polarities

We all also have experience that there are two polarities of magnetism - north-seeking and south-seeking, or just north and south. You probably are aware that like poles repel each other and that unlike poles attract. Notice the similarity between this statement and the one about electric charge. This also means that the 'north pole' of the earth, which attracts the north-seeking end of our compasses, is of course a magnetic south pole. The differing polarities can be accounted for by the varying orientation of atoms, as already seen in sketch outline in Figure 5   Open in popup window. The effect produced is identical in the case of the two atoms, but from our point of view, in this diagram the motion of the electrons in one of the atoms is reversed. The result is that the effect is apparently reversed. This is another workable approximation, complicated by the fact that electrons don't really spin. They do behave as if they did, though.

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The origin of light

Another interesting thing that has been discovered about electrons and their 'orbits' is that each of the 'orbits' represents a particular energy level. An electron in the innermost 'orbit' that it can occupy has the lowest level of energy that it can have - it is said to be in the ground-state. Have a look at Figure 6   Open in popup window. As energy is added to the atom, by (for example) heating it, the atom will vibrate more and more, taking up more space or expanding, but nothing else will happen until a certain key amount of energy is gained. Once this amount is exceeded an electron will hop up from its ground-state 'orbit' to the next one out.

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atomic energy states

Once the electron has moved the atom is described as being in an excited state, and a 'solar system' representation of this is shown in Figure 7   Open in popup window. The excited state is not a stable condition, and in due course (when the energy input stops) the electron will return home, and in doing this the excess energy is emitted as a pulse of electro-magnetic energy - see Figure 8   Open in popup window. It is called electro-magnetic energy because, as I have explained, a moving charge is associated with a magnetic change as well.

The word energy has a very specific meaning in this context, being the capacity to do work. Work also has a special meaning here; it is done when a force causes a movement in the direction of the force. In other words, energy has the capacity to cause movement. The emitted electro-magnetic energy pulse is called a photon. The photon is a complicated idea, but a simplified way of thinking about it is as a packet containing a wave. The wave is vibrating, and the frequency of the vibration depends on how much energy the photon has, and that depends on how many orbits the electron jumped over. If the emitted photon reaches our eye, and if it happens to be vibrating in our visual waveband, so we will see light. All lamps produce light in this way. Depending on the number of orbits-worth of energy released so the radiation will be visible light, ultraviolet light, infrared light or X-rays. The varying frequency of the vibration is what we perceive as color. The work that is done by the photon in this case is to move another electron to a different orbit in a chemical called rhodopsin in our eye. Remarkable, really!

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quantities or quanta?

The integral (that is whole-number - 1 unit, 2 units, 3 units... and so on) nature of this process is described in Quantum theory, and the idea can perhaps be grasped by glancing back at Figure 6   Open in popup window. A 'quantum' is a 'quantised' or if you like 'quantified' amount - an integer unit, 1 quantum, 2 quanta, 3 quanta... The statement at the beginning of the last century that atoms gained or lost energy in fixed amounts when they gained or emitted photons was what earned Einstein the Nobel prize . Anyway, we won't go into Quantum theory just now, other than to acknowledge its existence. It is worth noting that, at normal room temperatures, the atoms in a substance are receiving quite a lot of energy input, and so electrons are always jumping up and down the odd energy level and giving off photons. The light given off is at far too low a frequency of infra red for us to see it, but night-vision equipment uses this radiation.

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a newer atomic model

It's time to change atomic models. The actual model of the atom in use nowadays does not have the electrons neatly packaged whizzing round in circular orbits. A slightly better model is one made up of a series of concentric shells of spherical form (called Quantum shells), each surrounding the previous one like the skins of an onion, with the electron being a little packet of spinning busyness (except it doesn't spin!) somewhere on the surface of the shell, constantly moving at high speed Figure 9   Open in popup window.

