how was the periodic table arranged before the discovery of protons neutrons and electrons?
by atomic mass
many elements were placed in the wrong place as by being ordered by atomic mass it did not take into account their properties (metals and non-metals were mixed)
why did mendeleev leave spaces in his periodic table?
for undiscovered elements and so he could predict future elements as he found they fitted a pattern
how many elements were there in the first periodic table
