Mendeleev arranged elements primarily in order of:

1. Mendeleev's periodic table predates the modern one. 2. He used the increasing atomic masses of the elements he knew. 3. He left gaps for undiscovered elements (eka-aluminium, eka-silicon, etc.). 4. Atomic number became the basis only after Moseley. 5. So the original Mendeleev arrangement used atomic mass. _Source: NCERT Class 10 Science Ch 5 'Periodic Classification of Elements'_