Where exactly it is and how fast it is moving we cannot say. This is for two reasons. For a start, the process of measuring its position affects the system. This, if you think about it, is true for all measuring systems, but as you approach the Quantum level the measurement effects are such as to make measurement increasingly imprecise. However, the 'rules' of the atomic universe are such that, even if we could develop an incredibly refined measuring technique, it is not possible in principle to know both where the electron is, and where it is heading, at the same time. This was originally stated as Heisenberg's 'Uncertainty principle' - welcome to the weird world of the sub-atomic! All we can say definitely is that there is a higher probability of the electron being in one place than another. You needn't worry about this too much when you get out a test meter (later in this course!). Although we can't say much about any particular electron, the laws of statistics enable us to make very precise predictions about the actions of a few billion of them at once - which is the smallest quantity you'll ever practically be dealing with!

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thieving atoms?

There is another point I will mention here. Atoms seem to like to fill up their shells, if they get the opportunity, and are not above stealing electrons from other atoms, or sharing them (rather like a time-share deal) if necessary. We will go into this a bit later. We do not have the space (or probably the inclination) to continue to examine all the different atoms that we know of, which adds up to about 105 at the moment. I say at the moment, because new ones are being discovered fairly frequently, which do not occur naturally on the earth, to our knowledge - 90-odd different atoms do occur naturally [NOTE 6] .

Each atom has a different number of neutrons and proton/electron pairs in its nucleus and shells, and the varying permutations give rise to the different properties of the materials. Depending on how full a particular shell is (you will remember that each shell can only hold a particular number of electrons) so the atom is more or less stable. Gold, for instance, is an example of an atom with a very stable structure which is largely unaffected by its proximity to other materials, as are lead and platinum, whereas other atoms are highly reactive - examples are sodium and phosphorus, which are quite unstable and reactive to the extent that they are not usually found in pure form.

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Ions and fields

We started off with the amber rod, and I said that when rubbed with fur some electrons were transferred from one material to the other. This happens much more easily with some materials than others. The electron is very light, but carries the same (though opposite) charge as the proton. It can therefore easily be pulled away from its atom, in some substances .

Now if a particular atom starts off with a net charge of +1 plus -1 = 0, then removing an electron and its charge (-1) will leave a net (+1) or positive charge on the atom. The atom is then called a positive ion. It will try to regain its missing electron(s) and therefore will attract electrons to it. Equally, if the shells are not full, an atom will try to fill them. If it captures an electron that has been detached from another atom, it will gain a negative charge, and become a negative ion. This is shown in Figure 10   Open in popup window. Because the electron is so light but highly charged, this sort of thing is actually going on all the time.

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an electric field

In the case of the amber rod, after rubbing it with fur (which is doing work, and so adding energy to the rod and the fur) a lot of electrons have been transferred to the atoms in the amber, and many of them become negative ions. The tendency of the individual electrons to return to where they come from is multiplied by many billions of times, and the result is that we experience a physical force, what we call an electric field - even one electron has an electric field, but it is too small for us to notice it . Equally if a substance has lost a lot of its electrons, it will have an electric field. The surplus or deficiency attempts to cancel itself out, and attracts any substance with the opposite condition as a result. This manifests in one way as the static clinging effect I mentioned earlier.

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potential difference

Whenever there is a charge difference between two points, the electrons will attempt to move from the place where there is a surplus to the place where there is a deficiency. If they are unable to do so there will be a stationary or static electric field between the two places, an actual physical force, and we call this difference of potential, or more briefly potential difference. The potential difference depends on the charge, the distance between the two places and the ease of movement between the two. Specifically, more charge equals more potential difference, increasing the distance equals more potential difference, and the more difficult it is for electrons to move the greater the potential difference.

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electric current

If the electrons are able to move then there will be an electric current, a movement of electrons until the charges are equalised. It is not just the electrons that move, however. The atoms with a deficiency of electrons have an effective positive charge, and will move towards the place where there is an electron surplus, if they are able to do so. The negative ions will also try to move . They will both move much more slowly than the electrons, since they are much more massive. The change from a static electric potential difference to an electric current is most dramatically demonstrated by a lightning bolt. The movement of charge is always associated with a magnetic effect, so the lightning bolt also causes a strong magnetic pulse. This situation is illustrated in Figure 11   Open in popup window

We can now see why the choice of 'negative' and 'positive' was unfortunate. If you look at Figure 12   Open in popup window you will see a representation of what happens in an electric field. Predominantly it is the very light negatively charged particles that move, so electron flow or current is from negative to positive, although we by convention regard current as flowing from positive to negative!! A big error, but by the time it was discovered we were deeply entrenched. Don't worry, it doesn't cause that many problems in practice - after all, the flow of negatively charged particles from the negative to the positive is equivalent to the flow of positively charged particles from the positive to the negative.

