The first periodic table was organized according to atomic mass, but the current periodic table is arranged according to atomic number. why is the atomic number now the basis for the periodic table?

It helps identify trends and group elements with similar properties.

Explanation:

Arranging the elements according to the atomic mass wouldn't help us have groups and periods with predictable properties that we can have when arranging it according to atomic number. Don't forget that isotopes have different masses.

Not only on the basis of atomic number its acctually on the basis of their electronic configuration, chemicals properties and physical properties.

alchemy hadn't scientific methods of study, appeal to mysticism and fantasy, the knowledge of chemistry and physics were modest at that time.

alchemy was - but as new rules were discovered it either changed into chemistry or it became "unscientific".