Symbol Na for sodium,
because in latin sodium is called natrium symbol=Na
mass is used in case og ionic compounds that do not contain discrere molecular unit.
Formula mass is used in case of ionic compounds that do not contain discrete molecular unit.
Formula mass is the sum of all atomic mass unit of elements composing a compound.
Example:BaCl₂ = 1 Barium and 2 Chlorine Ba = 137.3 × 1 = 137.3 amu + Cl = 35.45 × 2 = 70.9 amu Formula mass of BaCl₂ = 208.2 amu
Hope it helps=)