There is a crater on the moon named after William Francis Gray Swann. He is widely recognized for his research into cosmic rays and high-energy physics. His book, The Architecture of the Universe, was published in 1934. In that seminal work Swann, referring to chemical elements, made the statement:

Starting from any one of them, and noting some property such as the melting point, for example, the property would change as we went along the row, but as we continued it would gradually come back to the condition very similar to that which we started ... The eighth element was in many respects like the first, the ninth like the second, the tenth like the third, and so on. Such a slate of affairs point[s] not only to a varied internal structure, but also to a certain harmony in that variation suggestive of some organized plan in building the atom.

The uniqueness of the individual elements and the recurring features that Swann describes are similar to the way our seven note diatonic scale plays out within our musical heritage, together with seven named colors of the rainbow. Of course there are other ways to segment a musical scale and the colors of the rainbow are clearly in the eye of the beholder. The naturally occurring and recurring relationships in chemistry and those that can be attributed to human preference may be worlds apart. But, why do we have such an affinity for sevens?