Final answer:
John Newlands' contribution to the modern periodic table includes the proposal of the Law of Octaves, suggesting that element properties repeat every eight elements, a concept that paved the way for later arrangements by atomic number.
Step-by-step explanation:
John Newlands' contribution to the development of the modern periodic table came through his formulation of the Law of Octaves. By arranging the known chemical elements in order of increasing atomic mass, Newlands noted that every eighth element had similar chemical properties, which he likened to the octaves of music. However, his model was limited and did not work for elements heavier than calcium. Although initially ridiculed, the concept of periodicity in element properties he proposed eventually contributed to the periodic table crafted by scientists like Mendeleev, who arranged elements by atomic mass, and Moseley, who later redefined the arrangement by atomic number. Newlands' work was a stepping stone that reflected the evolving understanding of atomic theory and chemistry in his time, underscoring the importance of recognizing patterns and periodicity in element properties.