THE ATOM 95 



attraction of the nucleus, repel each other. Again, 

 it is the gravitational mass of the sun that attracts 

 the planets, but in the atoms it is the charge on the 

 nucleus and not its mass that attracts the electrons. 

 Newton's law of gravitation therefore is replaced 

 by Coulomb's law of electrical attraction. Further, 

 whereas the planets of the solar system keep to their 

 several orbits, the electrons of an atom (responding 

 to excitation, and accompanied with marked changes 

 in energy) jump, or fall, from one orbit (or energy 

 level) to another. 



Again, because it was found that the application 

 of the classical theory led to inconsistencies with 

 the facts of the atom's stability, energy changes in 

 the atom are duly reckoned with on the quantum 

 theory. Thus there has resulted the new mechanics 

 of the atom; and in agreement with the finding 

 that in the series of the elements, beginning with 

 hydrogen, atomic number 1, with each succeeding 

 element one electron is added to the atom, the 

 modified Bohr atom pictures the successive binding 

 of the electrons in the field of the nucleus. 



In the modified Bohr theory, the requisite number 

 of electrons — the number permitted by the nucleus — 

 are grouped around the nucleus in concentric or- 

 bits, orbits of various types, and in successive 

 "shells," or "energy-levels," the K-shell screening 



