PHYSICS 



We have direct and convincing evidence that the 

 atoms of the chemical elements are made up of a 

 positively-charged nucleus, which contains almost 

 the whole of the mass of the atom, in association 

 with such a number of relatively light negatively- 

 charged electrons as are required to make the 

 whole structure electrically neutral. The simplest 

 atom, that of hydrogen, consists of a single 

 negative electron having ^ of the total mass in 

 association with a single nucleus having a positive 

 charge of equal magnitude and possessing a mass 

 equal to J-f^j of that of the atom. According to 

 Bohr's theory the electron and the nucleus are 

 in a state of relative motion similar to that of 

 the earth and the sun. According to the quan- 

 tum hypothesis, however, only certain orbits are 

 possible, namely, those which satisfy the quantum 

 conditions referred to in the last paragraph. Such 

 orbits are executed without loss of energy by 

 radiation, and are called by Bohr the stationary 

 states of the atom. There are an infinite number 

 of such states corresponding to all the positive 

 integral numbers. Each of these states is a poten- 

 tial hydrogen atom, but the state for which n = i 

 possesses much less energy, and is therefore much 

 more likely to be formed than any of the others. 

 This particular state is taken to represent the 

 ordinary hydrogen atom. The other states are 

 only likely to occur in appreciable numbers under 



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