Lookup:      ;INPUT: X = atomic number of the element of interest.
LDA PeriodicTable_Column,x
STA $20                    ;store column number in memory (I chose $20 arbitrarily, you can store it anywhere)
LDA PeriodicTable_Row,x
STA $21                    ;store row number in memory
RTS

PeriodicTable_Column:
db $ff,$01,$18,$01,$02,$13,$14,$15,$16,$17,$18,...         ;I don't need to write them all out, the concept is self-explanatory enough.
PeriodicTable_Row:
db $ff,$01,$01,$02,$02,$02,$02,$02,$02,$02,$02,...