isotope

Isotopes and atomic weights

For a given element, such as oxygen, the number of neutrons can vary. But no matter what the number of neutrons, the element keeps its identity, based on the atomic num- ber. Differing numbers of neutrons result in various isotopes for a given element.

Each element has one particular isotope that is most often found in nature. But all elements have numerous isotopes. Changing the number of neutrons in an element’s

nucleus results in a difference in the weight, and also a difference in the density, of the element. Thus, hydrogen  containing a neutron or two in the nucleus, along with the pro- ton, is called heavy hydrogen.

The atomic weight of an element is approximately equal to the sum of the num-

ber of protons and the number of neutrons in the nucleus. Common carbon has an atomic weight of about 12, and is called carbon 12 or C12. But sometimes it has an atomic weight of about 14, and is known as carbon 14 or C14.

Table 1-1 lists all the known elements in alphabetical order, with atomic numbers in one column, and atomic weights of the most common isotopes in another column. The standard abbreviations are also shown.

Electrons

Surrounding the nucleus of an atom are particles having opposite electric charge from the protons. These are the electrons. Physicists arbitrarily call the electrons’ charge negative, and the protons’ charge positive. An electron has exactly the same charge quantity as a proton, but with opposite polarity. The charge on a single elec- tron or proton is the smallest possible electric charge. All charges, no matter how great, are multiples of this unit charge.

One of the earliest ideas about the atom pictured the electrons embedded in the nu- cleus, like raisins in a cake. Later, the electrons were seen as orbiting the nucleus, mak- ing the atom like a miniature solar system with the electrons as the planets (Fig. 1-1). Still later, this view was modified further. Today, the electrons are seen as so fast- moving, with patterns so complex, that it is not even possible to pinpoint them at any given instant of time. All that can be done is to say that an electron will just as likely be inside a certain sphere as outside. These spheres are known as electron shells. Their centers correspond to the position of the atomic nucleus. The farther away from the nucleus the shell, the more energy the electron has (Fig. 1-2).

Electrons can move rather easily from one atom to another in some materials. In other substances, it is difficult to get electrons to move. But in any case, it is far easier to move electrons than it is to move protons. Electricity almost always results, in some way, from the motion of electrons in a material.

Electrons are much lighter than protons or neutrons. In fact, compared to the nu- cleus of an atom, the electrons weigh practically nothing.

Generally, the number of electrons in an atom is the same as the number of protons. The negative charges therefore exactly cancel out the positive ones, and the atom is electrically neutral. But under some conditions, there can be an excess or shortage of electrons. High levels of radiant energy, extreme heat, or the presence of an electric field (discussed later) can “knock” or “throw” electrons loose from atoms, upsetting the balance.

Ions

If an atom has more or less electrons than neutrons, that atom acquires an electrical charge. A shortage of electrons results in positive charge; an excess of electrons gives a negative charge. The element’s identity remains the same, no matter how great the ex- cess or shortage of electrons. In the extreme case, all the electrons might be removed

Table 1-1.    Atomic numbers  and weights.

Element name	Abbreviation	Atomic number	Atomic weight*
Actinium	Ac	89	227
Aluminum	Al	13	27
Americium**	Am	95	243
Antimony	Sb	51	121
Argon	Ar	18	40
Arsenic	As	33	75
Astatine	At	85	210
Barium	Ba	56	138
Berkelium**	Bk	97	247
Beryllium	Be	4	9
Bismuth	Bi	83	209
Boron	B	5	11
Bromine	Br	35	79
Cadmium	Cd	48	114
Calcium	Ca	20	40
Californium**	Cf	98	251
Carbon	C	6	12
Cerium	Ce	58	140
Cesium	Cs	55	133
Chlorine	Cl	17	35
Chromium	Cr	24	52
Cobalt	Co	27	59
Copper	Cu	29	63
Curium**	Cm	96	247
Dysprosium	Dy	66	164
Einsteinium**	Es	99	254
Erbium	Er	68	166
Europium	Eu	63	153
Fermium	Fm	100	257
Fluorine	F	9	19
Francium	Fr	87	223
Gadolinium	Gd	64	158
Gallium	Ga	31	69
Germanium	Ge	32	74
Gold	Au	79	197
Hafnium	Hf	72	180
Helium	He	2	4
Holmium	Ho	67	165
Hydrogen	H	1	1
Indium	In	49	115
Iodine	I	53	127
Iridium	Ir	77	193
Iron	Fe	26	56

Krypton                            Kr                                       36                                   84

Lanthanum                     La                                       57                                  139

Lawrencium**                Lr or Lw                           103                                 257

A good example of an ionized substance is the atmosphere of the earth at high altitudes. The ultraviolet radiation from the sun, as well as high-speed subatomic par- ticles from space, result in the gases’ atoms being stripped of electrons. The ionized gases tend to be found in layers at certain altitudes. These layers are responsible for long-distance radio communications at some frequencies.

Ionized materials generally conduct electricity quite well, even if the substance is normally not a good conductor. Ionized air makes it possible for a lightning stroke to take place, for example. The ionization, caused by a powerful electric field, occurs along a jagged, narrow channel, as you have surely seen. After the lightning flash, the nuclei of the atoms quickly attract stray electrons back, and the air becomes electrically neu- tral again.

