Large amounts of energy can be measured in, say, millions of barrels of oil equiva­lent or kilotons of TNT equivalent. A more familiar household unit is the kilowatt- hour (1,000 Wh), which is used in our electric bills. A 100-W bulb will use 100 W h of electricity every hour. Since there are 3,600 seconds in an hour, a watt-second (which is called a joule) is 1/3,600 of a watt-hour, or the energy used by a 1-W cell phone in 1 s. These are units that we use on a human scale. When we talk about atoms, however, we have to use much smaller units because atoms are very small. There are some 100,000,000,000,000,000,000,000 atoms in a teaspoon of water. So the energy of an atom would be that much smaller than the energy units, like a watt-second, that we encounter in real life.

First, let’s find a way to avoid writing all those zeroes. Scientific notation is an easy shorthand to do this. The large number above has 23 zeroes and is written as 1023, where the superscript, called an exponent, tells how many zeroes follow the 1. A thousand (1,000) would be written as 103 and pronounced “ten to the third power” (or ten cubed in this case). Three thousand would be 103 multiplied by 3, written as 3 x 103. 3,600 would be written 3.6 x 103, and so forth. This works also for fractions if we use negative exponents. One thousandth (1/1,000) would be 10-3. Two hundredths would be 2 x 10-2. The only thing to note is that if we write decimals, 1/1000 would be 0.001, and the number of zeroes is one less than the exponent. But you need not worry about that; just remember that 10-3 is a thousandth, 10-6 is a millionth, 10-9 is a billionth, and so forth.

How much energy is released in fusing two hydrogen atoms? It is approximately 3 x 10-18 J. Joules are too large when dealing with atoms. A more convenient unit of energy is in order. The unit used is the electron-volt, or eV, which is more like the size of the energies of atomic particles. One electron-volt is 1.6 x 10-19 J. Now we can use eVs and stop counting zeroes. Since we will be talking about atoms in the next few chapters, we will use eVs and not worry about changing to more familiar units until we have to design reactors.

Let’s get an idea of how big 1 eV of energy is. Molecules, CO2 for instance, are held together with an energy of about 1 eV. An atom is a nucleus surrounded by electrons, equal in number to the protons in the nucleus. The outermost electron in an atom is bound to the nucleus with about 10 eV. A fusion reaction yields about 10 million eV or 10 MeV. A fission reaction yields about 100 MeV. The advantage of nuclear power is now obvious. Chemical reactions involve molecules and atoms, as in the burning of gasoline. These reactions yield eVs of energy each, and therefore a large number of molecules (read tankfuls of gasoline) are needed in normal use. Chemical energy is already very efficient. Witness monarch butterflies going 2,000 miles from Canada to Mexico or demoiselle cranes going from Russia to India over the Himalayas with no food or stopping. But chemical energy is infinitesimal compared with nuclear energy. Nuclear reactions yield tens to hundreds of millions of eVs each, so that the fuel needed for even a large power plant occupies a relatively small volume. Some think of hydrogen fusion as “burning” water. To do this in a chemical sense means that you first have to separate the hydrogen from H2O and then ignite the hydrogen. The energy you get is relatively small, since it is a chemical reaction. In any case, you can’t get any more energy out than it took to separate the hydrogen from the oxygen in the first place. But “burning” the hydrogen in a nuclear sense yields many million times more energy than in chemical burning.