You are right (slaps forehead), and I was not thinking about the historical context. The earliest tables were based on "valence", or the ratio with which an el

You are right (slaps forehead), and I was not thinking about the historical context. The earliest tables were based on "valence", or the ratio with which an element tended to combine with other elements in compounds. Two of the most consistent and easily obtained compounds are those of hydrogen and oxygen, which nearly always combine with other "monovalent" elements in ratios of 1 and 2, respectively. When a new element was discovered, the ratios with which it formed oxides and hydrides could be determined, and this would allow its placement on the table. A student from the late 1800s would only be able to think in terms of equivalents, not electrons and their orbitals - as you point out. The inclusion of hydride and oxide formula templates would have been an integral part of using the table.