Lookup:
;input: D0.W = the atomic number of interest.
LEA PeriodicTable,A0
ADD.W D0,D0        ;we're indexing a table of words, so double the index.
MOVE.W (A0,D0),D0  ;D0.W contains row number in the high byte and column number in the low byte.
RTS

PeriodicTable:
DC.W $FFFF ;padding since arrays start at zero in assembly.
DC.W $0101 ;HYDROGEN
DC.W $0118 ;HELIUM
DC.W $0201 ;LITHIUM
DC.W $0202 ;BERYLLIUM
DC.W $0213 ;BORON
DC.W $0214 ;CARBON
DC.W $0215 ;NITROGEN
DC.W $0216 ;OXYGEN
DC.W $0217 ;FLUORINE
DC.W $0218 ;NEON
;etc.