ATOMIC ENERGY — JOHNS 179 



This is equal to the energy that would be generated by the total 

 electric power industry in the United States (as of 1939) running 

 for approximately 2 months. Burning this amount of coal would 

 give us 8.5 kilowatt hours of heat energy, so that the ratio is about 

 3,000 million to 1. No wonder the tiny losses of matter could not 

 be detected, and there was no confirmation of Einstein's prediction 

 for 25 years, though he had suggested that radioactive substances 

 should give it. 



THEORY OF ATOMIC STRUCTURE 



If we are to understand how atomic energy is released we must 

 know something of atomic structure, and form some picture or con- 

 struct some model of an atom. At dinner tonight Mrs. Johns re- 

 marked that the Chinese thought in pictures for theirs is a picture 

 language. One woman under a roof is their ideogram for peace. 

 That picture appeals to us; fundamentally we are all alike. Our 

 models of the atom may not be wholly correct, but if they help our 

 thinking, their creation is justified. We believe now that all atoms 

 are in the main constructed out of three fundamental bricks, the 

 electron, the proton, and the neutron, and from these atoms the 

 whole universe is built up. The electron or Beta-particle is very light 

 and exceedingly small, since 50,000 million placed side by side would 

 stretch across a period at the end of a sentence on a printed page. 

 It carries a unit negative charge of electricity. The proton carries 

 a unit positive charge of electricity, is smaller in volume than the 

 electron, but weighs 1,847 times as much. The neutron was dis- 

 covered in 1932 by Chadwick in England, has a mass close to that 

 of the proton and, as its name would indicate, carries no charge at 

 all. This unique characteristic of neutrons delayed their discovery, 

 prevents us from observing them directly, makes them very pene- 

 trating and so important in nuclear change. 



From these three basic cosmic units, the 92 elements of chemis- 

 try are built and range from the lightest, hydrogen, to the heaviest, 

 uranium. We conceive the atom as consisting of a central nucleus 

 made up of an approximately equal number of protons and neutrons ; 

 and about the same number of electrons revolving as satellites around 

 the nucleus. To be balanced electrically there must be just as many 

 electrons carrying negative charges as protons carrying positive 

 charges. This number is the atomic number of the atom. The total 

 number of protons and neutrons in an atom is its atomic weight ap- 

 proximately and is called its mass number. Thus the hydrogen atom 

 has 1 proton as nucleus, and 1 satellite electron. So its atomic number 

 is 1 and its mass number 1. The helium atom has a nucleus consisting 

 of 2 protons and 2 neutrons, and 2 satellite electrons. Its atomic num- 



