Number: A mole is an amount of a substance equal to a quanity. of 602 sextillion particles (atoms, molecules, or unit cells). Written as 602,000,000,000,000,000,000,000 = 6.02 x 1023. Mass: A mole is the atomic mass (element), molecular mass. (covalent compound) or the formula mass (ionic compound) of a. substance expressed in grams. Since atoms, molecules and unit cells are extremely small, a mole is a huge number of particles but a small mass.