What was the property used to organize elements in the modern periodic table?

The primary organizing characteristic is Atomic Mass. As the elements pass in the Periodic Table, the atoms' mass increase.

However a qick glance shows the elements are not presented in a long string, they double back to form a grid with rows and columns.

The rows (left to right) have the same number of atomic orbitals which fill up as the row movers from left to right. Starting a new row starts a new orbital.

The groups are the columns of elements (up and down) that have similar characteristics based on how full the orbital is. As an example every element in the firat column has one electron in its outer orbital. A warning the transitional elements do not follow this rule exactly but are in separate rows and column.

Finally at the top of the column are two single elements hydrogen (H) and Helium (He). They are made up by filling the smallest and most interior orbital (S) which has space for only two electrons. As a consequence hydrogen is stacked with the reactive metals and helium with the noble gases