Structure of Matter

One topic which we shall certainly discuss is the structure of matter. We shall find that there is a simple, general pattern in the structure of all the solid materials which principally concern us, and that electrons are a key part of that pattern.

There are ninety-two separate chemical substances from which the world is made; substances such as oxygen, carbon, hydrogen, iron, sulphur and silicon. Often, of course, they are combined together to make more elaborate materials, such as hydrogen and oxygen in water. But these are the ninety-two elements; elements which the chemists have grouped in a list called the Periodic Table and which, in various combinations, go to make up all the millions of compounds that exist.

If, however, an atom of any one of these elements is examined, it will be found to consist of an assembly of three different kinds of particle: protons, electrons and neutrons. The atoms of some elements may contain several hundred particles, while other elements may have less than ten particles in the atom.

Before we look at the way in which these particles makeup an atom, we need to know something of their two most important properties: mass and electrical charge. The proton and the neutron both have approximately the same mass, and the mass of the electron is very much less - about 1/1840 of the mass of the proton. The electron is negatively charged, and the proton has a charge of the same size but of positive sign. The neutron carries no charge.

Any object, from an atom upwards in size, will normally contain equal numbers of protons and electrons, and will thus have no net electrical charge. If electrons are removed in some way from the object, it will be left with a net positive charge. If electrons are added to the object, it will become negatively charged.

Two objects which are electrically charged exert a force on each other which is inversely proportional to the square of their distance apart. If the two charges have the same sign, then the objects repel each other, and if the charges are of opposite sign, then they attract each other. In particular, a proton and an electron will attract each other, and the closer they are together, the greater will be the force.

In general, an atom has a central core called the nucleus, which consists of protons and neutrons. Surrounding this nucleus is a cloud of electrons. The number of protons in the nucleus is equal to the number of electrons in the cloud. The total positive charge on the nucleus due to all the protons is just balanced by the total negative charge of all the electrons in the cloud, and the atom as a whole is electrically neutral.

To get an idea of the principles common to the structure of all atoms we shall start by considering the simplest atom-that of the element hydrogen.

Hydrogen is the lightest of all the atoms, with a single electron in the electron `cloud' and a single proton as its nucleus., All other elements have neutrons as well as protons in the nucleus; for example, helium, the next simplest atom, has two electrons in the cloud, with two protons and two neutrons in the nucleus.

In the hydrogen atom the electron rotates round the nucleus-in this case a single proton-rather like the Earth rotates round the Sun. If we think of the circular electron orbit, then its radius is such that the electrical attraction between the positively charged proton and the negatively charged electron is just sufficient to provide the force needed to bend the electron path into a circle, just as a stone whirled round on a string would fly off at a tangent if it were not constrained to its circular path by the tension in the string.

This simple system with the electron rotating round the nucleus has a certain amount of energy associated with it. First, there is the kinetic energy of the moving electron. Any moving body possesses kinetic energy, and the amount of energy is proportional to the mass of the body and the square of its velocity. Thus a small, high-speed body like a bullet may have more kinetic energy than a much larger body moving slowly.

The second kind of energy associated with the hydrogen atom is potential energy, due to the fact that a positive and negative charge separated by a certain distance attract each other, and could be organised to do work in coming together. In the same way, a lake of water at the top of a mountain has potential energy associated with it, because it could be organised to do work by running through turbines down into the valley. The higher the lake is above the valley, the greater will be the potential energy. Similarly, the farther the electron is from the nucleus in the hydrogen atom, the greater will be the potential energy.

The total energy associated with the hydrogen atom. is the sum of the potential energy and the kinetic energy. This total depends upon the radius of the orbit in which the electron rotates and has a minimum value for hydrogen atoms in the normal state. If a normal hydrogen atom is in some way given a little extra energy, then the electron moves out into an orbit of greater radius, and the total ; energy associated with the atom is now greater than it was in the normal state. Atoms possessing more than the normal amount of energy are said to be `excited'.

The way in which atoms receive extra energy to go into an excited state, and the way in which they give up that energy in returning to the normal state, is of fundamental; importance and we shall discuss it later as the quantum: theory. In particular, we shall find that energy can only be given to an atom in packets of certain sizes and that energy is emitted by excited atoms in similar packets. The wrong size packet will not be accepted by an atom, and an excited atom will never emit anything other than one of a limited set of packet sizes.

The fact that an atom of any particular element can emit or absorb energy only in packets of certain sizes is' one consequence of a set of rules which governs the behaviour of electrons in atoms. These rules also give rise to two other important general properties of the' electrons in atoms.

The first of these is that, within the atom, an electron may only possess certain energies-there are certain `permitted energy levels' for the electron. This concept of permitted energy levels is extended later from the single isolated atom to the electrons in solids, where the electrical properties are largely determined by the permitted energy levels and which of them are possessed by electrons.

Another consequence of the rules for electron behaviour within the atom is one which concerns us immediately, because these rules establish a set of patterns into which the electrons are arranged in more elaborate atoms than those of hydrogen.

The hydrogen atom is very simple, with its single electron rotating in a circular orbit, which is of a fixed radius for the normal state of the atom.

Helium is the next simplest atom, with two electrons and, of course, two protons in the nucleus to balance the negative charge of the electrons. The nucleus also contains two neutrons, and the two electrons circulate round this nucleus in the same orbit.

When we come to the next element, lithium, which has three orbital electrons, the electrons are arranged in a new way round the nucleus. The nucleus now contains three protons and some neutrons.