An element might be both an ion and an isotope different from the usual isotope. For example, an atom of carbon might have eight neutrons rather than the usual six, thus being the isotope C14, and it might have been stripped of an electron, giving it a positive unit electric charge and making it an ion.

Compounds

Different elements can join together to share electrons. When this happens, the result is a chemical compound. One of the most common compounds is water, the result of two hydrogen atoms joining with an atom of oxygen. There are literally thousands of dif- ferent chemical compounds that occur in nature.

A compound is different than a simple mixture of elements. If hydrogen and oxy- gen are mixed, the result is a colorless, odorless gas, just like either element is a gas separately. A spark, however, will cause the molecules to join together; this will liber- ate energy in the form of light and heat. Under the right conditions, there will be a vi- olent explosion, because the two elements join eagerly. Water is chemically illustrated in Fig. 1-3.

1-3    Simplified diagram of a water molecule.

Compounds often, but not always, appear greatly different from any of the ele- ments that make them up. At room temperature and pressure, both hydrogen and oxy- gen are gases. But water under the same conditions is a liquid. If it gets a few tens of degrees colder, water turns solid at standard pressure. If it gets hot enough, water be- comes a gas, odorless and colorless, just like hydrogen or oxygen.

Another common example of a compound is rust. This forms when iron joins with oxygen. While iron is a dull gray solid and oxygen is a gas, rust is a maroon-red or brownish powder, completely unlike either of the elements from which it is formed.

Molecules

When atoms of elements join together to form a compound, the resulting particles are molecules. Figure 1-3 is an example of a molecule of water, consisting of three atoms put together.

The natural form of an element is also known as its molecule. Oxygen tends to occur in pairs most of the time in the earth’s atmosphere. Thus, an oxygen molecule is some- times denoted by the symbol O2. The “O” represents oxygen, and the subscript 2 indi- cates that there are two atoms per molecule. The water molecule is symbolized H2O, because there are two atoms of hydrogen and one atom of oxygen in each molecule.

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Insulators    11

Sometimes oxygen atoms are by themselves; then we denote the molecule simply as O. Sometimes there are three atoms of oxygen grouped together. This is the gas called ozone, that has received much attention lately in environmental news. It is written O3.

All matter, whether it is solid, liquid, or gas, is made of molecules. These particles are always moving. The speed with which they move depends on the temperature. The hotter the temperature, the more rapidly the molecules move around. In a solid, the molecules are interlocked in a sort of rigid pattern, although they vibrate continuously (Fig. 1-4A). In a liquid, they slither and slide around (Fig. 1-4B). In a gas, they are lit- erally whizzing all over the place, bumping into each other and into solids and liquids adjacent to the gas (Fig. 1-4C).

Conductors

In some materials, electrons move easily from atom to atom. In others, the electrons move with difficulty. And in some materials, it is almost impossible to get them to move. An electrical conductor is a substance in which the electrons are mobile.

The best conductor at room temperature is pure elemental silver. Copper and alu- minum are also excellent electrical conductors. Iron, steel, and various other metals are fair to good conductors of electricity.

In most electrical circuits and systems, copper or aluminum wire is used. Silver is impractical because of its high cost.

Some liquids are good electrical conductors. Mercury is one example. Salt water is a fair conductor.

Gases are, in general, poor conductors of electricity. This is because the atoms or molecules are usually too far apart to allow a free exchange of electrons. But if a gas be- comes ionized, it is a fair conductor of electricity.

Electrons in a conductor do not move in a steady stream, like molecules of water through a garden hose. Instead, they are passed from one atom to another right next to it (Fig. 1-5). This happens to countless atoms all the time. As a result, literally trillions of electrons pass a given point each second in a typical electrical circuit.

You might imagine a long line of people, each one constantly passing a ball to the neighbor on the right. If there are plenty of balls all along the line, and if everyone keeps passing balls along as they come, the result will be a steady stream of balls moving along the line. This represents a good conductor.

If the people become tired or lazy, and do not feel much like passing the balls along, the rate of flow will decrease. The conductor is no longer very good.

Insulators

If the people refuse to pass balls along the line in the previous example, the line repre- sents an electrical insulator. Such substances prevent electrical currents from flowing, except possibly in very small amounts.

Most gases are good electrical insulators. Glass, dry wood, paper, and plastics are other examples. Pure water is a good electrical insulator, although it conducts some current with even the slightest impurity. Metal oxides can be good insulators, even though the metal in pure form is a good conductor.

Electrical insulators can be forced to carry current. Ionization can take place; when electrons are stripped away from their atoms, they have no choice but to move along. Sometimes an insulating material gets charred, or melts down, or gets perforated by a spark. Then its insulating properties are lost, and some electrons flow.

An insulating material is sometimes called a dielectric. This term arises from the fact that it keeps electrical charges apart, preventing the flow of electrons that would equalize a charge difference between two places. Excellent insulating materials can be used to advantage in certain electrical components such as capacitors, where it is im- portant that electrons not flow.

