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Niels Bohr was the first to propose ( 1923 ) that the periodicity in the properties of the elements might be explained by the periodic filling of the electron energy levels , resulting in the electronic structure of the atom .
 Niels Bohr
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Niels Bohr