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The nuclear family

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elements and compounds

So far we have looked at atoms in isolation. Atoms normally don't (can't!) remain in isolation - they get together with other atoms, either of the same sort or of different sorts, and form molecules, a molecule being the smallest part of a substance that can exist by itself. A substance consisting of molecules made up of atoms of one type only is called an element, whilst one that has molecules made up of more than one type of atom is called a compound.

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atomic bonding

Molecules are glued together (for our purposes) by two types of interaction. The first of these happens when two ions, one positive and one negative, get together, and this is called ionic bonding. The other type is when atoms share electrons to fill up their shells, and this is called co-valent bonding. You will remember that I said there were several concentric shells in which electrons might be located (have another look at Figure 6   Open in popup window). The most important electrons from our point of view are those in the outermost shells, which are known as valence or mobile electrons. These are the ones that take part in the interactions between atoms, and that cause an atom to be easily or otherwise involved in reactions with other atoms. These also are the electrons available for movement as an electric current.

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electron sharing

Atoms (as I mentioned before) like to fill up their shells when they can. In Figure 13   Open in popup window there is a representation of several similar atoms of a substance called silicon, in molecular form. Silicon happens to have four valence electrons in the outer orbits, but room for eight. What tends to happen is that the atoms share their electrons, so that at any one moment a particular atom may count on having more than the four it started off with. The electrons are not 'personal', as it were, they whiz around one atom and then head off on a circuit around the next and so on. This is a molecular structure for an element, and an example of co-valent bonding.

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mixed molecules

Atoms of differing kinds also get together to form molecules. For example, atoms of iron get together with oxygen to form rust, or iron oxide. Atoms of hydrogen get together with oxygen to form di-hydrogen oxide, or water . Let's have a quick look at water before we go on. Oxygen is an interesting (not to say crucial from our viewpoint!) substance which is normally very 'poisonous' to us in pure form because it's highly reactive. What this means is, because oxygen has six electrons in its outermost shell - which has a capacity of eight - it tries extremely hard to fill this shell. It will steal or borrow two electrons from almost any other atom it runs into. When it runs into hydrogen, which only has one electron per atom, but room for two, it will link with two hydrogen atoms at once, producing a molecule as shown in Figure 14   Open in popup window. In this way each of the atoms completes its shell, and they become locked together. A molecule at room temperature does not stay still, it is continuously vibrating a little like the edge of a struck bell - it only stops moving at a temperature called absolute zero (roughly -273 degrees Celsius).

For what we are trying to do, that's enough on atoms for now: having established that atoms form molecules, we only have a couple more aspects to look at before getting on to applications - what we can make electrons, ions, atoms and molecules do.

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The phases of matter

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solid, liquid, gas

We are all familiar with the three basic states or phases that matter adopts; solid, liquid and gas. The essential difference between the three is temperature (which is another way of saying energy level).

In a solid the molecules have bound themselves by their attractive forces in a neat tight array, as close together as their size and characteristics will permit. All true solids have a crystalline structure, and if they are broken they will tend to break into regular patterns (have a look at a sugar crystal or a snowflake). There are a few substances which really are liquids but masquerade as solids, glass is one good example - if you look at old windows you can see that they are thicker at the bottom since the glass is slowly flowing downwards.

In a liquid the molecules are still vibrating and still locked together by their attractive forces, but they are not locked into a crystalline state, they can change positions. Because of this a liquid can fill any vessel (yours, if the seacocks leak).

In a gas the molecules are much farther apart than in solids and liquids, and they are moving at much higher speeds and regularly colliding with each other. The forces holding liquids and solids together do not apply, so a gas can expand to evenly fill any vessel that contains it.