Two of the electrons are in the same orbit, but the third one is in a different orbit, farther away from the nucleus. The rules say that the innermost orbit, or shell, is completely filled when it has two electrons. Atoms like lithium, with more than two electrons, must start a second orbit of greater radius to accommodate the third; fourth, etc., electrons. When this second orbit has eight electrons in it, there is no room for more, and a third orbit of still larger radius has to be started. Thus sodium, which has eleven orbital electrons, has two in the innermost orbit, eight in the next orbit and one in the outermost orbit.

In the illustrations of electron orbits for various atoms, the number with the positive sign indicates the number of protons in the nucleus and thus is equal to the number of orbital electrons. This number is called the atomic number of the element. There are neutrons in all the nuclei except hydrogen. The chemical properties of an element are determined by the electrons, and in this respect it is not surprising that the outermost electrons are most important because, when two atoms come together in a chemical reaction, it is the electrons in the outside orbits which will first meet and interact. It is worth noting from the illustrations that hydrogen, lithium and sodium, all with a single electron in their outside shell, have a general similarity of chemical behaviour. Similarly, many of the atoms with two electrons-or with three, etc.- in the outer shell, can be grouped together as being related chemically.

Atoms do not normally have a separate existence, and the simplest form in which we meet matter in the natural world is the gas. This consists of single molecules of the substance moving about at random and largely independent of each other except when they collide.

Typical gas molecules are oxygen (O₂) and hydrogen (H₂). In each of these molecules, two atoms of the element are joined together to make the stable unit found in natural oxygen or hydrogen. The atoms in a molecule are bound together by forces due to complex interactions between the electron systems of the individual atoms. The exact nature of these forces does not concern us as we shall be more interested in solids than in gases. However, it is important to notice that molecules, like atoms, emit or absorb energy in packets of certain sizes and that the electrons in the molecular system also have only certain permitted energies.

Gas molecules may contain atoms of more than one element, e.g. carbon dioxide (CO₂), and in the case of some organic gases may contain many atoms of several different elements and be very large.

The solid is in many ways similar to a very large molecule. Most of the solids that are important in electronics are crystalline, and the characteristic feature of a crystal is a regular arrangement of atoms. The atoms may all be of the same element, as in copper, or they may be of different elements, as in common salt (NaCl) or copper sulphate (CuSO₄).

In the crystal, as in the molecule, the interaction of the orbital electrons binds the individual atoms into the characteristic pattern or lattice.

There are certain permitted energy levels for electrons in the solid, just as there are in the atom and the molecule, and these energy levels will be different in different materials.

One special aspect of electron properties in solids is that certain materials, particularly metals, contain some electrons which are able to move away from their parent atoms. If a battery is connected to such a material, electrons will flow through the material, and it is called a conductor of electricity. If the application of a battery to a material does not cause a flow of electric current, then the material is called an insulator.

As they flow through a conductor, the electrons which make up the current being driven round the circuit by the battery give up some of their energy to the main solid structure of the material, which thus becomes hot. If electrons lose much energy in passing through a conducting material, then that material is said to have a large resistance to current flow. A given current flowing: through a high-resistance conductor will generate much more heat than the same current flowing through a low resistance conductor. Thus the heating element of an electric fire will be made of high-resistance material, usually a metal alloy. The wires carrying the current under the floor to the fire will be made of low-resistance material, invariably a metal. Materials which offer low` resistance to the passage of an electric current are called good electrical conductors.

This, then, is the way in which the chemical energy stored in a battery is converted into heat through the action of a flow of electrons in a conductor. If a metal is heated to a sufficiently high temperature by the current, as in the tungsten lamp filament, then the same mechanism can provide light - another form of energy.

Sometimes, however, it will be necessary to encourage electrons actually to escape from the surface of the metal so that, for instance, they can be formed into a beam passing' down a cathode-ray tube to paint a picture on a screen.

The emission of electrons from a material occurs only if the electrons inside are in some way given sufficient energy to break through the surface where a sort of energy barrier exists, called the work function of the material. Materials with low work functions emit electrons easily because less energy is required for an electron to overcome the energy barrier and escape.

If a solid is heated, then it receives extra energy. This may be shared between the regular crystal lattice and some of the electrons which can escape from their parent atoms. If the electrons thereby acquire sufficient energy to escape from the surface of the solid, then the process is called thermionic emission because it is brought about by heat. Thermionic emission provides the electrons which move through the vacuum in a radio valve, but not the electrons in a transistor, which always remain inside the solid material of which the transistor is made.

Another way in which electrons may be given sufficient energy to escape is by shining light onto the surface of the material. If electrons escape as a result of receiving energy from incident light, then the process is called photoelectric emission. This is the basis of many light sensitive devices using photoelectric cells.

For a material with a given work function there is a certain critical wavelength for photoelectric emission to occur. If the wavelength is too long, there will be no emission. In general, therefore, ultra-violet or blue light, which has a short wavelength, causes photoelectric emission from more materials than the longer wavelength red or infra-red radiation.

Whether an electron stays inside a solid or escapes depends on the amount of energy the electron possesses. But inside the solid the electrons have a number of permitted energy levels, and very often important properties of the material depend on which of these energy levels are occupied. Thus a material may be an insulator if its electrons are in low energy levels, but it can be made to conduct electricity if sufficient energy is given to it to raise the electrons to higher permitted levels. Furthermore, if electrons in high energy levels give up energy and fall to lower permitted levels, then the energy emitted may be useful in special ways - lasers, for instance, depend upon such emission.

We have seen that electrons are fundamental to the structure of matter. Moreover, that the part played by an electron in the structure is very much connected with the energy levels permitted to it, and with the energy it actually possesses. In particular, we have seen that the electrons flowing as electric current through a solid may give up energy to the crystal structure, so that heat and perhaps light may be given off by the material.