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Name: Alex
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How are scientists able to obtain energy from splitting an atom?

Dear Alex,

The energy really comes from splitting the nucleus of an atom into two lighter nuclei. The heaviest nuclei, like uranium 235 are not so tightly bound since they have many positively charged protons (92 for uranium) which repel each other. The products of the fission include two nuclei, each with about half as many protons which are therefore more tightly bound.

In a fission reaction, the products of the fission have a total of about 0.5% less mass that the parent nucleus. Using E = mc^2, this means that some mass has been changed to some other form of energy.

0.5% may seem like not much, but it is about 1,000,000 times larger than the mass reduction in a chemical reaction.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Thanks for your question! As you probably know, atoms are very small -- and they are very strange also. One of the strange things about atoms is that all of the particles with positive charges (the "protons") are all very close to each other in the central part of the atom called the "nucleus". This is strange because the the protons have the same charge (they are all positive) and like charges do NOT attract each other -- in fact, they repel! If you have ever experimented with magnets, you know that like poles (north and north, for example) repel each other and opposite poles (like north and south) attract each other. In a similar manner, two protons will repel each other, while an electron (which has a negative charge) and a proton will attract each other.

So it is very strange that all the positive parts (the protons) are all together in the nucleus of an atom. It takes a special nuclear force, called the "strong force" to keep all of these positive charges close to each other. Sometimes, when a neutral particle (meaning it has no charge -- it is not positive or negative), called a "neutron", enters the nucleus of the atom the atom splits (or "fissions") into two smaller nuclei and a couple of neutrons. These smaller nuclei require a smaller amount of energy ("binding energy") to hold them together, so some energy is released also. The extra energy is released when the atom fissions in the form of heat.

The total amount of energy from one fission is pretty small. But if the other neutrons also enter nearby nuclei, you can get a chain reaction that causes a rapid, exponential increase in the amount of energy being released. Imagine dominoes lined up. If the first domino knocks down 2 others, then those 2 knock down 4, then those 4 knock down 8, you can see how this would grow to a very large number of dominoes falling down in a short period of time.

If the reaction continues uncontrolled, you get the explosive energy associated with nuclear weapons. If the reaction is controlled, you can remove heat from the reaction with a coolant (such as water) and use the heated water for things like generating electricity or driving an aircraft carrier through the ocean.

For more information about how this works in nuclear power plants, see this website:


Todd Clark


A heavy atom has more mass than all of the pieces after being split. Until Albert Einstein's work with relativity, scientists believed that mass always stays the same. If you break a ten pound block into two pieces, you still have ten pounds of material. Splitting an atom reduces mass. It would be like having only nine pounds of material after splitting the ten pound block. The mass becomes usable energy according to Einstein's E=mc^2 relationship.

At the level of individual atoms, mass acts like a form of energy. Mass energy can change into other kinds of energy. Two common forms are heat and light. Burning something actually uses this same principle, but on a larger scale. Carbon has a mass. Oxygen has a mass. A carbon dioxide molecule has a slightly smaller mass than a carbon atom and two oxygen atoms. The slight decrease in mass becomes heat energy. This is not nearly as powerful as splitting an atom, but it is based on E=mc^2.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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