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temperature as energy

The factor that determines whether a particular material is in one state or another is its temperature, or energy level. Most materials can be made to adopt any one of the states by adding or subtracting energy from them, that is heating or cooling them. Solids, liquids, and gases all come into the study of electricity, in good time, but we will finish off this section with a look at some electrical properties of solids.

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Conduction and non-conduction

The most important feature of a substance for our purposes is its capacity to conduct electricity. The ability to conduct electricity is called conductivity, and there are substances which conduct electricity well (called conductors), badly (insulators), and in between (semi-conductors). Interestingly most good electrical conductors also conduct heat well, and vice-versa. In a conductor the electrons are not bound to the atom (or rather the molecular entity) very tightly, so that they can easily be detached. Well known conductors are mostly metals, with silver being the best and copper and aluminium not far behind.

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non-metallic conductors

Non-metallic substances can also conduct electricity, graphite (which is a form of carbon) for example, and whilst pure water is an insulator, any ionic impurity in water converts it into a fair to good conductor. Since water is also the most powerful and universal solvent that we know of, that means that almost all water everywhere is, in practice, a pretty good conductor because of the ions dissolved in it. Some plastics and ceramics also conduct well, and a new breed of organic conductors will probably be significant in the future.

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insulators

Insulators have their electrons very tightly bound, so that they can move to a limited degree but require large forces to actually pull them away from the atom. Some common insulators are glass, most ceramics and plastics, rubber, dry wool or cotton, air, oils, waxes and so on. Semiconductors are substances that do not conduct electricity well but are not insulators, either. Their great contemporary use lies in that their conduction characteristics can be altered in various special ways, which we will go into later.

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Summary and review

  1. There will be a physical force called an electric field between them
  2. There will be a potential difference between them
  3. If there is a conducting path available the electrons will flow from the surplus to the deficiency

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Footnotes

 [Note 1] Why 'stubbing your toe'? Because Doctor Johnson - whose school I went to, regrettably - refuted the Idealist claims of Berkeley by kicking a rock, whilst stating 'I refute it - so'. He suffered from foot problems later in life. The man was clearly an idiot :).

 [Note 2] The constituents of the atom, to the extent that they actually exist, follow the same sorts of ratios of 'matter' to space within themselves as the atom does as a whole, so there's not a whole lot of 'stuff' in there really. You may also recall 'E=mc2' which is inaccurate (in this form) but well known. The point is - matter is (equivalent to) energy anyway. This is one of the reasons I'm not fond of Doctor Johnson's refutation. 'I refute him - so!' I say, whilst pointing to a hydrogen bomb or nuclear reactor. He did invent the dictionary, though, so it's not all bad. Very nearly, though - I may describe the school, one day.

 [Note 3] As noted in a previous footnote, mass is equivalent to energy as in Einstein's famous equation. It can all get quite tricky if you want to get pedantic

 [Note 4] Some of the other forces that hold matter together and also keep it apart - other than the electric field (properly known as the electromagnetic force) - have been labeled as the force of gravity, the weak atomic force and the strong atomic force. The electron/nucleus distances adjust until a null is achieved between the forces. I can't say much more about it without looking it up, so if you are interested you might as well look it up yourself. The net effect acts a little like the spinning balls (with a charge, of course)!

 [Note 5] The neutrons and protons are part of a family of related particles called baryons, whist electrons are from a family called leptons. They can be taken apart, and they might turn out to be made out of sub-particles (the word particle definitely breaks down here) known as 'quarks' (from James Joyce), whose extraordinary properties have been labeled as 'strangeness', 'charm'... though they may well not exist. Try a book on advanced physics for the layman for a read as good as any novel...

 [Note 6] The reason the other atoms are not commonplace hereabouts, although we can manufacture them and they are commonplace elsewhere in the universe - in stars, for example - is that the other atoms mostly are radioactive and quickly break down (by radioactive breakdown) into simpler atoms which are less radioactive, or not radioactive. These are the ones we find lying around the place.

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