Porcelain or glass can be used in electrical systems to keep short circuits from oc- curring. These devices, called insulators, come in various shapes and sizes for different applications. You can see them on high-voltage utility poles and towers. They hold the wire up without running the risk of a short circuit with the tower or a slow discharge through a wet wooden pole.

Resistors

Some substances, such as carbon, conduct electricity fairly well but not really well. The conductivity can be changed by adding impurities like clay to a carbon paste, or by wind- ing a thin wire into a coil. Electrical components made in this way are called resistors. They are important in electronic circuits because they allow for the control of current flow.

Resistors can be manufactured to have exact characteristics. Imagine telling each person in the line that they must pass a certain number of balls per minute. This is anal- ogous to creating a resistor with a certain value of electrical resistance.

The better a resistor conducts, the lower its resistance; the worse it conducts, the higher the resistance.

Electrical resistance is measured in units called ohms. The higher the value in ohms, the greater the resistance, and the more difficult it becomes for current to flow. For wires, the resistance is sometimes specified in terms of ohms per foot or ohms per kilometer. In an electrical system, it is usually desirable to have as low a resistance, or ohmic value, as possible. This is because resistance converts electrical energy into heat. Thick wires and high voltages reduce this resistance loss in long-distance  electrical lines. This is why such gigantic towers, with dangerous voltages, are necessary in large utility systems.

Semiconductors

In a semiconductor, electrons flow, but not as well as they do in a conductor. You might imagine the people in the line being lazy and not too eager to pass the balls along. Some semiconductors carry electrons almost as well as good electrical conductors like copper or aluminum; others are almost as bad as insulating materials. The people might be just a little sluggish, or they might be almost asleep.

Semiconductors are not exactly the same as resistors. In a semiconductor, the ma- terial is treated so that it has very special properties.

The semiconductors include certain substances, such as silicon, selenium, or gal- lium, that have been “doped” by the addition of impurities like indium or antimony. Perhaps you have heard of such things as gallium arsenide, metal oxides, or silicon rectifiers. Electrical conduction in these materials is always a result of the motion of electrons. However, this can be a quite peculiar movement, and sometimes engi- neers speak of the movement of holes rather than electrons. A hole is a shortage of an electron—you  might think of it as a positive ion—and it moves along in a direction opposite to the flow of electrons (Fig. 1-6).

1-6    Holes move in the opposite direction from electrons in a semiconducting material.

Static electricity   15

When most of the charge carriers  are electrons, the semiconductor is called N-type, because electrons are negatively charged. When most of the charge carriers are holes, the semiconducting  material is known as P-type because holes have a positive electric charge. But P-type material does pass some electrons, and N-type material car- ries some holes. In a semiconductor, the more abundant type of charge carrier is called the majority carrier. The less abundant kind is known as the minority carrier.

Semiconductors are used in diodes, transistors, and integrated circuits in almost limitless variety. These substances are what make it possible for you to have a computer in a briefcase. That notebook computer, if it used vacuum tubes, would occupy a sky- scraper, because it has billions of electronic components. It would also need its own power plant, and would cost thousands of dollars in electric bills every day. But the cir- cuits are etched microscopically onto semiconducting wafers, greatly reducing the size and power requirements.

Current

Whenever there is movement of charge carriers in a substance, there is an electric current. Current is measured in terms of the number of electrons or holes passing a single point in one second.

Usually, a great many charge carriers go past any given point in one second, even if the current is small. In a household electric circuit, a 100-watt light bulb draws a cur- rent of about six quintillion (6 followed by 18 zeroes) charge carriers per second. Even the smallest mini-bulb carries quadrillions (numbers followed by 15 zeroes) of charge carriers every second. It is ridiculous to speak of a current in terms of charge carriers per second, so usually it is measured in coulombs per  second instead. A coulomb is equal to approximately 6,240,000,000,000,000,000 electrons or holes. A cur- rent of one coulomb per second is called an ampere, and this is the standard unit of electric current. A 100-watt bulb in your desk lamp draws about one ampere of current.

When a current flows through a resistance—and this is always the case because even the best conductors have resistance—heat is generated. Sometimes light and other forms of energy are emitted as well. A light bulb is deliberately designed so that the resistance causes visible light to be generated. Even the best incandescent lamp is inefficient, creating more heat than light energy. Fluorescent lamps are better. They produce more light for a given amount of current. Or, to put it another way, they need less current to give off a certain amount of light.

Electric current flows very fast through any conductor, resistor, or semiconductor. In fact, for most practical purposes you can consider the speed of current to be the same as the speed of light: 186,000 miles per second. Actually, it is a little less.

Static electricity

Charge carriers, particularly electrons, can build up, or become deficient, on things without flowing anywhere. You’ve probably experienced this when walking on a car- peted floor during the winter, or in a place where the humidity was very low. An excess or shortage of electrons is created on and in your body. You acquire a charge of static

electricity. It’s called “static” because it doesn’t go anywhere. You don’t feel this until you touch some metallic object that is connected to earth ground or to some large fixture; but then there is a discharge, accompanied by a spark that might well startle you. It is the current, during this discharge, that causes the sensation that might make you jump.